US2541436A - Automatic electric toaster - Google Patents

Description

Feb. 13, 1951 B. G. OLVING 2,541,

AUTOMATIC ELECTRIC TOASTER Filed May 5, 1945 4 Sheets-Sheet 1 4 IN VEN TOR.

B. G. OLVING AUTOMATIC ELECTRIC TOASTER Feb. 13, 1951 4 Sheets-Sheet 2 Filed May 5, 1945 Patented Feb. 13, 1951 AUTOMATIC ELECTRIC TOASTER Bror G. Olving, Elgin, Ill., assignor to McGraw Electric Company, Elgin, 111., a corporation of Delaware Application May 5, 1945, Serial No. 592,135

12 Claims.

My invention relates to electric crisping and toasting devices and particularly to timing means for such appliances.

An object of my invention is to provide a timing means and an electric circuit therefor which are dependable and which will stand up in service for a relatively long period of time.

. Another object of my invention is to provide a timing means, the performance of which depends upon the inherent electrical characteristics of its component parts and the operation of which is not dependent on or controlled by anyfienergy supplied from mechanical or thermal storage devices.

- Still another object of my invention is to provide a timing means and an electric circuit therefor including an electron tube to obtain the above mentioned operation.

Yet another object of my invention is to provide an electric circuit comprising an electron tube as well as means for gradually charging an electrostatic condenser connected to the tube to thereby cause an increasing current to traverse the tube, which current operates electric relays to predetermine the time period of crisping and of toasting in that order.

' Other objects of my invention will either be apparent from a description of one circuit embodying my invention or will be pointed out during the course of such description and set forth in the appended claims.

: In the drawings,

"Figure 1 is a view in side elevation of a toaster embodying my invention,

Fig. 2 is a front elevational view of the devlce shown in Fig. 1, Figs. 1 and 2 being shown on a reduced scale,

Fig. 3 is a vertical, longitudinal, sectional view taken on the line 3--3 of Fig. 4,

Fig. 4 is a vertical, lateral, sectional view taken on the line 4-4 of Fig. 3,

Fig. 5 is a fragmentary horizontal, sectional view taken on the line 5--5 of Fig. 4,

Fig. 6 is a diagram of connections for the timing elements shown in Figs. 3, 4 and 5,

Fig.7 is a graph showing time periods for various grid bias voltages of the electron tube of Fig. 6,

j'Fig. 8 is a graph showing current values for various grid bias voltages Of the electron tube shown in Fig. 6, and,

'j Fig. 9 is a series'of graphs showing plate characteristics of the electron tube shown in Fig. 6.

wf Re1'erring first of all to Figs. 1 to 5 inclusive,

I have there shown a domestic two-slice toaster designated generally by numeral II which includes a bottom frame member l3 which may,

preferably but not necessarily be made of molded composition material. I provide positioned against the upper surface of member i3, a metal bottom or base plate l5, as well as an outer casing l'l having an open top and bottom, and acover 19 suitably secured to the top portion of the outer casing I1.

I have elected to show a two-slice toaster which is provided with a pair of openings 2| through the cover l9 through which fresh shoes of bread may be dropped into the toaster casing and through which toasted slices of bread may be removed, all as is well known in the art.

I provide two pairs of planar, vertically-extending toast heating elements 23, each one of which comprises one or more sheets 25 of electric-insulating material such as mica, on which is wound a strip 21 of a suitable electric resistance material. The means for and method of supportingcally upwardly and downwardly in a slot in the rear intermediate wall 29 while the front end portion of each of said carriers is secured to rearwardly bent end portions of a rear carriage plate 31. I provide further a front carriage plate '39 which is spaced from and secured to the rear carriage plate 31 by a plurality of headed studs 4|, each of which is secured in substantially a corner portion of the front carriage plate 39. I provide further a plurality of grooved rollers 43 on each of the headed studs 4| between the front and the rear carriage plates, which rollers are adapted to move upwardly and downwardly against-the inner surface of a pair of vertical rods 45, the lower end portions of which are adapted to fit into the base plate I5 while the upper reduced end portions thereof are adapted to fit into the front end portions of a pair of frame plates 4! and in across bar 49 provided to insure that the horizontal distance between the two rods 45 will not vary during the life of the appliance.

