20 amp power supply question

Discussion in 'The Projects Forum' started by yourownfree, Aug 5, 2014.

  1. yourownfree

    Thread Starter Active Member

    Jul 16, 2008
    I am building a power supply. The question I have is what is the difference of using a pass transistor's base, hooking it up at the input of say a 15 volt regulator vs the base of the pass transistor hooked to the output of the regulator. I have seen both ways, have tried both ways but I don't know which way is preferred, or better. My test show you get more output when the output of the regulator is connected to the base of the pass transistor. when I mean more output, I mean more current at the output.
    Last edited: Aug 5, 2014
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    The usual way is to have a series resistor on the input of the 15Vreg.
    The current flowing thru the resistor and Vreg, causes a voltage drop across the resistor.

    The pass transistor is a PNP type, Emitter to the 'high' side of the resistor and Base to the Vreg input, Collector to the Vreg output.

    The voltage across the resistor controls the pass transistor current.
    If you need a diagram, ask.

    Could you post a diagram showing your two methods of connecting the pass transistor.?
  3. yourownfree

    Thread Starter Active Member

    Jul 16, 2008
    what I have done is use a 12 volt regulator with three rectifiers in series coming off the ground/common leg of the regulator. I have a resistor on the input of the regulator and the output goes directly into the base of the pass transistors. I have succeeded in gaining 4 more amps over using a 7805 ic in the same configuration. My total output now from 14 volts drops to 12 volts at 16.3 amps. This is a modified Kenwood PS-30 power supply. My next step is to finish the over voltage circuit. Kenwood doesnt have one for this supply. What happens is the pass transistors will short giving the full 22 volts to the load, killing the radio attached. My circuit will shut it down at 16 volts. I have removed the original regulator circuit and replaced it with mine. The regulator is a radio shack 7812c. The rectifiers are 1n4007's. I will be adding some voltage adjustable control to vary the voltage slightly and to allow the user to set the high max limit of the over voltage. It will be on the front panel as taking off the top lid is just nuts to make adjustments. As you can see the front panel has been fitted with a readout of voltage and amps and it works fine. So far so good on this one. The rear of the unit has been fitted with new pass transistors using artic silver heat compound. Also the heat sink has been fitted with a fan the same size as the heatsink. I have to get to work now so maybe after work I will have time to show you the diagram of what I have so far. I will show you the two different approach's I originally was wondering about. I did see the theory behind the series resistor and again I saw the theory behind using just a zener and a resistor which uses up too much power, vs using a transistor to drive the base, where you calculate the gain of the pass transistor to get the value of drive current needed to run it to saturation. when I tried then series resistor type circuit where the base was hooked to the input of the regulator I couldn't get the current I needed. Seems to be a little more on the conservative side.Just an opinion, but I never had time to tweek on the resistors much. I can see a voltage drop drop across it(series resistor) and possibly better regulation, but for what I need a couple of volts drop at the output will be ok. It drops 2 volts(from 14 volts) at 16.3 amps. That is all the unit will do. I tried 20 volts and drops to 12 same as before. It was just an experiment. It dropped 8 volts. The result is 12 volts 16 amps is the magical number it likes, so I will run it at 13.8-14 volts, when and if it reaches the full load capacity it will have some room to drop.
  4. MrChips


    Oct 2, 2009
    Yoh! Can you put some paragraph breaks and extra lines in your post?

    Then maybe I will consider reading it.
    Last edited: Aug 6, 2014
    djsfantasi likes this.
  5. crutschow


    Mar 14, 2008
    The saying is "A picture is worth a thousand words". That certainly applies here. :rolleyes:
  6. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    What was posted:

    What I saw: