0-30 Vdc Stabilized Power Supply

[MonSSter]

well . . the output voltage of u2 is the same as before thats .. around 200mV and he's still very hot. The u2 is tl081

[patrick01]

Had the same problem with U2 getting hot
The voltage devider circuit (ResR13,R14)that is suppose to hold the Transistor Q1 off is keeping it on and thats why U2 is getting hot.
Get the voltage to the negative so that Q1 is off and only swithes on when AC is removed.
Problem solved

[MonSSter]

..and how did you do that?

[audioguru]

Here is how Q1 works and how the negative 5.6V supply works:

[patrick01]

I think your assumption about the voltage is wrong
If you messure with a voltmeter the voltage is 26,4 volts and not the too voltages together
If you make R13 a 46K then the voltage on Q1 is less than 0.6 volts to switch it on so it stays off
Only swithes on as the AC is removes as you say the positive voltage takes longer to get to 0V

[audioguru]

I think your assumption about the voltage is wrong
If you messure with a voltmeter the voltage is 26,4 volts

Where is 26.4V?
If the transformer is only 24VAC (much too low for this project) then its peak voltage when loaded is 33.9V. The rectifier bridge drops it about 2V at full load and the main filter capacitor will have a little ripple making a positive supply of about 32VDC.

If you make R13 a 46K then the voltage on Q1 is less than 0.6 volts to switch it on so it stays off

46k is much too high for R13. With R13 as I show then Q1 is off when the supply is on and quickly turns on when needed.

[patrick01]

I really dont want to get in a argument about Ohm's law but my multimeter measure the 32v minus the 5,6V which gives you the 26.4.
What I said and measured is that Q1 was on with the values in the circuit diagram which pulled the output of U2 down which caused it to get hot!!!
So you have to switch Q1 off to get the supply working
Worked for me
Hopefully this is the end of the story :-X

[audioguru]

Hi Patrick,
The +32V and the -5.6V are in series and add to 37.6V across the total of 11.5k resistors.
Q1 is off in this circuit.

[patrick01]

You were right I was wrong
Seems I got Ohm's Law all wrong but nothing better to get the old books out again. At least you got me thingking again
What might have happened is that I have selected a value close to R14 but a bit higher which will give you a positive voltage on the base of Q1.

Thanks

Patrick :-\

[glockshotz]

Well I have finished my power supply and it works like a Charm :)

But I would like to add a digital panel meter that requires 5V power supply, does anyone have and good suggestions on how I should go about doing this.

P.S. this is my first project and I'm still learning

[audioguru]

Well I have finished my power supply and it works like a Charm :)

That's great! ;D
Did you use the original little 2N2219 transistor as Q2? Doesn't it get too hot when the project's output current is high and its voltage is set low?
Did you use the original 24V transformer? Does the project give more than about 25VDC at 3A without ripple?

[MP]

Well I have finished my power supply and it works like a Charm :)

But I would like to add a digital panel meter that requires 5V power supply, does anyone have and good suggestions on how I should go about doing this.

P.S. this is my first project and I'm still learning

I have seen a good answer to this from ante in the past. He coils a wire and attaches it to the top of the transformer for an additional low voltage tap. Perhaps he can give you a little more instruction regarding this since I am only giving the information second hand.

MP

[glockshotz]

That's great! ;D
Did you use the original little 2N2219 transistor as Q2? Doesn't it get too hot when the project's output current is high and its voltage is set low?
Did you use the original 24V transformer? Does the project give more than about 25VDC at 3A without ripple?

I did use the original 2N2219 transistor as Q2.  I have a heatsink on it, should I use a diferent transistor?

I used a 24V transformer with 100VA so it can draw more current than I want or need.(I'll have to limit this some how)

I noticed that I can only get the 0-30V when I have a load attached to the output. Is this because of the negative supply rail?

[audioguru]

I did use the original 2N2219 transistor as Q2.

[glockshotz]

I noticed that I can only get the 0-30V when I have a load attached to the output. Is this because of the negative supply rail?

It should work fine without a load. What happens without a load?

I get about 15.4v to 30v and the 2N3055 transistor is connected correctly

[audioguru]

I get about 15.4v to 30v and the 2N3055 transistor is connected correctly

Then Q2 or Q4 leaks. Probably Q2 from being fried.

Measure the output voltage at pin 6 of U2. It is probably negative in its attempt to reduce the output voltage down to normal.

[glockshotz]

Then Q2 or Q4 leaks. Probably Q2 from being fried.

Measure the output voltage at pin 6 of U2. It is probably negative in its attempt to reduce the output voltage down to normal.

Come to think of it, it was working fine until I exceeded 3A.  The output on pin 6 is 30v to -0.15v.  I couldn't find a TIP31A but I did find a TIP32C 140V 5A 40W will it work the same? 

[glockshotz]

Sorry I see the TIP32C is PNP I'll have to keep looking

[glockshotz]

Ok it works!
I replaced Q2 with TIP31C and Q4 with a new 2N3055.

[audioguru]

Ok it works!
I replaced Q2 with TIP31C and Q4 with a new 2N3055.

Great! You made a good fix. ;D

[Roelli]

Hello!

I just build the cool power supply and it works great. Many thanks for this nice project!  8)

I can regulate the current with a hot-wired Output from 0,01A to 4,6A. Works great.

But how can I protect the output transistor when there is a short at the output. I fear I will demolish the 3055 when making a short with maximum current.
Using some 3055 parallel? I don't want to use a heatsink which's bigger than my workbench  ;)

Any ideas about a protection circuit or so? Maybe a relais which turns off the voltage when the LED is on or so?

Many Thanks!

Roelli.

[audioguru]

Hi Roelli,
Welcome to our forum. ;D
It is good to hear that your project works great, but it is too bad that you are worried about it dying.

The LED lights whenever the load current exceeds the setting of the current-regulation pot. Maybe you will need its current regulation and don't want it to shut off by your relay idea.

Shorting the output when its current-regulation is set to a high current is nearly the same as setting a low voltage of up to 6V at full current (3A). The Q2 and Q4 transistors will still have a high voltage across them which when multiplied by the current makes a lot of heat. With the recommended 30VAC transformer, Q4 must dissipate 117W with its 3A output shorted. So a pretty big heatsink or fan cooling is needed and Q4 should be two paralleled 2N3055 transistors (with emitter resistors to match them).
Q2 must dissipate a max of about 4.8W which is way too high for a little 2N2219, so a TIP31A real power transistor on a finned heatsink is recommended as its replacement.

We have discussed how its original little 2N2219 for Q2 will die due to overheat, and the same for a single 2N3055 for Q4.

[Roelli]

Cool! Many thanks.

What a nice forum - and nice people here  ;D
And I thought I'll be executed because of not reading EVERYTHING of the 69 pages thread.
Which emitter resistors can you advise?  0,1R/2W Resistors each?

Thank you very much audioguru!

[audioguru]

Hi Roelli,
I'm glad to help fix this project.
Some people made a supply with two 0.1 ohm resistors and one 2N3055 was hot and the other was cold. Transistors are all different. One had a high current gain and the other had a low current gain. Changing the resistors to 0.33 ohms allowed the mis-matched transistors to have much better balance.

Here is my recommended modified parts list:

[Tedy]

Hi again  ;D
Is that normal that R2 get hot ~60