GB2034501A - Regulated power supply - Google Patents

Abstract

In a regulated dc power supply, a series transistor 18 is controlled by the difference between an adjustable reference (from potentiometer VR1 across a Zener diode D1) and a feedback signal, from resistor R2 and diode D2. If the output at 14, 16 is short circuited, diode 30 conducts to limit the output current, for use with a model railway, optional components C1 C2 and VR2 simulate braking and acceleration. <IMAGE>

Description

SPECIFICATION Improvements in and relating to electrical power supplies Field of the invention This invention concerns electrical power supplies and in particular converter units for transforming alternating current to direct current and providing adjustable voltage power supplies for model toys such as electric trains. Although the invention is primarily directed to such applications it is to be understood that the invention is by no means limited to such applications.

Background to the invention The most common drive unit for electrically powered models and toys comprises permanent magnet commutated motors which require a source of direct current to drive them. Although dry cell batteries are a common source of power these are unsuitable for high current devices such as electric trains which commonly require in excess of a tenth of an amp and commonly a quarter of an amp. and since most toys of this nature are relatively static and are normally used indoors it is more convenient to provide a direct current supply derived from the alternating current electric supply to the house.

Typically a converter comprises a transformer for reducing the supply voltage which is typically at 240 volts to a lower and much safervoltagetypically in the region of 12 to 24 volts, together with a rectifying circuit and current limiting device such as a potentiometer or rheostat for introducing additional resistance into the circuit and reducing the current to the motor to allow a degree of speed control to be obtained.

When a relatively high internal resistance source is used as a source of direct current, short circuit volts do not cause undue damage or overheating since the internal resistance of the source limits the short circuit current. However where the source has a very low internal resistance as may well be the case where a full wave rectifying circuit employing low internal resistance solid state devices is used in conjunction with a high current low voltage secondary winding on a mains transformer the short circuit current can be very considerable and can easily cause overheating of the transformer if it is not detected and the appliance switched off. Various short circuit devices have been devised and incorporated into power supplies for models and toys but in the past these have tended to be electromagnetic trip-out devices or thermally operated switches.

Problems have arisen with both types since in the case of a magnetically operated tripping device the spurious current surges during operation commonly cause the device to trip out to the aggravation of those using the model ortoy and in the case of a thermal operating device, relatively high currents can be maintained for relatively long periods of time before the device heats up sufficiently to cause the bimetal strip or other device to move sufficiently to open circuit the device.Consequently when this form of short circuit protection is incorporated the transformer and rectifying components have tended to be up rated considerably relative to the normal full load current which the unit is rated to deliver in order to accommodate the surges and/or high short circuit current overloads which can occur (only in the case of a model railway regularly occur) due to accidental short circuiting between the rails.

Objects of the invention It is one object of the invention to provide an improved form of power supply typically but not exclusively for models and toys in which a short circuit condition is detected virtually instantaneously and the short circuit current is reduced virtually to zero as soon as the short circuit is detected.

It is another object of the present invention to provide an improved power supply typically but not exclusive of the models and toys in which the voltage can be reduced automatically and according to a given decay characteristic so as to simulate braking in a toy train or toy motor car.

The invention According to one aspect of the present invention in a D.C. power supply having positive and negative output terminals to which a load is connected the current flows through a base controlled junction of an active solid state device and the value of the controlling voltage is itself governed by the potential difference between the output terminals so that when the latter falls to a low value as in a short circuit condition the control voltage and therefore the base current of the active solid state device is significantly limited and in the case of a short circuit condition virtually cut off.

According to a preferred feature of the invention the control voltage is derived from an adjustable voltage D.C. source the output of which is dependent on the potential difference between the output terminals.

Preferably a polarity sensitive circuit device is incorporated in a connection between the said output terminals and the adjustable voltage D.C.

source so that when the potential difference exceeds the voltage from the adjustable voltage D.C. source the latter is unaffected by the actual potential difference between the output terminals but in the event that the said potential difference between the output terminals falls below the voltage of the adjustable voltage D.C. source the latter is limited to the lower voltage determined by the potential difference between the output terminals.

