LEDs find wide use as indicators and as light emitters in devices such as optocouplers. In some applications, the LED or the emitter may be located remotely at some distance from the main unit. Typical examples are dashboard-mounted automotive indicators and industrial optosensors. In critical applications, you may require some means of monitoring the integrity of the LED. Using just four transistors and six resistors, this circuit provides switchable, constant-current drive for an LED and indicates both open- and short-circuit fault conditions (Figure 1). And there's a bonus, too.
 |
||
| Figure 1. | This LED driver doubles as a fault monitor and limits short-circuit current to boot. | |
Control signal VCONT switches the LED on and off. When VCONT is high, Q1 and the LED are off. When VCONT is as low as 0 V, Q1 turns on and sources a constant current to the LED. Because most LEDs have a forward-voltage drop of at least 1.2 V, adequate base-bias voltage exists for Q3, which turns on, thereby providing a conduction path for Q2. This conduction, in turn, provides bias for Q4, which turns on and pulls high, thus indicating a healthy LED.
Because Q2 and Q4 are both now on, the base potential of Q1 sits at roughly two VBE drops below the positive-supply rail, VS, thereby placing one VBE drop across R1. Consequently, with R1 = 68 Ω, Q1 sources a steady current of approximately 10 mA to the LED. Provided that the value of R2 is large enough, little of the LED's forward current diverts into Q3’s base. As long as the LED remains undamaged, /FAULT stays high, signaling normal drive conditions. Should the LED go open-circuit, Q1’s collector load becomes just R2 in series with Q3’s base. Because R2 is much larger than R1, Q1 saturates, the voltage across R1 falls to around 20 mV or so, and the emitter potentials of Q1 and Q2 rise toward VS. With insufficient base drive, Q4 now turns off, and the output falls to 0 V to signal the fault condition.
On the other hand, if a fault puts a short circuit across the LED, Q3 immediately turns off and deprives Q2 of collector current. Q2’s base-emitter junction now behaves like a diode, clamping Q1’s base to a potential dictated mainly by Q2’s VBE drop and by the ratio of R3 to R4. Because R4’s value is smaller than that of R3, Q2’s emitter potential now rises toward VS. Once again, Q4 turns off and goes low to indicate the fault condition. With the resistor values shown in Figure 1, Q1’s base now sits at approximately 4 V, leaving only 200 to 300 mV across R1. Therefore, the short-circuit current is effectively “choked back” to less than a third of the normal value, thereby saving power – the bonus. Under normal conditions, with the LED on, Q1 conducts more current than Q2, causing its VBE drop to be slightly larger than that of Q2. Consequently, the potential across R1 is slightly less than a diode drop, and you may need to experiment with the value of R1 to set the desired LED current.
You must select R3 to satisfy the base-current requirements of Q1 and Q2 when VCONT is low. Tests on the prototype circuit produced good results with R3 = 39 kΩ, although a smaller value may be required, depending on the LED current and the current gain of Q1 and Q2. When the LED is on, both Q2 and Q3 are fully on, so a reasonably large value of R5 is required to limit their joint collector current to an acceptable level. However, R5 must not be too large, or Q2 will be unable to furnish the current that R4 and Q4’s base require. Making R5 approximately four or five times larger than R4 is a good starting point.
Although the circuit in Figure 1 has a 5 V supply, you could use other voltages, provided that you scale the resistor values accordingly. Operation at lower voltages is possible as long as Q1 has adequate “headroom” to stay out of saturation, but beware of problems if you use a blue or a white LED, because these devices tend to have relatively high forward-voltage drops. The transistor types are not critical; most small-signal devices with high current gain should be adequate, although Q1 may need to be a power device if your design requires a high LED current, a high supply voltage, or both.
