JPS61254018A - Overcurrent protection circuit for restricting current flowing through transistor switch - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates generally to overcurrent protection circuits, and more particularly, in which a transistor is connected to switch current through a load electrically connected in series with the collector-emitter path of the transistor. The case relates to an overcurrent protection circuit used to limit the current flowing through that transistor.

It is known to use keyboards that include illuminated push-button keypads in conjunction with one type of control and/or monitoring system. For example, such a keyboard is
Used in a plasma display keyboard used in conjunction with a plant safety monitoring system installed by the applicant in conjunction with an atomic couplant. In this system, the keyboard is a 32 illuminated push button keypad consisting of two 16 push button banks. One button from each bank is illuminated at any one time to indicate the button last selected by the operator; a keyboard illuminated in this manner greatly assists the operator in skillfully using the keypad. .

For obvious reasons, it is extremely important to have all light sources of the keyboard operational at all times. The light sources for the plasma display keyboard described above are 12 volt incandescent lamps, each driven by a transistor switch. Incandescent lamps have a finite lifespan and therefore require a periodic analogy. Replacing a burnt-out lamp is a simple act for an experienced practitioner, but it is still a simple task for those not particularly familiar with the system, such as by shorting the ends of the lampholder together or By inserting a defective lamp with a resistance that is too high, there is a great potential for serious damage to the associated switching transistors.

If a short circuit were to occur across the lampholder, the collector current of the transistor could exceed its maximum rating and destroy the transistor. Although the transistor itself is relatively inexpensive, the disruptive effort in replacing a transistor can be relatively expensive. Therefore, the cost of the lamp and/or transistor is inconsequential compared to the human time required to replace a keyboard transistor.

Furthermore, if a lamp of poor resistance is installed in the circuit, the transistor is forced to operate in its active region rather than its saturation region, which can generate unwanted heat and damage the transistor. is possible.

An object of the present invention is to provide an overcurrent protection circuit which prevents excessive current from flowing through the transistors connected as described above when a short circuit occurs across the load.

Another object of the invention is to provide an overcurrent protection circuit for transistors connected to switch current through an incandescent lamp, in which a lamp with a defective resistance is inserted into the circuit. The transistor remains operating in its saturation region even if

Yet another object of the invention is to provide a common emitter connection line of a plurality of transistors, each connected to switch current through a respective load, in which case an overcurrent condition exists in one of the transistors. In addition to limiting the current, the load of the bad resistance is
An object of the present invention is to provide an overcurrent protection circuit that can maintain a transistor in its saturation region even when placed in one circuit.

The foregoing and other objects provide an overcurrent protection circuit formed in conjunction with a transistor connected to switch current through a load electrically connected in series with the collector emitter path of the transistor. and has first and second inputs, each receiving an enabling and disabling signal, and an output connected to the base of the transistor, the first input being an enabling signal from an external circuit. and base circuit means arranged to receive a selected one of the disable signals to turn on the transistor when an enable signal is present on each of the first and second inputs; and said base circuit means operative to turn off said transistor when a disabling signal is present on either one of said first and second inputs, and a terminal connected to an emitter of said transistor. a reference resistor having a voltage proportional to the current in the collector-emitter path and detectable at the terminal, such that the current flowing through the transistor develops across the reference resistor; comparator means having an input connected to said terminal of a reference resistor for receiving said voltage developed across said reference resistor, said comparator means having a voltage threshold corresponding to a maximum current allowed in said transistor; said comparator means having an output for producing an output signal that changes from a first state to a second state each time a voltage at an input of the means increases to meet said voltage threshold, indicating an overcurrent condition in said transistor; , pulse means having an input connected to an output of said comparator means and an output connected to a second input of said base circuit means, the pulse means comprising a quiescent signal amount being an enabling signal and a disabling signal. producing an output signal comprising a pulse signal having a predetermined pulse width, and producing the pulse signal corresponding to the predetermined pulse width when the output signal of the comparator means changes from the first state to the second state; pulsing means such that the base circuit means turns off the transistor for a period of time.

According to further features of the invention, the base circuit means:
The base circuit means includes means for supplying current to the base of the transistor so as to maintain the transistor in saturation regardless of the magnitude of the electrical resistance of the load whenever the base circuit means is operated to turn on the transistor. .

