0-30 Vdc Stabilized Power Supply

[Sparkington]

Ok Im trying to fix my project using the following info.

    Received the following email from Ivan on 02/2010:

    Ok. I bulid your project about a day ago. Mounted all the parts on the pcb and then concluded that there is some serious problems in this schematics. First, 2N3055 will overheat, so you have to connect two of them in parallel with emitter resistors 0.1ohm/5w. Second, maximal voltage between '+' and '-' of TL081 is 36VDC.If you connect them as it is shown in this circuit diagram that voltage will be about 45VDC, so they will burn down immediately. To fix this problem you have to reconnect all pins number 7 of U1, U2 and U3, emitter of Q3 and 'upper' end of R19 to out of an 7809 with 18V zener diode between 'common' pin and '-' of 3300uF cap, and input of 7809 connect to '+' of the same cap. Now, on pin number 7 and mentioned parts you'll have 27VDC, and total voltage will be 32.6VDC. Third, instead of using 3300uF, use 4700 or 6800uF/63VDC to reduce the ripple on higher currents (2-3A). The rest of the circuit is perfect. I like it cause it is so inexpensive and easy to make with those simple reconstructions i mentioned.

I was wonder if I insert the LM7809 into the circuit properly, I dont really understand words sometimes, and if i got it wrong I was wondering you could supply the correct schematic.  I cant buy any other IC listed in fixes expect a MC34072DG and know it wrong IC but it all I got :/

[audioguru]

Sparkington,
You forgot to connect the driver and output transistors to the power supply (the input of the 7809 regulator).

Ivan used the original circuit and most of its parts then added a 7809 regulator and an 18V zener diode so that the maximum unregulated supply voltage to the opamps is only (+27V plus the -5.6V negative supply=) 32.6V then the original opamps rated at 36V maximum can survive.
But then since the positive unregulated supply is 27V then the maximum output voltage from the project at 3A is only about 20VDC, not 30VDC.

An MC34072 is two MC34071 opamps in one case. The surface-mount package might get too hot because it is very small.

[Sparkington]

Thank you for the quick reponse, I will remake the PCB and start again. I will use the upgraded parts on the rest circuit describe by one of your early post. 0-20V will get me out trouble for now, I will upgrade transformer to a 30V 3A later on. Time to go to the local electronic store by more parts lol

[peckcj]

I've built my PSU and everything works as expected  .  My only issue is as follows...

I have 2 LED panel meters that I will be using for voltage and current displays.  They are both PM129B units (http://www.circuitspecialists.com/images/PM129A&B.pdf).

I got the voltage meter to work fine, but the current meter is giving me a headache.  I am using R7 as the shunt resistor, and since I built the 5 amp version, that resistor is valued at .27 ohm.  When I connect the meter across R7, I get high reads...750-1200mV when I have a loading resistor drawing .5 amps.  The meter is set at the 200mA range.

Should I add another shunt resistor into the scheme that is better suited for this 200mA meter?  I was thinking .40 ohms.

[audioguru]

..... the current meter is giving me a headache.  I am using R7 as the shunt resistor, and since I built the 5 amp version, that resistor is valued at .27 ohm.  When I connect the meter across R7, I get high reads...750-1200mV when I have a loading resistor drawing .5 amps.  The meter is set at the 200mA range.

The meter is a voltmeter so it does not have mA ranges.
If it is set for the 200mV range then it will read 200mV which is too low when a current of (200mV/0.27 ohms)= 0.74A. when the current is 5A then it will try to read over-range at (5A x 0.27 ohms)= 1350mV.
If it is set to 20V range then when the current is 5A it will read (5A x 0.27 ohms)= 1.35 which is too low.

Set it to the 20V range and add a multi-turn cermet 500k calibration trimpot in series with its RA. Adjust the trimpot so that its reading is accurate.

It might need its power supply 0V connected to the power supply 0V, not the output 0V. 

[peckcj]

The meter is a voltmeter so it does not have mA ranges.

Yes....my mistake   

Thank you

It might need its power supply 0V connected to the power supply 0V, not the output 0V. 

Could you elaborate on this please?  What is the difference between the output 0V and the PS 0V?

[audioguru]

R7 connects between the power supply 0V and the output 0V.
The output 0V becomes a positive current-sense voltage when there is load current.

[peckcj]

Thank you, I'll try that.

Also, my C1 capacitor is a 20,000 mF 63V electrolytic that is holding a charge after I power down, so I need to add a bleeder resistor across the terminals.

Do you have a recommendation on the size of resistor?

[audioguru]

C1 already has a bleeder resistor, R1. It is 2200 ohms so it will take about 3.7 minutes to discharge your capacitor but the electronics will also discharge it so maybe in 1 minute.
 

[peckcj]

Thank you.

Would you have any recommendations on how to deplete that standing voltage faster without having to change my C1 type?

[audioguru]

I don't know why you want to reduce the output voltage when the circuit is turned off.
C1 holds a large charge to reduce ripple in the output.

Maybe a second contact on the on-off switch can discharge it quickly through a low value resistor. 

[josko]

Hi,
I have made two modules of picmaster's version, but I have a little issue. First of my two modules is working flawlessly regulating voltage from 0-38V and so the current limiter.
But the other visually looking the same (all parts are the same and as well interconnections) is also regulating voltage but only 0-31V and also current limiter is not working at all in one position of current setting potentiometer I have managed to get limiting diode shut (it's around 1/4 from lowest current setting) but if I move it slightly higher or lower limit indicating LED begins to shine but it is not sudden blink as in the first module, but linear, like some bulb dimmer also it doesn't matter if I move it to max current or to min current and also voltage drops only by 2-3 volts when it limits current.
Any suggestions what might be wrong here?

[audioguru]

Hi Josko,
Picmaster's schematic does not list its transformer voltage. It should be 28VAC or 30VAC.
The circuit should regulate the output well up to 30VDC at the output, not 38V.

Opamp U3 regulates the output current by comparing the voltage set by the current-setting pot to the voltage caused by load current in R7. When the load current is higher than the current-setting then the output voltage of U3 drops which turns on transistor Q1 on Picmaster's schematic which lights the LED. When regulating the current, the output voltage is reduced by U3 pulling down D1 on Picmaster's schematic. The output voltage of the project can be reduced to zero when the output is shorted since the output of U3 can go down as low as minus 1V. Maybe your defective circuit does not have the -1.3V supply to U3.

[josko]

Thank you for your answer,  I have 2x 29,7V AC transformer (it is 1200W this makes it 600W for one 29.7V AC on C cores so it doesn't drop under any load, actually when I shorted it once it blow up secondary (and also primary) circuit breakers instead of transformer itself ).

Thank you for information about 30VDC I have managed to get 38VDC by adjusting RV3 (I have adjusted it to get 0 V DC when I turn voltage potentiometer to min) is it dangerous for correct operation of my circuit?

You found it! It is that -1.3V supply on defective circuit I measure something about +0.53V (I have -1.45V on correct one)  Do you have any clue what could be wrong?