Since it is desired to normally yielding-1y maintain the bread carriers 35 in their upper or nontoasting position, I provide a tension coil spring the upper end portion of which is secured to a lug or bracket 53 secured against the upper end portion of the front intermediate wall 3| while the lower end of spring 5| is secured to a rearwardly extending lug 55 constituting a part of the front carriage plate 39. This will therefore yieldingly bias the bread carriers into their upper or non-toasting positions.

Means for moving the bread carriers as well as the carriage plates to their lower or toasting position shown in Fig. 3 by broken lines, comprise a bar 51 secured against the front surface of the front carriage plate 39 and extending through a slot in the front wall of the outer casing I1 and being provided with an actuating knob 59 secured thereon outside of the casing I1. Pressure on the knob by an operator will cause downward movement of the bread carriers and the parts operatively associated therewith into their lower or toasting positions where they will be held by the engagement of a projection 6I secured against the front surface of the front carriage plate 39 with a hook-shaped end 63 secured to the upper end portion of a resilient bar 65 which has its lower end mounted on a bracket 61, of substantially U-shape, which is secured against the upper surface of base plate I5.

: A pair of fixed contacts 69 and H are supported on a block 13, of electric-insulating material, which is supported from the front intermediate wall by a bracket 15. A contact bridging member 11 is insulatedly supported as by a resilient arm 18 from the rear carriage plate 31 as by a block '19, of electric-insulating material. I provide further a pair of upper fixed contacts III and 83 which are supported on a block 85 of electricinsulating material, which block is supported by a bracket 81 secured against the front surf-ace of the front intermediate wall 3|. The purpose and use of the upper fixed contacts 8| and 83 will hereinafter appear.

Referring now to Fig; 6 of the drawings, I have there illustrated a system of control for an improved crisper and toaster, which system comprises a pair of supply circuit conductors 89 and 9| of which supply circuitconductor 09 is connected to one terminal of one of the toast heating elements 23, each pair of which positioned in a toasting chamber may be connected in series elec tric circuit with each other.

I provide further an electron tub 93 which is preferably a power amplifier tube of the triode type and which has a relatively high vacuum. The tube includes a plate anode 95, a control grid 91, a cathode 99 and a filament IOI. When the filament of such a tube is energized and direct current potential of the proper polarity is impressed upon the plate and cathode, current will pass through the tube if the negative grid-bias voltage is within the proper range. If the gridbias voltage is made less negative, more current will pass through the tube, that is a decrease in the negative grid-bias voltage will produce an increase in the'plate to cathode current.

,A voltage doubler bridge of th conventional type is provided which has been designated by numeral I03. This bridge comprises two halfwave rectifiers I05 and I01 and two electrostatic condensers I09 and III, the two rectifiers I05 and I01 being connected in opposition to each other in such a'manner that when an alternating current voltage is impressed across the terminals H3 and H5 thereof, direct current of approximately double the peak voltage of said A. C. line voltage will appear across the terminals H1 and H9, terminal H1 being of positive polarity and terminal H9 being of negative polarity. At the same time one-half of the above mentioned D. C. voltage will appear between terminals H1 and H5, terminal H1 being positive with respect to terminal H5 and the other half'of the voltage will appear between terminals H5 and H9, terminal H5 being positive with respect to terminal I I9 but negative with respect to terminal I I1. An adjustable potentiometer I2I has its two terminals connected to terminals H9 and H5 and is provided with a manually-adjustable contact arm I23.

I provide also a timing condenser I25 as well as a timing resistor I21, the timing resistor I21 having a negative temperature coefficient of resistance and being subject to heat from one or more of the toast heating elements 25.- The capacitance of the condenser I25 may beon the order of 6 microiarads and the resistance of resistor I21 may be on'the order of 10 megohms.

The timing condenser I25 has one terminal thereof connected to the manually-adjustable contact: arm I23 while its other terminal is connected to one terminal of the timing resistor I21 by a conductor I29, while the other terminal of timing resistor I21 is connected by aconductor I3I- with a conductor I33 which connects terminal 'I I'5- of the Voltage doubler. bridge with one terminal of the heated filamentv as well as with theterminal The other terminal of the timing condenser I25 ,is-alsoconnected to the control grid .91. Terminal. H3 of. th voltage, doubler bridge is connected: by azcondu'ctor I35':- with supply circuit conductor. 89:? I provide a third heating or crispingelement. I31 which,'las

is shown, particularly in Figs. 3 and 5' of the; drawings, may be split in two ,ha-lves, each half being positioned beloweach slice of .bread when the bread is in its. toasting position and mounted .1' on a strip I30, of electric-insulating material.- I; provide an electromagnetic .latch;1release which includes a coil I39, a verticallyemovable armature.

core I4I connected tothe free end of a bellcrank lever I43 which is pivotally mounted on azpivotpin I45 supported by member. 61; the longerzvere. tically extending armof bell crank lever I43 being: adapted to engage the hook-shaped endportion;

63 and cause it to move out of engagem'entwith member 6 I, ber 61.

I provide further two electromagneticrelays I46 and I41, of which -relay l46 includes acoll I49, a vertically-movable armature core I5I- and;

a contact bridging member I5 2, which is adapted to engage with and be disengaged-from the fixed contacts I53 and I55. One terminal ofthe crisping heater I31 is connected to one terminal of the second toastheating element 23 by a conductor I51, which conductor J51 is connectedby aconv ductor I59 with fixed contact I53, Fixed contact I55 is connected by a conductor 'lfiIfto the other terminal of thecrisping heating: element I31,

Relay I41 comprisesl'a coil I63, an' armature terminals of coils I49 and I63 arexconnected by a conductor I13, .while the lower terminal of coil I49 is 'connected bya conductor I15 withthe plate anode 95," while; th glowergterminal of' coil, I63

is connected by a con'duct'oni16wlth'terminall'll Coil I39 may-be supported by'memof the voltage doubler bridge. Fixed contact I59 of the second relay is connected to supply circuit conductor 89, by conductors I11 and I35, while fixed contact I1I is connected by a conductor I8I with the lower terminal of coil I39. The upper terminal of coil I39 is connected to fixed terminal 69 of the main switch by a part of a conductor I83 and a conductor I85, while the other fixed terminal H of the main switch is connected to the second supply circuit conductor 9I by a conductor I81. The fixed contact 69 is also connected to terminal I I3 of the voltage doubler bridge I03 by conductor I83 and to one terminal of the third or crisping heating element I31 by conductor I89. The upper fixed contact 8| is connected to conductor I89, while the other upper fixed contact 83 is connected by a conductor I9I with the upper terminal of timing resistor I21. The filament IUI has one of its ends connected to a resistor I93, the other terminal of which is connected to conductor I11. Upon closing the main switch the voltage doubler bridge I03 is therefore energized through a circuit traceable as follows: from supply circuit conductor 89 through conductor I35, through the bridge to terminal I I5; through conductor I83, through the fixed contacts 69 and H and contact bridging member 11, through conductor I81 to the other supply circuit conductor 9|; The three toast heating elements 23, 23 and I31 are energized through a circuit traceable as follows: from supply circuit conductor 89 through toast heating elements 23, 23 and I31 through conductor I89, through the engaged fixed contacts and contact bridging member and through conductor I81 to the other supply circuit conductor 9|.

"The second relay I41 has connected in shunt circuit therewith a fixed resistor I95 in addition to an adjustable resistor I91, which latter resistor has a manually-adjustable contact arm .I 99 which is connected to the lower terminal of coil I63, while the upper terminal of coil I63 is connected to one terminal of resistor I95.

Let it now be assumed that a user of the crispingand toasting appliance desires to make use thereof. He will then drop one or two slices of bread into the toaster and then push downwardly on knob 59 to start the crispin operation. Current will flow from supply circuit conductor 89 through toasting heaters 23 and the crisping heater I31 in series, then through conductors I89 and I85, contact bridging member 11 and conductor I81 to the other supply circuit conductor 9|. Since the crisping heater I31 is fully energized while the toasting heaters 23 are not, .crisping of the slice or slices of bread will take place to a desired degree, as will be hereinafter set forth, when the crisping heater is short circuited so that the toasting heaters will be fully energized and cause browning or toasting of the slice or slices of bread being treated to a desired degree, all as will be hereinafter set forth.

