0-30V Stabilized Power Supply

[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?

 

[audioguru2]

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.

View attachment 41634

View attachment 41635

 

[audioguru2]

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:View attachment 41636

 

[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?

 

[audioguru2]

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?

 

[audioguru2]

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?

View attachment 41640

View attachment 41641

 

[audioguru2]

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.

 

[audioguru2]

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:
View attachment 41642

 

[benny8902]

All OpAmps are the same, MC34071. You know, it doesn't bother me much that the LED is on, as long as the rest is working like a charm. 

View attachment 41643

 

[audioguru2]

The output of U3 and Q3 should never turn the LED on dimly.
Maybe if the current-setting pot is at minimum, there is no load and U3 has a fairly high input offset voltage then the LED will be turned on. R17 sets the amount of minimum current regulation so it can be increased to 47, 56 or 68 ohms to increase the minimum current setting which will turn off the LED.

 

[Kiroid]

Hello I'm Kirill from Ukraine. Help me please, I have a trouble with 0...30V Stabilized Power Supply.  I've just finished upgrading construction from original circuit to new one (with new TLE2141 opamps and trimpots connections fixed and BD139 instead of 2n2219). I have transformer rated 30V, 6A and 10mFx63V capacitor for C1. And I've got such problem: voltage regulation works fine, except that fact, that I can't establish zero output voltage when Vreg pot is set to 0 - I have about 25 mV on output terminals. But that fact doesn't bother me: I suspect that trimpot I used is slightly poor Other thing drives me mad: output current never rises more, than 1.2 A It seems that current limiting part working correct, because under the load LED glows only when current limiting pot is at 1/3 of full range from minimum (1/3 - it's exactly ~1.2 A), than it fades, just like source has out from current limiting mode. But the output current still remains 1.2 A. For more clean scene: I set output voltage to 35V, and set current limiting threshold to 0 - LED switches on, as it supposed to. Then I connect load - 5 Ohms resistor to output and increase current limiting threshold. Voltage on the load rises to about 6 Volts, and current, respectively, to 1.2 A. Voltage at U3 pin 3 = 0.56V, as it should be. LED switches off, but voltage and current still remains the same, even if current limiter is set to max, as though something else limiting the output current %) Any ideas?

 

[audioguru2]

C7 is an electrolytic capacitor on the output. electrolytic capacitors have dielectric absorption where they store a voltage for a while. Try using a film capacitor instead then adjust the input offset voltage trimpot on u2 for an output of 0v when the voltage pot is at zero.

If the BD139 or the output transistors have their pins connected wrong (emitter and collector reversed) then their current gain is very low which reduces the max output current.

R7 senses the output current. Its voltage is compared to the setting of the current pot by U3.
The output of U3 goes only low enough to turn on Q3 and the LED and to reduce the output voltage through D9. With a very high load current or shorted output then the output of U3 might go 0.7V below 0V so that D9 can reduce the output voltage almost to 0V. 

 

[xristost]

Hello,

I also have а couple of questions. First, the original article states:

In order to make it possible for U2 to control the output voltage down to 0 V, it is necessary to provide a negative supply rail and this is done by means of the circuit around C2 & C3. The same negative supply is also used for U3. As U1 is working under fixed conditions it can be run from the unregulated positive supply rail and the earth.

But in the improved schematic (latest version) pin 4 of U2 is connected to ground ("-" pin of C1). Is this done on purpose or by oversight?

Second, what is the behaviour of the output voltage when the power is switched off with Q1 removed from the schematic? Is any harm to put Q1 back?

Thanks.

 

[audioguru2]

1) The original article states:



But in the improved schematic (latest version) pin 4 of U2 is connected to ground ("-" pin of C1). Is this done on purpose or by oversight?

The original circuit had a few serious problems. It used TL081 opamps that need a 5.6V negative supply for their inputs to work properly and its absolute maximum allowed total supply was only 36V. The project used a 24V transformer which produced an unregulated +32V which is not high enough for the output to be 30V at 3A, its max output might be only +25V. Without a load the 24V transformer might produce +34V so the opamps operate with a total of 34V + 5.6V= 39.6V which is higher than their max allowed total supply voltage.

TLE2141 and MC34071 opamps were selected for the fixed version because their maximum total supply is 44V and their inputs work perfectly without a negative supply voltage. Without a load and with a 30VAC transformer the total supply for U2 is +42.4V so it will be fine. A 28VAC transformer can be used instead and the supply for U2 will be +39.6V which gives more headroom. It is easy for the output to produce +30V at 3A.

2) What is the behaviour of the output voltage when the power is switched off with Q1 removed from the schematic? Is any harm to put Q1 back?

TL081 opamps have a problem called Opamp Phase Inversion where the output suddenly goes high if an input voltage becomes too close (within a few volts) to their negative supply voltage. The negative supply in this project was small so it collapsed first when the power was turned off which caused the output voltage of U2 to rise which caused the output voltage of the project to rise. Q1 was used to short the output of U2 to 0V when the negative supply begins to drop so the output voltage could not rise.
The new opamps do not have this problem so Q1 is not needed.
U3 needs to have a small -1.3V negative supply so that its output in series with D9 can go low enough to cause an output voltage of close to 0V when the output of the project is shorted.  View attachment 41648

 

[xristost]

TL081 opamps have a problem called Opamp Phase Inversion where the output suddenly goes high if an input voltage becomes too close (within a few volts) to their negative supply voltage. The negative supply in this project was small so it collapsed first when the power was turned off which caused the output voltage of U2 to rise which caused the output voltage of the project to rise. Q1 was used to short the output of U2 to 0V when the negative supply begins to drop so the output voltage could not rise.
The new opamps do not have this problem so Q1 is not needed.
U3 needs to have a small -1.3V negative supply so that its output in series with D9 can go low enough to cause an output voltage of close to 0V when the output of the project is shorted. 

There was a video posted on some page of this tread, where output voltage raise briefly. And that was with these new opamps. That's why I was thinking to put back Q1.