Conveniently the adjustable voltage D.C. source comprises a potentiometer having an adjustable tapping for delivering an adjustable D.C. voltage between two terminals and one of the two terminals is connected via a polarity sensitive device such as a diode to one of the output terminals and other of the two said terminals is connected to the other output terminal. By connecting the diode correctly so a potential difference between the said output terminals which is less than the voltage between the two terminals of the adjustable voltage D.C. supply will cause the diode to conduct and clamp the voltage from the adjustable voltage D.C. source at the output voltage between the said output terminals.

Alternatively the adjustable D.C. source may comprise a potentiometer having tappings and a switch for selecting between the tapping to provide different voltages between the said two terminals.

Where an infinitely variable or switched potentiometer is used preferably a voltage stabilizing device in the form of a zenor diode or the like is connected across the supply to the potentiometer so as to stabilize the potentiometer voltage and therefore the output from the said adjustable voltage D.C. source.

According to another preferred feature of the invention, a capacitor may be included in the adjustable D.C. source and the voltage across the capacitor determines the voltage between the two terminals of the adjustable voltage D.C. source and a discharge circuit is provided together with a switch for discharging the capacitor in accordance with a given decay characteristic determined by the characteristics of the discharge circuit when the switch is closed. By arranging that the switch in the form of a push button control switch so a model train or other toy such as a car which is travelling along at a speed determined by the particular setting of the adjustable voltage supply can be apparently braked by simply depressing the push button switch thereby causing the capacitor within the D.C. source to be discharged according to the decay characteristic.By providing a relatively slow decay characteristic i.e. a relatively slow discharge rate so the braking effect can be gradual and by increasing the rate of discharge and therefore the steepness of the decay characteristics so the braking can be more and more fierce. Preferably the rate of discharge of the discharge circuit is adjustable typically by means of a variable resistor and/or different fixed or preset circuits can be selected by a number of different push button switches so that different rates of decay and therefore different braking simulations can be obtained.

By using a push buttom type switch, the braking effect can be removed simply by releasing the switch and since this will automatically allow the capacitor to become recharged to the normal operating vol tagewithin the adjustable voltage source (and thereby allow the output voltage from the latter to rise to its original value before the discharge circuit was connected across the capacitor) the model train or car will begin to pick up speed in accordance with the charging characteristic of the capacitor. By arranging that the latter is charged through a charging circuit having a siginficant inherent resistance, so a relatively slow rate of acceleration can be simulated which improves the realism of an operating model.It will be appreciated that by providing an adjustable charging circuit so the rate of acceleration from a braked condition can be controlled and set to a desired level consistent with the model which is being controlled. Thus in the case of a car the rate of acceleration could be high whereas in the case of a model train the rate of charge could be set lower so as to simulate the normally lower acceleration associated with trains.

The invention will now be described by way of example with reference to the accompanying drawings.

In the drawings Figure 1 is a block circuit diagram of one embodiment of the invention, Figure2 is a circuit diagram of the embodiment shown in Figure 1, Figure 3 is a block circuit diagram of another embodiment of the invention, Figure 4 is a circuit diagram of the embodiment shown in Figure 3, Figure 5 is a block circuit diagram of a further embodiment of the invention and Figure 6 is a circuit diagram of the said further embodiment of Figure 5.

Detailed description of the drawings General Each ofthe embodiments represents a source of direct current which is controllable for typically supplying a model railway and in which in accordance with the invention a circuit is provided to protect the device against high short circuit currents caused by short-circuiting the output terminals of the load connected thereto. In each case only the current controlling circuit and short circuit sensing circuit and the associated controls are illustrated.

Each of the circuits includes input terminals 10 and 12 which in practice will be connected to the output of a D.C. supply typically comprising a transformer having a primary adapted to be connected to the electricity supply mains and a secondary for delivering an open circuit voltage typically in the region of 15 to 25 volts and a full wave rectifying circuit and associated smoothing components for producing direct current in the range of 1/2 to 1 amp. at 12 to 15 volts.Each of the embodiments shown in the drawings provides a control for limiting the current to a pair of output terminals which in each of the embodiments are labelled 14 and 16 and each of the embodiments also incorporates in accordance with the invention a short circuit sensing device for ensuring that the current which can be drawn from the output terminals 14 and 16 is significantly reduced in the event of a short circuit condition prevailing between the two output terminals 14 and 16.

Figures 1 and2 The first embodiment comprises a control element 18 which as shown with reference to Figure 2 comprises a p.n.p. power transistor TR1,the output current available from terminals 14 and 16 being determined by the current flowing between the emitter collector which in turn is controlled by the base current to the device. This is controlled by a comparator circuit element 20 which with reference to Figure 2 will be seen to comprise an n.p.n.

transistor TR2 the emitter collector current of which serves to control the base current available for TR1.

The base current for TR2 is derived from a variable reference voltage source 22 which as shown in Figure 2 comprises a potentiometer made up of R1 and VR1 and a zenor diode D1 producing a stabilized and variable voltage at junction 24 the actual value of which will be determined by the setting of the slider on the variable potentiometer VR1.

By selecting relative values of R1 and VR1 and an appropriate zenor diode D1 so the maximum voltage to which junction 24 can rise will still limit the base current available for TR2 to a value such that the emitter collector current through TR2 is the maximum base current required for TR1 to produce the design emitter collector current flow through the transistor TR1 consistent with its normal continuous mode of operation. TR1 is therefore selected from devices having a rated continuous emitter collector current slightly in excess of the maximum current which is to be made available from the terminals 14 and 16 and for model railways this is typically of the order of 1/2 to 1 amp.

Basic control of the line current is obtained by deriving a second signal for the comparator circuit 20 from a sampling circuit 26. Referring to Figure 2 this will be seen to comprise a diode D2 connected so as to conduct between the positive and negative rails and a series connected resistor R2. The junction 28 is connected to the emitter of the transistorTR2.

The voltage at junction 28 will follow the voltage between output terminals 14 and 16 so that in the event that a high current is drawn through the device 18 thereby increasing the potential drop across the collector emitter junction so the voltage available for the comparator circuit transistor 20 will be reduced and the base current available for TR1 will also be reduced. This will tend to limit the current available from the output terminals 14 and 16 since if too high a current is drawn the device 18 will simply be moved into a lower conduction state so that the high current demand is not met and the current is maintained at a safe value consistent with the rated operation of the device TR1.

In addition to the sampling circuit 26 a short circuit sensing device is connected between output terminal 14 and junction 24 so that in the event of a short circuit the comparator 20 is caused to operate in a condition such as to produce virtually no current flow through the control circuit 18. The short circuit sensing device is denoted in the block circuit diagram by reference numeral 30 and as will be seen from Figure 2 this typically comprises a diode D3 connected between the positive output terminal 14 and the junction 24. In the event that the voltage v (see Figure 2) between teminals 16 and 14 drops below the voltage between junction 24 and terminal 16 (i.e. the output voltage from VR1) D3 will conduct and clamp the potential of junction 24 to the voltage V.In the event that voltage V is very small (i.e. in the case of a short circuit condition) junction 24 will be reduced virtually to a potential of the negative supply line (i.e. output terminal 16) thereby effectively cutting off transistor TR2 and simultaneously transistor TR1 thereby cutting off the current flow across the emitter collector junction of transistor TR1.

It will be seen therefore that in the event of a short circuit condition developing across terminals 14,16, the transformer and rectifying circuit connected to the output terminal 10, 12 is immediately effectively disconnected from the output terminals and fully protected from the short circuit current which would otherwise flow.

The advantage of the circuit provided by the invention is that as soon as the short circuit condition is removed, the full output current from terminals 14 and 16will once more be available without any delay as is commonly the case when magnetic and/or thermal devices are used. it will also be seen that no reset button or reset switch will be acquired to cause the output current to once again be available.

In the event that visual or audible indication of a short circuit condition is required, an aiarm device typically in the form of a low voltage filament lamp L (see Figure 2) is included in series with the diode D3 so that in the event that the latter conducts significantly (as will be the case in the event of a short circuit conditon) the lamp Lwill be illuminated.

