Simple transistor regulated power supply

Thread Starter

Rob_1486819725

Joined Feb 11, 2017
3
this schematic is designed to give an output from 35-50v. The question is which part do i change for use with a 24v transformer, so i can regulate the output under 35v??
at this point i have the circuit running with the 24v transformer but it only gives out 35v(the pot doesnt change the output) .
thank you!
 

panic mode

Joined Oct 10, 2011
1,865
if your regulator is not responding to potentiometer position change, something is wrong with your circuit, for example if you short MPSA42 collector and emitter, you should see output drop to about 10V. if that does not happen, check if other two transistors are ok and wired correctly
 

Dodgydave

Joined Jun 22, 2012
9,641
Don't see the point of the 0r22 resistor, its not being used to sense current, i would replace the two transistors for Tip121 Darlington pair.
 

AlbertHall

Joined Jun 4, 2014
10,538
if your regulator is not responding to potentiometer position change, something is wrong with your circuit, for example if you short MPSA42 collector and emitter, you should see output drop to about 10V. if that does not happen, check if other two transistors are ok and wired correctly
The transformer has been changed to 24V secondary so the regulator cannot achieve its minimum 35V output.
 

AnalogKid

Joined Aug 1, 2013
8,677
The three resistors between POS and GND and the 12 V zener diode combine to set the output voltage. The zener plus the base-emitter voltage of the MPSA42 combine for approx 12.6 V. This is the minimum regulated output voltage possible without changing the zener diode to a smaller value. If 12.6 V is an acceptable minimum output voltage, then changing the 10 K and 2.2 K fixed resistors will change the max and min adjustment range. If you are familiar with Ohm's Law, that is what is used for this.

Another way to view this circuit is a non-inverting opamp with 12.6 V on the non-inverting input, and the base of the MPSA42 as the inverting input. The three resistors between POS and GND form the series and shunt feedback resistors, with the ratio changing as the pot is adjusted.

ak
 

panic mode

Joined Oct 10, 2011
1,865
I suspect the transformer puts out a little more than the rated 24 V.
yes.. or his line voltage is slightly higher than usual. either way, this is slight change, has no impact on circuit performance.

bigger issue is why the output is not adjustable (even if resistor values are not modified for lower range).
there could be several causes such as blown power transistor, bad feedback (for example burned out potentiometer or not not connected correctly etc.), diode is not really a zener diode or it is for higher voltage, etc.
 

GopherT

Joined Nov 23, 2012
8,012
Note: the 10000uF capacitor should not connect to the pot wiper! I'm guessing you have a wiring issue and you need to adjust the value of the resistor south of the pot as @AnalogKid pointed out.
PS: when wires cross on a schematic, a dot at the intersection means they are electrically connected, otherwise it is just two separate wires.
 

Thread Starter

Rob_1486819725

Joined Feb 11, 2017
3
Thank you for the feedback everyone!
I understand that i have to play around with the voltage divider values, will post a photo of the pcb and everything, after i do some more tests
 

panic mode

Joined Oct 10, 2011
1,865
I calculate a minimum output voltage of about 44.5V for the original circuit (pot all the way to the top).
That causes 12.7V at the transistor base (pot wiper).
What did you come up with?

i calculate max 12.7V at the base of feedback transistor regardless of potentiometer setting. not sure what your point is.
 

crutschow

Joined Mar 14, 2008
25,634
i calculate max 12.7V at the base of feedback transistor regardless of potentiometer setting. not sure what your point is.
That to get 12.7 volts at the base for the output to be in regulation, the output can be no lower than 44.5V with the resistor values shown and the pot all the way to the top, i.e. the minimum regulated output voltage is 44.5V.
 
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