0-30V Stabilized Power Supply

[audioguru]

hi every one. i create the original version and work fine
but i have 2.5A at 30v when turn voltage volume under
7v current get to 1,5 A and overload protection
do not work
 please help me
 excuse me for poor english

The original version has many errors that cause it to be unreliable and not meet its specs.

[thenr.]

hi every one. i create the original version and work fine
but i have 2.5A at 30v when turn voltage volume under
7v current get to 1,5 A and overload protection
do not work
 please help me
 excuse me for poor english

Hi Ajax007, forget the original version. You should see the xristost's schematic the following link:
http://diyfan.blogspot.com.br/2012/02/adjustable-lab-power-supply.html

[ajax007]

thank you for reply

please help me i dont have new component to create new version .
my circuit work fine only problem is current not constant. when current
volume is max: 30v 3A (short circuit) when current volume is
max and voltage volume is 7v current :2A (short circuit)
my transformer is 24v 3A

[audioguru]

When the original circuit has a 24VAC/3A transformer then the transformer can supply 24V x 3A= 72VA. But 24V rectified produces 24V x 1.414V (-2V for the rectifier bridge) which is +32V.
But the circuit needs at least 40VDC instead of only 32V so the voltage from the original transformer is also too low. The transformer must be rated at 42V x 3A= 127VA.

The tiny old 2N2219 driver transistor gets too hot when the output voltage is low and the current is high. The improved circuit uses a modern power transistor with a real heatsink.
The single 2N3055 output transistor also gets too hot. The improved circuit uses two output transistors (with emitter resistors) to share the heat.

When your output is 7V at 2A then maybe the driver and/or output transistor gets too hot and fails.   

[ajax007]

thank you
2N2219 and 2n3055 temperature is normal
i test circuit this way:
1-turn current and voltage(30v) volume max
2-connect ammeter and read 3.2A
3-turn voltage volume to 30 to 8v
connect ammeter and read 3.2A
4-turn voltage volume to 7v  connect ammeter and read 2.6A
5-turn voltage volume to 3v  connect ammeter and read 1.5A
you think transformer voltage is low i test 28v and i get same result

[audioguru]

With a +32VDC supply and a 3A load, the voltage across R7 reduces the output voltage 1.41V.
The maximum output of the TL081 opamp U2 is +29.1V but it could be as low as +27.6V.
The maximum output of the 2N2219 driver transistor Q2 is +28.1V but it could be as low as +26.3V.
The maximum output of the 2N3055 output transistor is +27.1V but it could be as low as +24.8V.

But the overloaded transformer probably cannot produce rectified 32VDC so the output voltage will probably be less and be unregulated with a lot of ripple.   

[thenr.]

I'm making my power supply 0-30V inside an old ATX power supply, and I have 2 Fans. How should I select the side of the flow?
Should I make throwing air at the heatsink or throwing the air which is inside the power supply out?
Sorry about my poor english.

[audioguru]

One fan can blow cool air into the heatsink and the second fan can blow hot air out.
We don't know if the fans can cool the output transistor well enough.

That is why the improved version of this project has TWO output transistors to share the heat.

[thenr.]

Thanks audioguru, I read the answers about to use two 2n3055. Will one BD139 be able to support both tranzistors ?

[audioguru]

Thanks audioguru, I read the answers about to use two 2n3055. Will one BD139 be able to support both tranzistors ?

Yes because each output transistor has half the total current then their current gain is higher so the BD139 driver uses less current and is cooler. Even the opamp that drives the BD139 will use less output current and will be cooler.

[thenr.]

OK Thank you audioguru... When the PS is finished I post the results here.

[vendaf]

i have a problem about my voltage adjust(potentiometer), the problem is that the voltage is not balance and not linear, my minimum voltage is 0.7v and max with 32v, but if i adjust the potentiometer 1/2 the voltage is only 1.3v, in 3/4 of my potentiometer i can adjust the voltage properly start by 3v-32v but i really wonder why the voltage is gather in the 3/4 of my potentiometer? i use 10k pot, i already replace it 3x,, but the same thing happened., i also replaced it with 1k value, but the max voltage lower, only 7.4v,,.. can you help me sir???thank you!!

[audioguru]

The pot is supposed to be a linear pot. You are probably using a pot with a logarithmic audio taper (used as a volume control).
A pot with an audio taper has an output level of about 1/10th or 1/20th when it is half-way.

[vendaf]

ok sir,, thank you!