Since the lamp L will automatically introduce a voltage drop it may be preferred that a voltage sensitive circuit is employed (not shown) which is sensitive for example to the voltage across diode D3 which will normally be high but which in a short circuit condition will become very small. By employing an appropriate reversal of phase in the voltage sensing circuit so this drop in voltage across D3 can be translated into voltage increase or current increase in another circuit so as to cause a lamp (not shown) to be illuminated or an alarm (not shown) to be sounded.

Figures 3 and 4 In most respects the embodiment shown in Figures 3 and 4 is similar to that shown in Figures 1 and 2 and the same reference numerals have been used to denote those parts common to the two circuits.

The essential difference lies in the provision of a selector switch in the variable reference source 22 and the difference is best seen in Figure 4. The selector switch is switch S1 and instead of providing an infinitely variable potentiometer VR1 this is replaced by a series of three fixed resistors R2, R3 and R4 connected to tappings 32, 34 and 36 of the selector switch S1. By selecting tapping 32 so the maximum base current for transistor TR2 and therefore TR1 can be obtained and full current will be available from the output terminals 14 and 16. By selecting terminals 34 or 36 in turns so lower values of base current are obtained and lower output currents from terminals 14 and 16 likewise obtained.

Figures 5 and 6 The embodiment of Figures 5 and 6 is essentially the same as that shown in Figures 1 and 2 and to that end components which are common to both embodiments are denoted by the same reference numerals.

The basic difference between the two embodiments is in the provision of a variable braking effect simulator circuit designated by reference numeral 38 which controls the decay of the voltage from the variable reference source 22 in the event that a switch 40 is operated. The latter is typically but not necessarily a push button operated switch.

Detail of the circuit 38 is shown in Figure 6.

Essentially the circuit comprises a capacitor C1 which is connected in parallel with the zenor diode D1 and in parallel with the capacitor is connected a series circuit of switch S1 and variable resistance VR2.

With switch S1 open the capacitor C1 simply charges up to the voltage across zenor diode D1 and maintains the voltage irrespective of the setting of VR2. The speed of operation of the model or toy will thus be completely under the control of VR2 the setting of which will determine the output current available from terminals 14 and 16.

By closing switch S1 the capacitor C1 will be discharged through variable resistance VR2. If this is set at a low value the rate of discharge will be high and the result will be that the voltage across the potentiometerVR1 will decay rapidly to a low value determined by the relative values of R1 in combination with VR1 and VR2 and this will cause the base current available for transistor TR2 and therefore the base current TR1 to decay similarly thereby bringing about a significant decay in the output current available from terminals 14 and 16. This will simulate rapid braking of the model or toy.

If the R2 is increased in value, closing switch S1 will still cause capacitor C1 to discharge thereby producing a decaying voltage across VR1 but the rate of decay will be less than the rate of slowing down and therefore effective braking of the toy or model will be reduced.

It will be seen that the moment S1 is opened capacitor C1 can once again charge up to the voltage determined by zenor diode D1 but by assuring that R1 has a high value relative to VR1 and VR2 (which will normally be the case) the rate at which capacitor C1 will recharge will be less than instantaneous so that the model or toy will apparently pick up speed and simulate acceleration which would normally apply to a vehicle such as a car or train. Provided S1 remains open, the final speed of the model or toy will simply be determined by the setting of VR1 and by changing the setting of VR1 so the speed of the toy or model can be adjusted at will.

A further refinement may be provided in the form of a second capacitor C2 shown in dotted outline only in Figure 6. This second capacitor is connected between the slider of the potentiometer VR1 and the negative line connected to output terminal 16. By connecting C2 through a switch S2 so the capacitor can be brought into or out of circuit. When switch S2 is open the overall circuit will respond and operate in the manner previously described. With S2 closed, the size of capacitor C2 will determine its influence on the circuit. If it is only a small value capacitor the influence will be very small and hardly noticeable.