In a preferred embodiment of the invention, an overcurrent protection circuit is formed in conjunction with the transistor connected to switch current through a load electrically connected in series with the collector emitter path of the transistor. , each having first and second inputs for receiving enable and disable signals, and an output connected to the base of the transistor, the first input being an enable signal from an external circuit. base circuit means arranged to receive a selected one of enable and disable signals to turn on the transistor when an enable signal is present on each of the first and second inputs; , and is operative to turn off the transistor when a disabling signal is present on any one of the first and second inputs;
A reference resistor having a terminal connected to the emitter of the transistor, the current flowing through the transistor creating a voltage across the reference resistor that is proportional to the current in the collector-emitter path and detectable at the terminal. a reference resistor configured to produce a maximum current allowed in the transistor; and comparator means having an input connected to the terminal of the reference resistor for receiving the voltage developed across the reference resistor; a voltage threshold corresponding to the voltage threshold and changing from a first state to a second state each time a voltage at the input of said comparator means increases to meet said voltage threshold and indicate an overcurrent condition in said transistor. pulsing means having an output for producing an output signal; an input connected to the output of the comparator means; and an output connected to a second input of the base circuit means, the pulse means being an enable signal. static signal amount,
generating an output signal comprising a pulse signal having a predetermined pulse width serving as a disabling signal, and generating the pulse signal when the output signal of the comparator means changes from the first state to the second state; period corresponding to the pulse width,
said base circuit means adapted to turn off said transistor; another transistor connected to switch current through another load; and first and second inputs and outputs. another base circuit means, a first input of said another base circuit means being arranged to receive a selected one of an enabling and disabling signal from said external circuit; A second input of said further base circuit means is connected to the output of said pulse means and an output of said further base circuit means is connected to the base of said another transistor, thereby The emitter of one transistor is connected to a terminal of the reference resistor, and the comparator means
in response to an overcurrent condition of each of said transistors;
An overcurrent protection circuit is provided which causes said pulsing means to generate a pulsed signal applied to a second input of each of said base circuit means to turn off transistors in which an overcurrent condition exists.

l of t Referring to FIG. 1, a lamp 1 is connected between the collector of the transistor 3 and the terminal 5 via the terminals A and B, respectively.
are shown connected. Terminal 5 is connected to a positive voltage source (not shown). The base of the transistor 3 is connected to receive current via a resistor 7, which is connected via a terminal 9 to another positive voltage source (not shown). Capacitor 11 is connected between the base of transistor 3 and ground to filter out unwanted high frequency signals.

NOR gate 13 has two input leads 15 and 17, each receiving a logic input, and an output 19 connected to the base of transistor 3.

Lead 17 is arranged to receive logic inputs from external circuitry (not shown in FIG. 1);
5 is arranged to receive a logic input including a feedback signal from an overcurrent protection circuit according to the invention as described below.

Reference resistor 21 has a terminal 23 which is connected to the emitter of transistor 3 and to the input of comparator 25 . The other end of the reference resistor 21 is grounded. The output of the comparator 25 is connected to the input of a one-shot multivibrator (hereinafter referred to as one-shot) 27, which has its output connected to the input lead 15 of the NOR gate 13. Comparator 25 has a threshold voltage that, when reached, changes the output of comparator 25 from one voltage state to another. The change in voltage state of comparator 25 triggers one-shot 27 to generate a positive output pulse having a predetermined pulse width, after which the output of one-shot 27 returns to zero voltage amount.

To explain the operation of the circuit of FIG. 1, assume that lamp 1 is a conventional 12 volt lamp and that when transistor 3 is on, the 12 volt voltage source Vcc at terminal 5 draws 77 MA. Further assume that reference resistor 21 is equal to 4.3 ohms and that the threshold voltage of comparator 25 is 0.8 volts.