The voltage doubler bridge is then energized and direct current voltage is impressed across the potentiometer IZI and a charging current begins to flow to the timing condenser through a circuit traceable as follows: from the rectifier terminal II5, through conductors I33 and I3I to and through the timing resistor I21, through conductor I29 to one terminal of the timin condenser I and from there to the adjustable arm I23 and back to the potentiometer. This charging current produces a voltage drop across the resistor I21, which voltage drop isimpresSed between the grid 91 and the cathode 99 of tube 93.

6 The polarity of the adjustable arm I23 of the potentiometer is negative with respect to that of the terminal I I5 and the voltage impressed on the grid 91 is therefore negative with respect to the cathode 99. When first connected there is no charge on the timing condenser I25 and a relatively large charging current flows through the timin resistor I21 causing a voltage drop in timing resistor I21 equal to the impressed potentiometer voltage. As the charging of the timing condenser I25 progresses, the charging current: decays or decreases and the terminal voltage across the timing condenser increases in accordance with the well-known logarithmic law depending upon the time-constant of the capacitance and resistance of the circuit. The drop across the timing resistor I21 and therefore the grid-bias voltage at any time is equal to the difference between the voltage impressed by the po-ztentiometer I2I and the voltage existing across the condenser I25 at the instant of consideration. When the condenser I25 has been fully charged to a voltage equal to the potentiometer voltage no more current flows to the condenser and the grid-bias voltage is then zero.

Referring now to Fig. 9 of the drawings, I have there shown a series of graphs or curves from which it will be seen that as the grid-bias voltage on the tube decreases, that is becomes less negative, the current through the plate circuit increases. For instance, considering a plate voltage of 150 volts in a load line indicated by 295 equal to 2,009 ohms which is the sum of the resistances of coils I49 and IE3, it will be seen that if the grid-bias voltage is more than -40 volts negative,

no current will flow in the plate circuit, but if the grid-bias voltage is decreased to -30 volts; the plate current is approximately 8 milliamperes,

at 20 volts it is 18 milliarnperes, at l() volts it is 31 milliarnperes and at I) grid-bias voltage, a current of about 50 rnilliamperes will flow in the plate circuit. This variation of plate current with the negative grid-bias voltage is shown by graph 291 in Fig. 8 of the drawings.

located at the bottom of a bread toastingcompartment is shorted by conductors I59 and IBI and the contact bridging member I52 engaging the fixed contacts I53 and I55, so that full supply" circuit voltage is impressed on the conventional toast heating elements 23. When relay I41 closes, the electromagnetic release mechanism is energized from the supply circuit conductor 9|- through conductor I81, through the closed main switch comprising fixed contacts 99 and H and the contact bridging member 11-, through conductor I85, coil I39, through conductor I8I, the

engaged contacts I69 and HI with contact bridging member I61, through conductors I11 and I35;

to the other supply circuit conductor 89.

The coil I63 of the second relay I 11 is shunted by a circuit comprising fixed resistor E and a variable resistor I91. The resistance of coil I63 may be on the order of 1090 ohms and that of resistor I95 may be on the order of 2999 ohms and 5 that of the variable resistor I91 may be on the While I give specific values order of 68,000 ohms. of resistances, it is to be understood that these are illustrative only as the actual values must be matched with the plate resistance of the electron tube employed. The purpose of the resistances" I95 and I91 connected in shunt with coil IE5 is to 7, delaythe closing oithe secondrelay which terminates the toasting operation by'deenergizing the entiretoaster;

The'first relay I45 may be set to close at 7.5 milliamperesand the relay I41 may be set to close at. 30 milliamperes when the adjustable arm l99 of the variable resistor I91 is at its lowermost position, that is when all of the resistors I95 and I91 are in circuit. The current traversing the tube 93 and coil I49 of relay I46 is divided into two parts, the larger part traversing coil I63 while the smaller part traverses the seriesconnected resistors I95 and I91, the values of these currents being inversely proportional to the respective resistances of the two circuits all as well understood. If, for instance, the resistance of coil I93 is 1000 ohms and that of the connected portion of resistor I91 in series with resistor I95 is 9000 ohms, of theplate current will traverse the coil I63 and 1 6 will traverse the resistors I95 and I91. As the relay I41 is set to close at 31 milliamperes, the plate current must increase to 35 milliamperes before the current through coil IE3 is sufficiently large to operate the relay. If the adjustable arm I99 is moved still further in a counter-clockwise direction, the plate current must be still higher before relay I49 closes.

