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Simple 3-Transistor Voltage Regulator (with current limiting) 2011-12-17

This article describes a simple 3-transistor voltage regulator with current limiting. It is based on a conventional design of unknown origin. The component values quoted below were chosen to provide a stabilised 12V 40mA output from a 17V DC supply; but I show how they can be calculated for a customised output.

The circuit


A suggested layout is:-

How it works

Voltage regulation
Tr1 is the main pass transistor, whose base receives current via bias resistor R1. The base voltage is controlled to give the required output voltage Vreg. A fraction of Vreg is tapped off from the voltage divider chain R3, VR, R4, any high frequency transients are damped by capacitor C, and the result is applied to the base of Tr3. If Tr3 base voltage, less the base diode drop (Vbe, about 0.6V), exceeds the zener diode voltage then Tr3 conducts and draws current via R1, thus reducing the base voltage of Tr1 and hence reducing Vreg so as to stabilise it. (VR is wired wiper-to-end-contact so that any intermittent wiper contact does not cause total loss of regulation).

Current limiting
The load current flows through R2. If the resulting voltage drop across R2 exceeds about 0.6V, base current flows in Tr2 and turns it on. Tr2 collector current is drawn via R1, thereby reducing Tr1 base voltage and so tending to turn off Tr1, thus lowering the output voltage and current to compensate.

Component values

Tr1 = NPN silicon, medium power (I used a BD535, simply because I had a salvaged one handy). Tr2 = Tr3 = any general purpose low power NPN silicon transistor. R1 = 1k (0.5 W). ZD = 5.6V. R2 = 10 Ohm. R3 = 4k7. R4 = 3k9. (R2-R4 are all 1/8 W). VR = 4k7 preset. C = 2uF, 25V.

The above values enable the output voltage to be set in the range 10.7V to 14.5V, with current limiting beginning at 60mA. With the output shorted to ground, the current is limited to about 80mA. At this current, R1 dissipates about 250mW and Tr1 dissipates about 1.4W, making a small heat-sink advisable.


Assume we want an output Vreg and current limiting to begin at Imax. R2 will then drop about 0.6V; hence R2 = 0.6V / Imax. Tr1 needs some headroom - say 3V - , and so we need a supply Vin of at least Vreg + 3 + 0.6 (the R2 drop). At the current limit threshold, the Tr1 base voltage Vbase is Vreg + 0.6V (across R2) + Vbe. The maximum base current Ibase of Tr1 is Imax / hfe (where hfe is the Tr1 current gain - only about 20 for the BD535). So R1 should be, at most, (Vin - Vbase) / Ibase and should be rated to handle Vin squared / R1 Watts. Tr1 should be rated to handle a dissipation of, say, twice Vin x Imax Watts and may need heat-sinking. Tr3 needs to be able to cope with a collector voltage = Vin and a collector current of about half Ibase. Tr2 sees a maximum collector voltage of about 1.2V and has a maximum collector current of about half Ibase, so is non-critical. The zener diode voltage is chosen to be a standard value of about half Vreg. For any transistor of at least moderate gain, Tr3 base current will be negligible; so the divider chain R3, VR, R4, only needs to pass 1mA or so. The voltage adjustment range can be increased / decreased by increasing / decreasing the ratio of VR to the values of R3 and R4. C is non-critical. An additional smoothing capacitor may be added at the Vreg output.

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