During normal operation, the one-shot 27 is stationary and N-
It outputs a low voltage that is an enable (on) logic signal to lead 15 of OR gate 13. If the shield 17 is also low voltage (
NOR gate 13 will produce a high voltage signal on its output lead 19, which in turn saturates the base of transistor 3, closes the collector-emitter path and closes the collector-emitter path, and the lamp 1 Turn on. If lamp 1 is of the correct size or type, it will flow 77 +++ A to reference resistor 21 as described above.
The output of one-shot 27 remains in a low voltage state since it will produce a voltage of 33 volts, which is less than the comparator threshold voltage of 0.8 volts.

If lamp 1 were to break and a short circuit was inadvertently created across terminals A and B during the lamp replacement process, the voltage across reference resistor 21 would rise rapidly and the voltage at terminal 23 would be 0.8 The comparator threshold voltage in volts is reached. This causes the comparator to switch states, triggering the one-shot 27 to issue an output pulse which will then cause the output of the NOR gate 13 to be at a low voltage, the low voltage output of the NOR gate 13 immediately Transistor 3 is turned off and remains off for a period corresponding to the pulse width of the output pulse from one-shot 27. One shot 27 at the completion of the output pulse
The output of NOR gate 13 returns to its quiescent low voltage state.
puts the output into a high voltage state. As a result, transistor 3
becomes saturated again and turns on. If a short circuit is maintained at terminals A and B, the voltage at terminal 23 rises again to the threshold voltage of comparator 25, triggering another output pulse from one-shot 27;
Then again transistor 3 will be turned off, and transistor 3 will continue to turn on and off at a frequency determined by the time constant RC of the one-shot 27.

FIG. 2 shows terminal 23 across reference resistor 21 when a short circuit is maintained across terminals A and B of FIG.
FIG. 2 is a signal waveform diagram showing the voltage of FIG.

The low voltage period between voltage pulses in Figure 2 is one shot 2
This corresponds to the period of the output pulse from 7.

At the completion of the output pulse from one shot 27.

The output of NOR gate 13 is at a high potential, turning on the transistor until the voltage at terminal 23 reaches the comparator threshold voltage of 0.8 volts. The overcurrent protection circuit of FIG. 1 turns off the transistor at the point when the overcurrent protection circuit of FIG.

Until the short circuit across terminals A and B is removed, or the other input lead 17 of NOR gate 13 receives a high voltage (off) signal (this high voltage signal also has the effect of lowering the output of NOR gate 13 to a low voltage). oscillates on and off as shown in the signal waveform diagram of FIG. In one form of the circuit of FIG. 1, one-shot 27 was configured to produce 7.9 microseconds between pulses of FIG. 2, and the pulses had a duration of 2.1 microseconds.

The circuit according to the invention also operates in an advantageous manner even if a defective lamp is installed which has a lower or higher resistance than a normal lamp.As can be seen from the analysis below, if a defective lamp is installed If the circuit is
As long as the lamp resistance is above a certain level,
It operates to maintain the transistor in saturation and to cause the transistor to oscillate on and off if the resistance of the faulty lamp is below a predetermined level, as in the case of a short circuit. Even if a normal lamp is installed, it is important to never allow the transistor to operate in its active region. This is because in that region of operation of the transistor the heat generated by the collector current will quickly damage the transistor.

The maximum collector current ICl4A allowed to pass through the transistor is given by: Icx^x=
Vr/Rr
j (Equation 1) Here, VT is the threshold voltage of the comparator 25, and Rr is the resistance value of the reference resistor 21. As assumed before, if VT=0.8 volts and Rr=4
．． 3, then ICMA=186 PlowA.

For lamp resistances RLO1 or less, as if there were a short circuit across terminals A and B,
The lamp resistance at which the circuit causes the transistor to oscillate is calculated by the following formula: RLO= (Vcc −Vca
-VT )/I CHAK (Formula 2) where, Va
e is the supply voltage at terminal 5 in FIG. 1 and VCS is the collector-emitter voltage drop when the transistor is saturated. If Vec=12 volts, Vc
B = 0.4 volts, VT = 0.8 volts, and I CM
If AX=186 wheels A, then RLO=58 ohms. Therefore, if the resistance of the lamp is less than 58 ohms, the transistor will oscillate on and off, as explained earlier in the case of a short circuit across terminals A and B in FIG. The comparator 25 changes state to turn off the transistor each time the ICMA is reached.