However if a relatively large size of capacitor is used, the effect will be to introduce a time lag into the response characteristic of the controller and toy or model. It will be seen that if VR1 is adjusted so as to increase the voltage at junction 24 the capacitor C2 will not immediately follow the increase in the setting of VR1 since it will have to charge up to the new voltage corresponding to the new setting of VR1 and particularly at lower speed setting this charging process will have to take effect through the large part of VR1 which will now appear in series with capacitor C2. Likewise if VR1 is reduced rapidly from a high setting to a low setting, capacitor C2 will not immediately discharge but will cause the base current TR2 to decay in accordance with the discharge characteristic of C2 taken in conjunction with the particular portion of VR1 which is in parallel therewith and the result will be much more gradual changes of speed than would otherwise be the case.

Claims (13)

1. A D.C. power supply having positive and negative output terminals to which a load is connected in which the current flows through a base controlled junction of an active solid state device and the value of the voltage controlling the base is itself governed by the potential difference between the output terminals so that when the latter falls to a low value the control voltage and therefore the base current of the active solid state device is limited.

2. A D.C. power supply as claimed in claim 1 in which the control voltage is derived from an adjustable voltage D.C. source the output of which is dependent on the potential difference between the output terminals.

3. A D.C. power supply as claimed in claim 2 in which a polarity sensitive circuit device is incorporated in a connection between the said output terminals and the adjustable voltage D.C. source so that when the potential difference exceeds the voltage from the adjustable voltage D.C. source the latter is unaffected by the actual potential difference between the output terminals but in the event that the said potential difference between the output terminals falls below the voltage of the adjustable voltage D.C. source the latter is limited to the lower voltage determined by the potential difference between the output terminals.

4. A D.C. power source as claimed in claim 2 or 3 to which the adjustable voltage D.C. source comprises a potentiometer having an adjustable tapping for delivering an adjustable D.C. voltage between two terminals, and one of the two terminals is connected via a polarity sensitive device to one of the output terminals and the other of the two said terminals is connected to the other output terminal.

5. A D.C. power source as claimed in claim 4 in which the polarity sensitive device is a divide which is connected so that a potential difference between the said output terminals which is less than the voltage between the two terminals of the adjustable voltage D.C. supply will cause the diode to conduct and clamp the voltage from the adjustable voltage D.C. source at the output voltage between the said output terminals.

6. A D.C. power source as claimed in claim 2 or 3 in which the adjustable D.C. source comprises a potentiometer having tappings and a switch for selecting between the tappings to provide different voltages between the said two terminals.

7. A D.C. power source as claimed in claim 4 or 6 in which a voltage stabilizing device is connected across the supply to the potentiometer so as to stablize the potentiometer voltage and therefore the output from the said adjustable voltage D.C. source.

8. A D.C. power source as claimed in claim 7 in which a capacitor is included therein and the voltage across the capacitor is used to determine the voltage between the two terminals of the adjustable voltage D.C. source and a discharge circuit is provided together with a switch for discharging the capacitor in accordance with a given decay characteristic determined by the characteristics of the discharge circuit when the switch is closed.

9. A D.C. power source as claimed in claim 8 when serving as the power supply for a motorized model wherein the switch is in the form of a push button operated switch, so that the model which will normally travel at a speed determined by the particular setting of the adjustable voltage supply can be braked by simply depressing the push button operated switch, thereby causing the capacitor within the D.C. source to be discharged according to the decay characteristic.

10. A D.C. power source as claimed in claim 9 in which the rate of discharge of the discharge circuit is adjustable so that different rates of decay and therefore different braking simulations can be obtained.

11. A D.C. power source as claimed in claim 9 or 10 in which the capacitor is charged through a charging circuit having a significant time constant so that a relatively slow rate of acceleration can be simulated which improves the realism of an operating model.

12. A D.C. power source as claimed in claim 11 in which there is provided an adjustable charging circuit so that the rate of acceleration from a braked condition can be controlled and set to a desired level consistent with the model which is being controlled.

13. D.C. power sources for model control constructed and arranged to operate substantially as herein described with reference to and as illustrated in Figures 1 and 2 or Figures 3 and 4 or Figures 5 and 6 of the accompanying drawings.