0-30V Stabilized Power Supply

[audioguru]

sorry but i CAN NOT attach any image or anything on this forum

It is simple to attach a schematic to your post:
1) During your reply, click on Additional Options.
2) Click on browse and locate the schematic's file on your hard drive.
3) Click on the file then click Open and it will be placed in the Attach box.
4) Post your reply then your schematic will be attached here.

Can't you increase the contrast since now it is pastel and pastel and is hard to see?
Can't you place parts closer together on the schematic so that all parts will look bigger?

Your new image that was linked looks better but it still lacks contrast.
Its parts are still far apart so it is huge which makes the parts too small. When enlarged it bigger as my neighbourhood since the parts are blocks apart.

Sorry but I can't spend hours scrolling through your huge schematic.

Here is the difference between its original image and the same image with contrast added:
 

[stelsing]

Hello. I create a power supply by scheme srecenvid. When adjusting the maximum current, in low position RV3 current reaches 10A(minimum), when increased resistance RV3 the current is increased. In what may be the problem?

[audioguru]

I can't read his schematic because it is fuzzy and has no contrast. If he draws the parts closer together then the schematic can have larger parts which will look clearer. Then he should increase the contrast.

I guess that RV3 is a variable resistance trimpot in series with the current pot.
Maybe you also cannot read his schematic then you have a resistor value wrong.

The current sense resistor is 0.47 ohms for the 3A power supply and is 0.27 ohms for the 5A power supply. 3A x 0.47 ohms produces 1.41V and 5A x 0.27 ohms produces 1.35V.

Adjust RV3 so that the slider of the current pot is 1.41V at a load of 3A or is 1.35V with a load of 5A. Then any more current will cause opamp U3 to reduce the output voltage which will regulate the current.

Maybe your opamps have different part numbers than the two that the revised circuit uses?
The negative supply for U3 is only -1.3V. Many opamps will not work with a negative supply that is so low. If you increase the negative supply voltage then opamp U2 will have too much total supply voltage which will cause it to fail.

[stelsing]

This schematic and part list.
Schematic i check tomorrow.

[audioguru]

This schematic and part list.
Schematic i check tomorrow.

It is my parts list and schematic.
With a 3A load then U3 should begin to reduce the output voltage and light the LED if the current tries to get higher.

I marked up the schematic to show the current regulator at 3A:

[stelsing]

Thank you. Everything works. 

[benny8902]

Hello.
I built it, it works, but it doesn't work correctly. Input voltage on C1 rises up to 57V!!! And I don't know why. Is it because R1 has 22k instead of 2.2k? The lady from the electronics shop messed up the values.
Output voltage varies from 0-30V, though. Should I be worried about the input voltage?

[audioguru]

Without a load a 28V transformer will be 29VAC and a 30V transformer will be 31VAC.
The peak voltage of 31VAC is 43.8V and the bridge rectifier drops the unregulated DC to 42.4VDC.

The absolute max allowed voltage for the latest recommended opamps is 44V.

Your 57VDC is 14.6V higher than normal. Maybe your transformer AC voltage is too high or maybe it is extremely cheap and its voltage is too high with no load then drops when loaded because it uses cheap thin wire with too much resistance. Measure it.

With 42.4VDC a 2.2k resistor draws only 19.3mA so a decent quality 30VAC transformer voltage will rise to maybe 30.9VAC.

[benny8902]

The transformer is just fine, it's a 30$ 150VA toroidal transformer. It's good quality. Something else was the problem. I have found it. I have integrated a multimeter on the same board, I have mounted a 7824, a 7812 and a 7805 for the uC and the LCD display. It wasn't a good solution. When I was connecting the display, the output voltage was rising quickly, so i've bought a 5W 20V zener diode and a 5W 220 Ohms resistor for voltage regulation. This way it worked just perfectly.

Now I have another problem. I have shorted the output to see the value of the current and I think I blew it. Now the output voltage is 39.6V, no matter what. Are the MC34071s blown? Or one of them?

[audioguru]

I have shorted the output to see the value of the current and I think I blew it. Now the output voltage is 39.6V, no matter what. Are the MC34071s blown? Or one of them?

U1 is simply an 11.2V reference voltage so it has nothing to do with current in the output.
U3 is used as a comparator to sense the voltage across R7 which represents the output current and compares it with the voltage set by the current-settingh pot. If the sensed current is too high then the output of U3 pulls down D9 to reduce the output voltage which should reduce the output current.
U2 is in the output amplifier circuit. D9 reduces its input voltage of U2 if the output current gets too high.

I think the driver transistor Q2 is shorted or the output transistors are shorted maybe because their heatsaink is too small and they got too hot. 

[benny8902]

No way the heat sink would be too small. See the pictures attached.

Also I have noticed that U2 gets hot. Any ideas, why?

[audioguru]

You forgot to add a heatsink to the BD139 driver transistor.
When the output is shorted then it will dissipate 1.5W if the output transistors have typical current gain or 5.5W if they have minimum current gain.
Without a heatsink it is at its max allowed temperature with a dissipation of 1.2W.

U2 will be very hot when the BD139 becomes shorted.   

[benny8902]

Problem solved. I have replaced the BD139 and added a heat sink. Now it works great.
Thank you for your help and for the design.

Just one more thing. My current limiting LED stays on all the time, though the current is not limited. When it goes into limiting stage, the LED lights brighter. What could I do to turn it off when it is not needed to be on? Voltage divider made out of 10K and 1K resistors? I have unconnected the LED and measured 19VDC on the pins in normal stage, 29VDC in current limit.

[audioguru]

My current limiting LED stays on all the time, though the current is not limited. When it goes into limiting stage, the LED lights brighter.

U3 and Q3 are a switch that is on or off, never half-way. Maybe you used an opamp for U3 that is different from a TLE2141 or MC34071?

I marked the current regulator circuit with notes that explain how it works: