Author Topic: Repair Generic Power Supply that can't go beyond 10V  (Read 7456 times)

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Offline OldSchoolTechCorner

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #25 on: September 19, 2015, 01:55:48 am »
Could be OP-AMP in current, or voltage control, or bad output transistor, which usually cause that issue. Without a schematic and measurements it a guess at best. Do you have a scope, or meter around to troubleshoot it? As you need to check taps off transformer, output transistors, relay section, big power resistors and OP-AMPS input and output. as for CV, you monitor the output voltage and feed this back into the inverting input of the op-amp driving the output, for CC, you monitor the voltage across a current sense resistor and feed this back into the same inverting input of the op-amp, is usually how it works as the circuit will automatically switch into CV or CV depending on which threshold is reached first. If current op-amp isn't working correctly, you won't have current control and voltage will drop at random, or no output at all.

First I would tap the board and make sure no cold solder joints and look for any obvious signs of damage components. Next step to take is to check transformer taps, relay section, big resistors, output transistors section and make sure no issues as shorted, or open transistors bad diodes, or resistors, ESR on capacitors and also getting correct voltages with the power section first and work your way back. Also check for excessive ripple. When you get to OP-AMP's assuming you check everything else. One trick is to remove the current OP-AMP and that will keep power supply from going into constant current mode and if you get stable voltage then current OP-AMP, or potentiometer is bad. Just make sure you don't draw more current then the PSU rated for, as you will blow the outputs, as you will have no current protection till OP-AMP is replaced.

To set current limit set power supply voltage to around 5 volts DC and short the + and - together to set current limit, then turn current pot to when the display read 25ma, so you don't blow LED, or put to little current in.

I watch the video, one thing I would like you to try is to short the + and - set voltage to 5 volts and turn the current knob and then tell us if it has a affect on current display, if current goes up, or down when you turn the knob? Plus if you can turn current knob down all the way on both coarse and fine and if CC light come on? One warning if current shoot up with no control pull cable, or turn off power supply. If OP-AMP is bad will cause lost of current control and issue with voltage dropping and output cutting off and on.
« Last Edit: September 19, 2015, 02:14:35 am by OldSchoolTechCorner »
 

Offline OldSchoolTechCorner

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #26 on: September 19, 2015, 02:20:22 am »
Watching your video do suspect current control OP-AMP is bad, or bad potentiometer, as notice CC light flashing when the relay clicks on, or off as it going in and out of constant current mode and did see you have current pot turn a little bit. Check the front panel potentiometer for current control and make sure they are not cracked, or broken, or dirty.
« Last Edit: September 19, 2015, 02:42:24 am by OldSchoolTechCorner »
 

Offline muhamadamru

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #27 on: September 19, 2015, 09:58:53 am »
Could be OP-AMP in current, or voltage control, or bad output transistor, which usually cause that issue. Without a schematic and measurements it a guess at best. Do you have a scope, or meter around to troubleshoot it? As you need to check taps off transformer, output transistors, relay section, big power resistors and OP-AMPS input and output. as for CV, you monitor the output voltage and feed this back into the inverting input of the op-amp driving the output, for CC, you monitor the voltage across a current sense resistor and feed this back into the same inverting input of the op-amp, is usually how it works as the circuit will automatically switch into CV or CV depending on which threshold is reached first. If current op-amp isn't working correctly, you won't have current control and voltage will drop at random, or no output at all.

First I would tap the board and make sure no cold solder joints and look for any obvious signs of damage components. Next step to take is to check transformer taps, relay section, big resistors, output transistors section and make sure no issues as shorted, or open transistors bad diodes, or resistors, ESR on capacitors and also getting correct voltages with the power section first and work your way back. Also check for excessive ripple. When you get to OP-AMP's assuming you check everything else. One trick is to remove the current OP-AMP and that will keep power supply from going into constant current mode and if you get stable voltage then current OP-AMP, or potentiometer is bad. Just make sure you don't draw more current then the PSU rated for, as you will blow the outputs, as you will have no current protection till OP-AMP is replaced.

To set current limit set power supply voltage to around 5 volts DC and short the + and - together to set current limit, then turn current pot to when the display read 25ma, so you don't blow LED, or put to little current in.

I watch the video, one thing I would like you to try is to short the + and - set voltage to 5 volts and turn the current knob and then tell us if it has a affect on current display, if current goes up, or down when you turn the knob? Plus if you can turn current knob down all the way on both coarse and fine and if CC light come on? One warning if current shoot up with no control pull cable, or turn off power supply. If OP-AMP is bad will cause lost of current control and issue with voltage dropping and output cutting off and on.
I post images for the PCB and the internal component at the first page for me i didn't see any obvious damaged component  and for Cold solder joints idk  i think all of them are ok but i will recheck them.

Next step to take is to check transformer taps "Where can i locate them  and the relay section and the big power resistor on the PCB ? "

Where i can find OP-AMPS input and output and how the readings must be ?

how to check the output transistor ?

For sure i have meter
When i turn   current knob down all the way on both coarse and fine and  i don't short + - the CC light is Off but but when i short  + - it will come on.

i thought i would be somewhat easier but i am very noob to know those techniques


 

Offline matseng

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #28 on: September 19, 2015, 10:14:37 am »

Since you have current limiting in the supply, you don't need the resistor to limit current for the LED (assuming you set the current limit properly.)

To test it, set the voltage to something higher than the LED voltage, doesn't matter much, but 5V should do. Then turn the current all the way down to zero and attach the led directly without a resistor.

The voltage will be low (perhaps even zero) because the current limiting has kicked in (which is what you want). slowly turn up the current dial, and you should see a glow around 3ma for a modern LED and the voltage will be roughly at the forward drop voltage of the LED. you can then ramp up the current slowly and see how the voltage affects it. You should be able to go up to 25milliamps with any modern LED without damaging it and it should be nice and bright, but if you are willing to sacrifice the led, feel free to turn it up slowly even more and watch the failure mode.

The current limiting on my cheap psu's kicks in so slowly that they will happily destroy both standard as well as 1 & 3 watt LEDs if I hook them up wile the power is on (having already set the current limit(s) to reasonable values previously). :-\
 

Online mikerj

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #29 on: September 19, 2015, 11:59:17 am »
The current limiting on my cheap psu's kicks in so slowly that they will happily destroy both standard as well as 1 & 3 watt LEDs if I hook them up wile the power is on (having already set the current limit(s) to reasonable values previously). :-\

Even on expensive PSUs it's unwise to connect such devices whilst the output is enabled,  I can clearly remember a technician at work blowing up a load of expensive 980nm pump lasers by doing this with an HP 6630 series.  The cheap PSUs are worse though since they very often have a fair size electrolytic cap wires straight across the output terminals.
 

Offline OldSchoolTechCorner

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #30 on: September 19, 2015, 12:57:27 pm »
It not the OP-AMP circuit, as I stated it could be earlier, as current control seem to work and does go into CC and CV modes fine and you have current control, as you shown in third new video on the current meter when shorting the leads together and setting voltage to 5 volts. More likely issue seems to be in relay section, or output section.
 

Offline OldSchoolTechCorner

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #31 on: September 19, 2015, 01:02:48 pm »

Since you have current limiting in the supply, you don't need the resistor to limit current for the LED (assuming you set the current limit properly.)

To test it, set the voltage to something higher than the LED voltage, doesn't matter much, but 5V should do. Then turn the current all the way down to zero and attach the led directly without a resistor.

The voltage will be low (perhaps even zero) because the current limiting has kicked in (which is what you want). slowly turn up the current dial, and you should see a glow around 3ma for a modern LED and the voltage will be roughly at the forward drop voltage of the LED. you can then ramp up the current slowly and see how the voltage affects it. You should be able to go up to 25milliamps with any modern LED without damaging it and it should be nice and bright, but if you are willing to sacrifice the led, feel free to turn it up slowly even more and watch the failure mode.

The current limiting on my cheap psu's kicks in so slowly that they will happily destroy both standard as well as 1 & 3 watt LEDs if I hook them up wile the power is on (having already set the current limit(s) to reasonable values previously). :-\

Because you get a spike as it falls in and out of regulation, when you first turn on, or off the power supply and charge store in capacitors. Reason why quality power supplies you have button to enable output manually, once power supply is turned on. Use power supply in CC mode and set current limit, as it removes the possibility of transient inrush current protecting the LED, or any other sensitive component from damage. The inrush current will make for a bad day and destroy some sensitive components with no issue.

« Last Edit: September 19, 2015, 01:39:07 pm by OldSchoolTechCorner »
 

Offline RJFreeman

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #32 on: September 20, 2015, 04:31:53 am »
Hang on, sequence is:
start at 19V
voltage drops off (capacitor discharge)
Voltage reaches around 9.8V
"Click" relay (tap changer?) kicks in
Voltage goes back to 19V
sequence repeats

now here's the thing, I would expect to hear relay kick in and kick out again (two clicks)
So the selector monitors the output voltage (and is usually independent of the regulator) so as teh voltage drops, I would expect it to drop out the higher voltage tap at probably 9.8Volts, which would mean the voltage wouldn't rise again until it went above maybe 10-11 Volts (which would presume the tap to be at 12V or so)

It is almost as though the tap selector is wired arse about, and so as the voltage drops, instead of dropping out the higher voltage tap, it kicks in the higher voltage tap, Then the output voltage has increased so the tap selector changes it mind and drops out immediately

So it all merges into the one click? (not sure, but think I can hear a slight echo?)

either way you need too open it up, check the taps from the transformer (measure the output voltage) and see what the relay is doing (is it kicking and dropping out immediately?)  then you might need to identify the drive circuitry for the relay (it will most likely have a transistor and channel on an op-amp comparing the output voltage with a reference voltage).
Unfortunately no one here can do this for you, and without a schematic we can't even suggest what to look at.

And to have any chance of finding a schematic, we need any detail (model numbers, brand names etc) of the power supply, there may be some  info on circuit boards inside....
« Last Edit: September 20, 2015, 04:34:16 am by RJFreeman »
 


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