If the lamp resistance RL is RLO (i.e. 5 in this example)
8 ohms), the transistor must be kept in its saturation region for the reasons mentioned above. In general, a transistor is saturated if the following equation holds: I a = I c/hps (Formula 3) where IB is the base current and hPB is the minimum current amplification factor of the transistor.

Since the collector current in the previous example is at a maximum I CMAX of 186 mA when the lamp resistance is equal to 58 ohms, and since the collector current Ic decreases as RL increases above 58 ohms, if the base current If 1B is sufficient to keep the transistor in saturation at I CMA, then the base current
8 is sufficient to keep the transistor in saturation. If 1+pH=50 (this value can be used as transistor 3 in implementing this invention, National Semiconductor
(which is the typical minimum current amplification factor for a 2N222 transistor manufactured by or), then from the above 23 the base current IB is 186A150 = 3.7mA
must be equal to or exceed .

The actual base current I B-actu output by the circuit of FIG. 1 at I CNA, i.e. just before the transistor begins to oscillate on and off.
al is expressed by the following formula.

I B-actual-= (VBB-VBB-VT)/Rs (Formula 4)
Here, VSa is the voltage at terminal 9 in FIG.
VBEE is the base-emitter voltage of transistor 3 and R9 is the resistance of resistor 7. VBB=5 volts, V
BB=0.4 volts, VT=0.8 volts and RB=7
50 ohms, then I B-actua
l=4. , '6 mA. In this example, I B-actual is I cNxx/hpa (
3.7mA), the lamp resistance R
It is clear that as L increases above RLO=58 ohms and at the same time the collector current decreases, the base current will continue to keep transistor 3 in saturation.

In short, if a defective lamp with a lower or higher resistance than a normal lamp is installed between terminals A and 8 of the circuit of FIG. That is, the circuit according to the invention will be protected in that it will: 1) cause the transistor to oscillate on and off when the lamp resistance is less than or equal to a predetermined value, such that the collector current is at the maximum allowed; 2) maintain the transistor in saturation if the lamp resistance is above a predetermined level;

FIG. 3 illustrates one aspect of the invention where the principles of the invention may be used in combination with a plurality of transistors connected to selectively turn on and off a plurality of lamps. There is. In FIG. 3, the same reference numerals as in FIG. 1 indicate those corresponding to or equivalent to those in FIG. 1. As shown in FIG.
A bank of b is connected to the collector of the respective switching transistor 3.3a and 3b. NOR
The base circuit of transistor 3, including gate 13, resistor 17, terminal 9 and capacitor 11, and the emitter circuit of transistor 3, including reference resistor 21, comparator 25 and one-shot 27, are exactly the same as shown in FIG. connected in a manner.

A base circuit of the transistor 3a including the NOR gate 13a, the resistor 7a, the terminal 9a and the capacitor lla,
The base circuit of the transistor 3b including the OR gate 13b, the resistor 7b, the terminal 9b, and the capacitor 11b is connected in the same manner as the base circuit of the transistor 3, respectively. The emitters of transistors 3.3a and 3b are each
Terminal 23 of reference resistor 21 connected to comparator 25
The comparator 25 is followed by a one-shot 27 in the same manner as shown in FIG.
．． 13a and 13b are connected to inputs 15.15a and 1.5b, respectively. The other input leads of the NOR gates, namely input leads 17.17a and 17b, are connected to two external circuits 29 which send an enabling signal to only one of the NOR gates at a time and to the other. N of
The OR gate is selectively operative to receive the disabling signal. Such external circuitry is generally known and does not form part of this invention.

In combination with the respective base circuits of transistors 3.3a and 3b, an overcurrent protection circuit comprising a reference resistor 21, a comparator 25 and a one-shot 27 protects each transistor when each of the transistors is turned on by an external circuit 29. 1 from overcurrent conditions and operates to maintain each transistor in its saturation region as described in connection with FIG.

Although FIG. 3 shows only three lamps and corresponding three transistor circuits connected in common emitter mode for ease of illustration and explanation, any number of lamps and associated transistor circuits may be used. It is clear that they can be connected with a common emitter arrangement as shown in FIG.

