Transistor regulated power supply problem

[KAKAH]

this is the schematic ive built. Its designed to give an adjustable output of 35-50v, the question is which part do i have to change to make it work below 35v. I have this connected to a 24v transformer and all it gives out is 35v(no matter the pot position). I know this is very simple for some.

 

[MikeMl]

Remove the 10K above the pot, and make the pot 10K instead of 2K. That will reduce the min output voltage to about 14V.

Watch the power dissipation in the 2SC5200. It should be on a large heatsink.

ps: I cobbled together this LTSpice sim to optimize the feedback voltage sensing range. Based on this, also change the bottom resistor (R4 in my sim) to 6.8K. This will give you a better range on the pot adjustment.

In the sim, p is the pot wiper position, going from 5% to 95%. The plot shows output voltage vs pot position.

If you would like a lower minimum voltage, then you could change the Zener to one with a lower voltage, say 5.6V. Then the minimum voltage would be ~(Vz+0.7)V, but R4 would change to yet another value...

 

[Nigel Goodwin]

I'd also like to see a resistor from the +ve supply feeding the zener diode, rather than relying on current through the transistor - or better still, a constant current source.

 

[oemcar]

substitute the 10k and 2R2 fixed for 1k's, and the 2k2 pot for 10k pot and see what happens- not sure of your current needs, but this should rectify your voltage range prob.
if you see voltage not well regulated at higher current loads with this mod, then sub the BD241C for a TIP120 darlington
jim

 

[spec]

It would also be wise to connect a 470 Ohm resistor between the base and emitter of the output transistor (2SC5200)..

spec

 

[oemcar]

It would also be wise to connect a 470 Ohm resistor between the base and emitter of the output transistor (2SC5200)..

spec

openly stating that I dont know it all-
it would seem to me that your mod has to do with current stability regarding this design?

 

[spec]

openly stating that I dont know it all-
it would seem to me that your mod has to do with current stability regarding this design?

Steady on oemcar, I stated no such thing. Please do not miss-interpret what I said.

As a general point, nobody knows it all.

By the way, the resistor across the output transistor base and emitter is to bypass leakage current and also to bias the driver transistor on with a small current so that there is not a wildly changing impedance at the output transistor base. The resistor also reduces the driver transitor hFE variations due to changes of collector current of the driver transistor. It also improves the frequency characteristics of the output stage. This is a standard design approach.

I have been working on this power supply, on and off, since the OP first posted and I thought you suggestion of using a TIP120 Darlington to increase the the current gain of the output stage was very good. You managed that without increasing the component count.

spec

 

[MrAl]

this is the schematic ive built. Its designed to give an adjustable output of 35-50v, the question is which part do i have to change to make it work below 35v. I have this connected to a 24v transformer and all it gives out is 35v(no matter the pot position). I know this is very simple for some.

Hello there,

My question would be, what range output do you really want here?
For example, 12 to 30 volts, 12 to 24 volts, 6 to 20 volts, 3 to 20 volts, etc.
The low end and high end are needed to be able to design this circuit so that you get the range you are looking for, otherwise it's a guessing game what components you need and you'll end up trying a lot of values to get what you want.
So if you state the range that would help everyone here help you to find the right components and values.
Note that the way the circuit stands at present there will be some minimum output possible, such as 2v or around there. Getting 0v would require some more mods.
Also, you should state what current levels you intend to try to get with this circuit too, such as 100ma, 200ma, 500ma, 1 amp, etc. With some combinations of current levels and output voltages you would need an impractically large heat sink.

To properly design a power supply circuit the designer needs to know as a minimum what the output voltage range will be and what the output current range would be.

 

[KAKAH]

thanks for the feedback everyone! Of course the resistors making up the voltage divider!

Note that the way the circuit stands at present there will be some minimum output possible, such as 2v or around there. Getting 0v would require some more mods.

I will do some further testing with the resistors, but i find this not true, since im stuck on 35v output(no load)

this power supply is inteded for an audio amplifier and needs to be in the range from ~18-24vdc 8amps peak, the 2sc5200 sits on a fairly large heatsink.

will report how it goes with changing the resistor values

 

[MrAl]

thanks for the feedback everyone! Of course the resistors making up the voltage divider!


I will do some further testing with the resistors, but i find this not true, since im stuck on 35v output(no load)

this power supply is inteded for an audio amplifier and needs to be in the range from ~18-24vdc 8amps peak, the 2sc5200 sits on a fairly large heatsink.

will report how it goes with changing the resistor values

Hi,

I was referring to the base topology which would allow for something like 2v min but of course that is after changing some values like the zener voltage. As it stands now with the 12v zener and with the right resistor values the min would be roughly 12v to 13v or thereabouts. Since you want 18v min then you should be ok with the 12v zener. If we figure around 12.7v operating point for the base, that means we need a ratio of about 1.4 which would mean say 14k total and 10k on the bottom to get 18v. To get 24v with that same 14k total we'd need about 7.4k on the bottom. We could tailor that resistor set to give good adjustment range by using a limit resistor on the bottom say 6.4k, then maybe a 2k pot, then a 5.6k resistor on top, just to quote some rough values.

If you had say 36v input however when adjusted to 18v output at 8 amps you would be dropping 18v and at 8 amps that is 144 watts. That is a LOT of heat and quite a waste of energy. If you could match the input to the output a little better you can bring that down quite a bit. If this is for a linear audio amplifier you loose even more energy because of that so you may even want to go to a buck switcher.

 

[dr pepper]

Something like a bulb or resistor to limit current would be a good idea during the testing stage just in case you get something wrong.