0-30 Vdc Stabilized Power Supply

[muffits]

About C4, R8 and R9 forget it.... my bad, R9 only takes down a bit amplitude of ripple voltage at low frequencies (something like to 10Hz). So I still don't understand R9 fuction.... I remeber that audioguru said that it isolates D9... does it mean that for D9  R9 seems to be source?

 

[audioguru2]

The input of the opamp U2 is megohms so R9 doesn't affect how it works, until the current regulator U3 needs to quickly reduce the voltage at the input of U2. Then the resistance of R9 isolates C4 from the input of U2 so that D9 can quickly reduce the voltage at the input of U2 without waiting for the filter C4 to discharge.

This project is regulated and shouldn't have ripple. If R4 is the original too high value of 4.7k instead of 1k, and D8 is a high current zener diode instead of a 5mA one, or if C1 is the original too small 3300uF instead of 12,000uF then the output will have ripple. 

 

[MonSSter]

helow! it's me again .. well i replace the Q2 and the u2(new one is still heating like mad) and now the output voltage is ~0.4V .. the voltage in q4 E,C and B is 32V+ ... so what is wrong now?

 

[audioguru2]

i replace the Q2 and the u2(new one is still heating like mad) and now the output voltage is ~0.4V .. the voltage in q4 E,C and B is 32V+ ... so what is wrong now?

Q4 is shorted and might have destroyed the new U2 and Q2.

Remove R15 and get Q2 and Q4 working properly first. As a test, connect two 10k resistors to the base of Q2. Connect one resistor to +34V and the other resistor to the output's 0V. Without a load, the output voltage should be about +15.7V. Without R15, U2 should be cool.

 

[MonSSter]

hmm .. I replaced q4  and u2(q2 looks ok)disconeted the r15 but nothing happend... output was i bit higer that's all ... so the thing still doesn't work ... any new directions(maybe you draw me the solution ). Tnx

eemmm ... q4 C = 34V, E = 0,5V B = 0,5V

 

[audioguru2]

Hi MonSSter,
In your reply #931 all 3 terminals of Q4 were at +32V and the output of U2 was at 0.4V. So I thought that Q4 was shorted.

Now with R15 disconnected the base and emitter of Q4 dropped to +0.5V which is normal.
The output voltage of U2 is high which is normal without R15.
Is U2 still hot?
Which part number is U2?

 

[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?

 

[audioguru2]

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

View attachment 39116

View attachment 39117

 

[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

 

[audioguru2]

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

 

[audioguru2]

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.

View attachment 39127

 

[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

 

[audioguru2]

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?

 

[MP1]

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?

 

[audioguru2]

I did use the original 2N2219 transistor as Q2.