It is to be understood that the above description of the invention is susceptible to various modifications, variations and applications, and the same is intended to be embraced within the meaning and scope of equivalency of the claims. has been done.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an overcurrent protection circuit connected according to an embodiment of the present invention, FIG. 2 is a signal waveform diagram used to explain the operation of the circuit of FIG. 1, and FIG. The figure is a circuit diagram showing an overcurrent protection circuit connected to a plurality of transistors according to another embodiment of the present invention. In figure 0, 1 is a lamp, 3 is a transistor, A and B are terminals, 1
゛3 is a NOR gate, 15 and 17 are input leads, 19
is an output, 21 is a reference resistor, 23 is a terminal, 25 is a comparator, and 27 is a one-shot multivibrator.

Claims (2)

[Claims] (1) An overcurrent protection circuit formed in conjunction with a transistor connected to switch current through a load electrically connected in series with the collector-emitter path of the transistor, each capable of first and second inputs for receiving enable and disable signals, and an output connected to the base of the transistor, the first input receiving enable and disable signals from an external circuit. base circuit means arranged to receive a selected one of said transistors when an enable signal is present on each of said first and second inputs; a reference resistor having a terminal connected to the emitter of the transistor, the reference resistor being operative to turn off the transistor when a disabling signal is present on any one of its inputs; and a reference resistor having a terminal connected to the emitter of the transistor; a reference resistor connected to the terminal of the reference resistor such that the current flowing through the transistor develops across the reference resistor a voltage proportional to the current in the emitter path and detectable at the terminal; comparator means having an input for receiving the voltage developed across the reference resistor, the voltage threshold corresponding to the maximum current allowed in the transistor and the voltage at the input of the comparator means increasing; an output for producing an output signal that changes from a first state to a second state each time the voltage threshold is met and indicates an overcurrent condition in the transistor; and an input connected to the output of the comparator means. and an output connected to a second input of the base circuit means, the pulsing means having a quiescent signal quantity serving as an enabling signal;
generating an output signal comprising a pulse signal having a predetermined pulse width serving as a disabling signal, and generating the pulse signal when the output signal of the comparator means changes from the first state to the second state; period corresponding to the pulse width,
An overcurrent protection circuit comprising: the base circuit means turning off the transistor; (2) an overcurrent protection circuit formed in conjunction with a transistor connected to switch current through a load electrically connected in series with the collector-emitter path of the transistor, each capable of first and second inputs for receiving enable and disable signals, and an output connected to the base of the transistor, the first input receiving enable and disable signals from an external circuit. base circuit means arranged to receive a selected one of said transistors when an enable signal is present on each of said first and second inputs; a reference resistor having a terminal connected to the emitter of the transistor, the reference resistor being operative to turn off the transistor when a disabling signal is present on any one of its inputs; and a reference resistor having a terminal connected to the emitter of the transistor; a reference resistor connected to the terminal of the reference resistor such that the current flowing through the transistor develops across the reference resistor a voltage proportional to the current in the emitter path and detectable at the terminal; comparator means having an input for receiving the voltage developed across the reference resistor, the voltage threshold corresponding to the maximum current allowed in the transistor and the voltage at the input of the comparator means increasing; an output for producing an output signal that changes from a first state to a second state each time the voltage threshold is met and indicates an overcurrent condition in the transistor; and an input connected to the output of the comparator means. and an output connected to a second input of the base circuit means, the pulsing means having a quiescent signal quantity serving as an enabling signal;
generating an output signal comprising a pulse signal having a predetermined pulse width serving as a disabling signal, and generating the pulse signal when the output signal of the comparator means changes from the first state to the second state; period corresponding to the pulse width,
the base circuit means adapted to turn off the transistor; another transistor connected to switch current through another load; and first and second inputs and outputs. another base circuit means, a first input of said another base circuit means being arranged to receive a selected one of an enabling and disabling signal from said external circuit; A second input of said further base circuit means is connected to the output of said pulse means and an output of said further base circuit means is connected to the base of said another transistor, thereby The emitter of one transistor is connected to a terminal of the reference resistor, and the comparator means
in response to an overcurrent condition of each of said transistors;
An overcurrent protection circuit wherein said pulsing means generates a pulsed signal applied to a second input of each of said base circuit means to turn off a transistor in which an overcurrent condition exists.