Author Topic: Repair Generic Power Supply that can't go beyond 10V  (Read 7517 times)

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Offline muhamadamru

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Repair Generic Power Supply that can't go beyond 10V
« on: September 17, 2015, 10:09:17 pm »
Hello
First i want to state that I am noob at troubleshooting and i got a generic power supply from a friend cause  i want to start studying practically "cause i am 3rd year student at electronics :-DD " electronic circuits and start doing alone some amplifiers, oscillators, voltage regulators, and so on.   
I made a video that explain what is happening  to the unit and i will try to write a description as much as  i can explain:

The power has fixed output which is 5V/2A and another variable output 0-30V 0-5A now :

1- the output of 5V is good and it maintain itself as 5V but the current using multi-meter Uni-t U33TA It shows me that it start at 3 Amps and then Drops to 1amp ( i don't know here may the multi-meter is not that good and doesn't has the load that need 2 amps idk "

2- on the variable output even if i don't connect any load   when i try to go over 10 it goes down and i hear voices like tick tick every time it drops :
https://www.youtube.com/watch?v=bvmcmP4toBg&feature=youtu.be

I made also some photos from inside so you can see maybe you could advice me :


Thank you in advance
« Last Edit: September 18, 2015, 09:36:44 am by muhamadamru »
 

Offline BAM5

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #1 on: September 17, 2015, 10:53:50 pm »
Hey Muhamadamru,

1- Sounds like when you wanted to check the amperage you shorted the supply with your DMM. This isn't how amperage works. If you want to be sure the amperage is in spec, you need to hook up a load to the power supply, and measure the amperage that goes through that. (In order to get a load to draw 2 amps from the supply you can use Ohm's law to figure out you need something with a resistance of 2.5 Ohms. But it will also need to handle 10 watts of power.)

2- Not sure what the clicking sounds are, they could be the relays on the first board J13 & J14. It's strange to me that the voltage is jumping around like that though. Actually, perhaps the circuitry is fine and the clicking you hear are relays switching between circuits for higher and lower voltages, and the cause of the voltage jumping around is a dirty pot?
« Last Edit: September 18, 2015, 01:01:03 am by BAM5 »
 

Offline muhamadamru

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #2 on: September 18, 2015, 01:03:19 am »
Hey Muhamadamru,

1- Sounds like when you wanted to check the amperage you shorted the supply with your DMM. This isn't how amperage works. If you want to be sure the amperage is in spec, you need to hook up a load to the power supply, and measure the amperage that goes through that. (In order to get a load to draw 2 amps from the supply you can use Ohm's law to figure out you need something with a resistance of 2.5 Ohms. But it will also need to handle 10 watts of power.)

2- Not sure what the clicking sounds are, they could be the relays on the first board J13 & J14. It's strange to me that the voltage is jumping around like that though.
yes i think i shorted it "i am stupid" i smelled something but i think that the power has protection in it because when i connect the it goes down the value on the screen of the volt to 0

10 watt  Resistor i think  this will cost me a  fortune 

now if i shorted it what i have to do ? replace the fuse ?

But i have to mention that the problem with the voltage drop and can't go over 10 and bounce before i do any ampere test

Also i have to mention that now when i read the voltage with volt-meter it reads exactly as it's on the screen +-1~2%
« Last Edit: September 18, 2015, 01:06:21 am by muhamadamru »
 

Offline BAM5

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #3 on: September 18, 2015, 01:15:52 am »
I added a bit to my first reply.

Alright as I stated before, you can't do an amperage test without a resistive load. What you're doing when you put the meter on the contacts in amp measuring mode is shorting them out which is not what you should be doing. I recommend you go to a site like khan academy and look up a highschool level physics class that explains the properties and laws electricity.
 

Offline RJFreeman

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #4 on: September 18, 2015, 01:48:38 am »
Quote
yes i think i shorted it "i am stupid" i smelled something but i think that the power has protection in it because when i connect the it goes down the value on the screen of the volt to 0

I would expect a bench supply like that to have current limiting built in, and you usually set the current limit by shorting out the supply and adjust the current control until you get the current you want.

the clicking sounds like 'tap switching relays' - these select different taps on the power transformer, based on the output voltage of the supply, this reduces the power dissipation in the regulator, so for example if the output voltage is, say 5 Volts (either because this is the voltage you have selected, or because the Constant current/current limit mode has kicked in) then rather than powering the regulator off the 30 Volt tap, the tap selector would switch down to a 12 Volt or maybe a 6Volt tap (depends on the supply).

so it might be that a higher voltage tap on the supply is open circuit, and so when the supply tries switching up it fails?

do you have any brand name or model number on teh supply?
 

Offline muhamadamru

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #5 on: September 18, 2015, 01:50:34 am »
I added a bit to my first reply.

Alright as I stated before, you can't do an amperage test without a resistive load. What you're doing when you put the meter on the contacts in amp measuring mode is shorting them out which is not what you should be doing. I recommend you go to a site like khan academy and look up a highschool level physics class that explains the properties and laws electricity.

Ofc i understood that this is not the the way that one could  measure the amp,  and we need a load  but i mean  mistake have been done and now if i have to fix it what i have to do ?  I mean that  i shorted the supply this means that i have to replace the fuse ?