Curve 291 in Fig. 8 shows the plate current in milliamperes corresponding to any negative grid-- bias voltage impressed on the electron tube 93. Ithas been hereinbefore set forth that the gridbias voltage in the circuit of 6 at any time is equal to the difference between the voltage impressed bythe potentiometer HM and the voltage existing across the condenser IE5 at the instant under consideration. When first connected to thecircuit, the condenser I25 has no charge and the grid-bias voltage at that instant is equal to thecharging voltage impressed by the potention eter. I2I, that it is relatively high. The charging of the condenser i225 begins immediately andthe grid-bias voltage starts to decrease logarithmically as is well known.

The time required for the grid-bias voltage to decrease from its initial maximum value to any desiredlower value is shown by the curves 209, 22 i and N3 of '7. For instance, referring to curve: 209, itwill be that he grid-bias voltage decreases from an initial voltage of -100 volts to -30volts in 60 seconds while it requires I20 seconds to drop to volts, 135 seconds to drop to volts and 150 seconds to drop to -5 volts. If the initial 5 id-bias voltage is -54 volts as shown by curve 2! it will 30 seconds to drop to-30 volts, 9-0 seconds to drop to -9 volts, 105 seconds to drop to -7 volts and 120 seconds to drop to -13 volts. If the initial grld-bias voltage is -30 volts as shown by curve 2! 3, it will require 60 seconds. to drop to -9 volts, '35 seconds to drop to '7 volts, and 90 seconds to drop to -5 volts.

Referring now to curve 209 of Fig. '1, it will be seen that if the initial grid-bias voltage which is the same as the actual potential tapped across potentiometer I2! of Fig. 6 -100 volts, it will drop to -30 volts in exactly 00 seconds. Referring now to Fig. 8, i will be seen that when the grid-bias voltage dropped to -30 volts, the plate is '1.5 milliamperes and ii the first relay is set to close at this current, this relay will automatically'close as soon as the grid-bias voltage drops to -30 volts and the auxiliary heating element I31 will be short-circuited thus terminating the first or crisping portion of the entire cycle. Itis to be understood that the resistance of the auxiliary lower resistor I31 is sufliciently high so that the vertically positioned heating elements, one at each side of a slice of bread, will not operate at a relatively high temperature so that little or no toasting of the sides of a slice of bread will be obtained during the first or the crisping,

portion of the: cycle. It is obvious that the entire slice of. bread will be subjected to a relatively low temperature conducted heat from not only the lower heater i3? but also from the two side heaters 23. As soon as the auxiliary heating element I31 is short-circuited by means of relay I46, full line voltage is impressed across the side toast heating elements 29 which then increase in temperature subjecting the dried-out slice or slicesof bread to comparatively large amounts of radiantheat for the finalbrowning or toasting portion ofthe cycle;

Referring again to curve 209 in Fig. '7, it will be seen that the grid-bias voltage continues to decrease from -30 volts to -59 volts in 60 additional seconds or from -30 volts to -1 volts in 75 additional seconds, or from -30 volts to -5 volts in additional seconds. Curve 201 in Fig. 8 indicates further that at a negative grid-bias voltage of -9 volts, the plate current is 31 milliamperes, at -'7 volts, it is 35 milliamperes, and at -5 volts, the plate voltage is 30 milliamperes. I03 is adapted to cause upward movement of contact bridgin member i631 when the current traversing it reaches 30.5 milliamperes.

It has been mentioned above that the plate" current from tube 93 will be divided into two parts when reaching the second relay I51, one

part of the current traversing coil I63 and the other part traversing resistors I95 and I91. The

resistance of coil I63 is on the order of 1000 ohms,

that of resistor I95 is 2000 ohms, that of resistor I91 to the point engaged by the movable contact arm I99 in Fig. 6 is 5000 ohms and that of the resistor 191 not in circuit is 63,000 ohms. When. the adjustable contact arm I99 is at its lowermost point, all of the resistance of resistor I91 is shunted across the coil I53, that is in series with resistor I95 so that only a small part of the plate current passes through resistors I95 and I91. the latter case when the current through coil I63 reaches its final value or 30.5 milliamperes, the

current through the shunted resistor is milliampere so that the relay closes when the plate current reaches 31 milliamperes which occurs exactly 60 seconds after operation of relay I45. If contact arm H9 is moved to substantially the position shown in Fig. 6 of the drawings, the total shunting resistance is 7000 ohms and when the 7 current through coil I63 reaches its final value of 30.5 milliamperes, the current through the shunted resistor is 4.5 milliamperes so that the relay' closes when the plate current reaches a total of 35 milliamperes which occurs exactly 75 seconds after relay I40 has operated. Further counterclockwise movement of contact arm I99 still further increases the length of time before the current value in coil I63 has reached the operating value of 30.5 milliamperes.

Referring now again to Fig. 8 of the drawings,

it will be seen that the maximum plate-current traversing the tube 93 occurs when the grid-bias voltage has dropped to zero volts and that this maximum current is 48 milliamperes. If new the resistance shunted across relay coil I 63 is less than 1600 ohms, the current traversing coil I63" (the resistance of which is 1000 ohms) would never reach its closing value of 30.5 milliamperes. The fixed resistor I95 having a resistance of 2000 ohmshas therefore been provided in series with The relay cell the variable resistor I91 and will assure closing of relay I even though the adjustable arm I98 of resistor I9'I may be turned to its extreme counter-clockwise position.

The resistance of the lower auxiliary resistor I31 is on the order of 3 ohms and that of the two resistors 23 is on the order of 11 ohms. At a supply circuit voltage of 115 volts, this means approximately 90 volts is impressed across the two heating elements 23, while 25 volts is impressed across resistor I31. This insures operation during the first part of the cycle of the main toast heating elements 23 at a relatively low temperature so that the slices of bread in the toasting compartment are subjected to very little radiant heat'but to a relatively large amount of convected heat supplied partly by heating elements 23 and partly from the lowermost heating element I31. The crisping part of the cycle is thus initiated as soon as the main switch comprising the fixed contacts 69 and H and the contact bridging member "I'I'has been closed.

Upon closure of the main switch, the filament of tube 93 and the voltage doubler bridge I83 are energized and a charging current begins to flow from the potentiometer I2I to the timing condenser I25 through the timing resistor I21, thus producing a negative grid-bias voltage between the cathode 99 and the control grid 91 of the tube and the initial grid-bias voltage is substantially A equal to the voltage supplied by the potentiometer I2I. This grid-bias voltage gradually decreases 'as thetiming condenser I25 is charged and when the grid-bias voltage has decreased to 40 volts, .a current starts to flow in the plate-circuit of the :tube through a circuit traceable as follows: from terminal I I5 of the voltage doubler bridge, through conductor I33, to the cathode 99 and then to the plate .95, through conductor I15, through coil I48,

'still further, the plate-current increases until the grid-bias voltage reaches volts at which time the plate-current has increased to 7.5 milliamperes and the first relay I46 operates to shortcircuit the auxiliary heating element I31 thereby permitting full line voltage to be impressed on the toast heating elements 23 and terminating the crisping portion of the cycle.

As the toasting operation now proceeds, the grid-bias voltage continues to decrease andthe plate-current becomes increasingly higher until :sufllcient plate-current flows in the circuit to permit a current of 30.5 milliamperes to traverse the coil I63, at which time the second relay is actuated and energizes the detent-release mechanism traversing a circuit as follows: from supply circuit conductor 9I, through conductor I81, through the main switch comprising fixed contacts '69 and H and the contact bridging member 11,..through conductor I85 and a part of conductor I83, through coil I38, the closed contacts of relay I41 and through conductors I11 and I35 to the other supply circuit conductor 89. When the contact bridging member 11 reaches its uppermost position, it will engage fixed contacts'fBI and 83 and the timing condenser is shortcircuited" so as to insure that there will be no residual charge therein when a subsequent toast- "ing operation is initiated. The circuit closed by engagement of contact bridging member I1 with the upper fixed contacts BI and 83 may be traced as follows: from the left-hand terminal of condenser I25 to the adjustable arm I23, the part of potentiometer I 2| connected in circuit, through conductors I83 and I89, to contact 8|, from there through contact bridging member 11 to the second fixed contact 83 and through conductors I9I and I29 to the other terminal of the condenser The crisping portion of the toast cycle can be varied by adjusting manually the position of the contact arm I23 of the variable potentiometer I2i as by a knob 2I5 (Fig. 5). The second or toasting portion of the timing cycle is changeable by the variable resistor I91, effected by a knob 2 I1 (Fig. 5), this toasting part of the cycle beginning when relay I is closed to short-circuit the auxiliary heating element I31 and ending when relay Ii closes to cause quick upward movement of the bread carriers and termination of a toasting cycle. It has been found that the time required to obtain a certain degree of toasting or browning by subjecting the slices of bread to radiant heat of relatively high temperature is not to a very great degree dependent on the prior crisping of the bread.

