Help with a voltage regulator circuit

Discussion in 'Electronics Resources' started by bytraper, Aug 5, 2011.

  1. bytraper

    Thread Starter Active Member

    Sep 28, 2010
    I am having some problems with the circuit posted below.
    The circuit is a basic regulator, that works in the following way:

    The LED provides a 1.8v reference at the base of the BD140 and the resulting 1.1v at its emitter causes 2.34ma
    to flow through its 470 Ohm emitter resistor and through the 3.3k resistor at its collector. This gives 7.7v at the BDX37's base and so 7v at it's emitter.

    This works fine for voltages up to 36v, but the problem is, with some 48v power sources (batteries OR power supplies), I get surges through the circuit,
    which causes overvoltage to the 100uF 25v capacitor which then blows.

    I've tried running bigger capacitors at the front end, but no matter what I do, occasionally when powered from 48v the circuit will allow a much higher
    voltage through which blows up all the stuff that is expecting no more than +7v !

    Can anyone suggest anything else I can try to stop parts blowing up when 48v is applied?
    It doesn't happen all the time, but occasionally it does, which makes the circuit unstable.

    Thanks for any and all input!

  2. mik3

    Senior Member

    Feb 4, 2008
    First, the reference voltage provided by the LED is not very constant. As the input voltage increases, the current through the LED increases and its voltage increases too. However, this is not the reason for blowing the output capacitor up. Improve the reference voltage by using a zener or a reference voltage IC.

    I believe that the problem is the leakage current of the BDX37 transistor. When, the output capacitor voltage reaches 7V, the transistor turns off to keep the voltage at 7V. However, there is a leakage current thorough the transistor which slowly charges the capacitor towards the input voltage. As soon as the voltage is greater than 25V, the capacitor fails.

    To solve the problem, put a 1K resistor in parallel with the output capacitor.
  3. bytraper

    Thread Starter Active Member

    Sep 28, 2010
    Thanks Mik,
    I'll give this a go, but I think you may have nailed it.