It has been hereinbefore stated that the timing resistor I21, which has a temperature resistance on the order of 10 megohms, has a negative temperature coefiicient of resistance and that it is subject to heat from one or more of the upper side toast heating elements. When the temperature of the toaster increases, the resistance of the resistor I21 decreases, the timing condenser charges at a higher rate and the negative gridbias voltage therefore also decreases at a faster rate thus shortening the length of the toasting cycle. The same comments apply also to compensation for exceedingly high and exceedingly low 'supply circuit voltages since at exceedingly high voltage applied to the main toast heating ele- -ments 23, the temperature of the heat radiated thereby against the timing resistor I21 causes a greater decrease in the resistance of the timing resistor and therefore also shortens the length .of a toasting cycle.

- ing from the spirit and scope thereof and all such modifications clearly coming within the scope of the appended claims shall be considered a part of my invention.

I claim as my invention:

1. An electron tube control system for an automatic electric toaster including electric toast heating elements, a normally open control switch for said heating elements, means to close said switch and an electromagnetic release means to cause opening of said switch, said system comprising a grid controlled electron tube, a timing resistor having a negative temperature coefficient of resistance and being subject to heat from said 'toast heating elements and a timing condenser connected in the grid circuit of said tube and two relays having their coils connected in series electric circuit withh each other and with the plate of tube, one of said relays controlling the crisping period of a slice of bread being treated and the second relay controlling the toasting period of a slice of bread being treated and a source of substantially constant direct current potential and means including said timing resistor, timing condenser and said source of direct current potential for causing a predetermined rate of increase of current traversing said tube and the coils of said relays to determine the lengths of the crisping and of the toasting periods and energization of the electromag- .gnetic release 'lilfiallfi to cause opening of the switch.

2. An electron tube control timer for .an automatic electric crisping and toasting device, c mprising a grid controlled electron tube of the negative grid-bias type, a timing resistor and a timing condenser in the grid circuit or the tube, two relays operating at difderent predetermined plate current values located in the plate circuit of th tube and adapted to control the crisping and toasting periods of the electric heating elements of an automatic electric crisping and toasting device and a variable resistor connected in parallel. circuit with the operating coil of one of said relays and adapted to predetermine the length of the toasting period of such a device.

3. An electron tube control system for an electric crisping and toasting appliance comprising a plurality of toast heating elements connected in series circuit for each slice of bread to be treated, a normally open switch for controlling said heating elements, means to cause closure of said normally open switch to start a crispingoperation, an electromagnetically releasable latch for holding said switch closed and a switch for shortcircuiting one of said heating elements, said system comprising a grid controlled electron tube, a timing resistor and a timing condenser in the grid circuit of the tube, a voltage doubler bridge source of direct current potential for charging said condenser, two relays in series circuit with .each other in the plate circuit of said tube, said source of direct current potential causing a predetermined rate of increase of current traversing said tube and. said relays, one of said relays controlling said short-circuiting switch and causing it to operate at a predetermined lower current .to terminate a crisping operation, and the other relay controlling said electromagnetically releasable latch to terminate a toasting operation.

4. A system as set forth in claim 3 and including a manually-actuable rheostat connected in parallel circuit with the operating coil of said other relay adapted to predetermine the length of the toasting period of said appliance.

5. An electron tube control system for an electric crisping and toasting appliance, comprising three electric toast heating elements for each toasting chamber, one of said toasting elements being positioned below and along the longitudinal center line of a slice of bread being treated, and the other two elements being positioned one at each side of a slice of bread and substantially parallel with the flat surface thereof, said system comprising a grid controlled electron tube, a timing resistor having a negative temperature coefficient of resistance and being subject to heat, from said toast heating elements, a timing condenser in the grid circuit of said tube and two relays having their coils connected in series electric circuit with each other and with the plate of said tube, a source of substantially constant direct current potential and means including said timing resistor, timing condenser and source of direct current potential for causing a predetermined rate of increase of current traversing said tube and the coils of said relays to determine the length of time a slice of bread is subject to low temperature convection heat from all of said toasting elements and then subjected to high temperatur radiant heat from the other toasting elements.

6. An electron tube control system as set forth in claim 1 in which said timing resistor is effec- '12 tive to shorten the length. of a toasting cycle as the temperature of the toaster increases.

7. An electron tube control system assetforth in claim 1. in which said timing resistor is efiective to shorten the length of a toasting cycle in case of increase of the supply circuit voltage to which said heating elements are connected.

8. In an electric toaster having a plurality of electric toast heating elements for bread crisping and bread toasting, an electric power circuit, an energizing circuit for said heating elements supplied by said power circuit, a timing circuit supplied by said power circuit including a grid controlled. electron tube, means supplying a substantially constant direct current potential for the tube grid, a timing resistor and a timing condenser operative to progressively vary the amount of potential delivered to the tube grid so as to cause a predetermined rate of change of plate current traversing the tube, and selectively operative control means for said energizing circuit in series with the tube plate and responsive to two different predetermined values of plate current to deter-mine the length of the crisping and toasting periods of the toaster heating elements.

9. In an electric toaster having a plurality of electric toast heating elements for bread crisping and bread toasting, an electric power circuit, an energizing circuit for said heating elements supplied by said power circuit, a timing circuit supplied by said power circuit including a grid controlled electron tube, means supplying a substantially constant direct current potential for the tube grid, a timing resistor and a timing condenser operative to progressively decrease "the amount of potential delivered to the tube grid so as to cause a predetermined rate of increase of plate current traversing the tube, and selectively operative control means .for said energizing circuit in series with the tube plate and to two ferent predetermined values of plate current to determine the length of the crisping and toasting periods of the toaster heating elements.

10. In an electric toaster having a plurality of electric toast heating elements for bread crisping and bread toasting, an electric power circuit, an energizing circuit for said heating elements supplied by said power circuit, a timing circuit supplied by said power circuit including a grid controlled electron tube, means supply-ing a-substantially constant direct current potential for the tube grid, a timing resistor and a timing condenser operative to progressively vary the amount of potential delivered to the tube grid so as to cause a predetermined rate of change of plate current traversing the tube, and a pair of control relays for said energizing circuit in series with the tube plate and being responsive to diffe 'ent predetermined values of the plate current respectively to determine the lengths of the crisping and toasting periods of the toaster heating elements.

11. In an electric toaster having a plurality of electric toast heating elements for bread crisping and bread toasting, an electric power circuit, an energizing circuit for said heating elements supplied by said power circuit, a timing circuit supplied by said power circuit including a grid controlled electron tube, means supplying a substantially constant direct current potential .fcr the tube grid, a timing resistor anda timing condenser operative to progressively vary theamount of potential delivered to the tube grid .so as to cause a predetermined rate of change of plate current traversing the tube, and selectively operative control means ior said energizing circuit in series with the tube plate and responsive to two different redetermined values of plate current to determine the length of the crisping and toasting periods of the toaster heating elements, a variable resistor connecting said grid potential supply means with said timing condenser and timing resistor for varying the grid potential and being operable to vary the crisping period and a further variable resistor interconnecting the tube plate and said selectively operative control means and being operable to vary the toasting period.

12. In an electric toaster according to claim 8, the plurality of electric toast heating elements being in series in the energizing circuit and being simultaneously energized during the crisp ing period, and said selectively operative control 14 means operating to tie-energize one oi said heating elements to initiate the toasting period.

BROR G. OLVING.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,008,413 Dawson July 16, 1935 2,337,124 Olving Dec. 21, 1943 2,356,649 Bucher Aug. 22, 1944 2,383,673 Olving Aug. 28, 1945- 2,415,963 Olving Feb. 18, 1947 2,451,508 Olving -1 Oct. 19, 1948 2,473,640 Faulk June 21, 1949

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