Or if it now gives me correct reading for the volts it's ok and nothing happened to him "despite the old problem with flickering output "

I attached for the 5V output and for the variable output :

« Last Edit: September 18, 2015, 09:39:28 am by muhamadamru »
 

Offline RJFreeman

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #6 on: September 18, 2015, 01:55:53 am »
Viewing it again, the supply does get to 30 Volts, and then the tap switcher kicks in, or drops out which seems to be when the supply drops back to 10 volts.
This would suggest that the comparator for the tap switcher is playing up. Although it seems to be intermittent?

the tap switcher usually uses a comparator to compare the output voltage with an internal reference voltage and uses that to decide when to switch taps.

The Tap switcher will be independent of the voltage regulator as it needs to reduce the supply when the current limit kicks in.
 

Offline RJFreeman

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #7 on: September 18, 2015, 02:07:08 am »
In the first 13 seconds of the video, it seems like the voltage slowly drops (which would be the Filter caps discharging) then the click would be the tap selector kicking in? then the voltage jumps up, suggesting a higher tap was momentarily selected and the capacitors charged up?

the voltage drop at 1:09 is due to the current limit kicking in (supplies might do this if you select the minimum current)  - this is indicated by the CV or Constant Voltage LED going out and the CC or Constant Current LED lighting up.

So yes this supply is deigned to be shorted out (at least in theory) and as I said before that is how you set the current limit.
 

Offline muhamadamru

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #8 on: September 18, 2015, 02:08:41 am »
Viewing it again, the supply does get to 30 Volts, and then the tap switcher kicks in, or drops out which seems to be when the supply drops back to 10 volts.
This would suggest that the comparator for the tap switcher is playing up. Although it seems to be intermittent?

the tap switcher usually uses a comparator to compare the output voltage with an internal reference voltage and uses that to decide when to switch taps.

The Tap switcher will be independent of the voltage regulator as it needs to reduce the supply when the current limit kicks in.

the Tap switcher is fed interdependently  from the regulator
Where can i find the  tap switcher in the PCB and do i have to replace it
Really this is the first time i hear about such new stuff  |O   
 

Offline muhamadamru

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #9 on: September 18, 2015, 02:09:31 am »
In the first 13 seconds of the video, it seems like the voltage slowly drops (which would be the Filter caps discharging) then the click would be the tap selector kicking in? then the voltage jumps up, suggesting a higher tap was momentarily selected and the capacitors charged up?

the voltage drop at 1:09 is due to the current limit kicking in (supplies might do this if you select the minimum current)  - this is indicated by the CV or Constant Voltage LED going out and the CC or Constant Current LED lighting up.

So yes this supply is deigned to be shorted out (at least in theory) and as I said before that is how you set the current limit.
i will make longer video for better analyzing
 

Offline muhamadamru

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #10 on: September 18, 2015, 02:12:17 am »
In the first 13 seconds of the video, it seems like the voltage slowly drops (which would be the Filter caps discharging) then the click would be the tap selector kicking in? then the voltage jumps up, suggesting a higher tap was momentarily selected and the capacitors charged up?

the voltage drop at 1:09 is due to the current limit kicking in (supplies might do this if you select the minimum current)  - this is indicated by the CV or Constant Voltage LED going out and the CC or Constant Current LED lighting up.

So yes this supply is deigned to be shorted out (at least in theory) and as I said before that is how you set the current limit.
if i want to set the current limit i have to shot the -  + outputs and change the current knob up to a chosen value ?
 

Offline RJFreeman

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #11 on: September 18, 2015, 02:19:56 am »
Quote
he Tap switcher is fed interdependently  from the regulator
Where can i find the  tap switcher in the PCB and do i have to replace it

The tap switcher will be circuitry around a Relay on the PCB. without a schematic I could not comment further as it might be an spare op-amp in a package with other parts of the circuit or could be a standalone comparator.
This is where any details on the supply might be useful,

Quote
Really this is the first time i hear about such new stuff

they are common in variable Linear power supplies
 

Offline RJFreeman

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #12 on: September 18, 2015, 02:21:32 am »
Quote
if i want to set the current limit i have to shot the -  + outputs and change the current knob up to a chosen value ?

Correct, that's usually how you set current limit on these types of supplies.
 

Offline ez24

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #13 on: September 18, 2015, 02:58:17 am »
this is what I see when I look at your pictures - never seen this happen before on this forum.  So my guess you are doing something "funny"

YouTube and Website Electronic Resources ------>  https://www.eevblog.com/forum/other-blog-specific/a/msg1341166/#msg1341166
 

Offline RJFreeman

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #14 on: September 18, 2015, 03:12:16 am »
Quote
i will make longer video for better analyzing

probably not much point, the thing now would be to find a schematic and start taking measurements.
 

Offline crispy_tofu

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #15 on: September 18, 2015, 05:46:04 am »
this is what I see when I look at your pictures - never seen this happen before on this forum.  So my guess you are doing something "funny"

Yeah, the pictures are acting up here as well. Can you please update them muhamadamru?  :)
 

Offline muhamadamru

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #16 on: September 18, 2015, 09:23:31 am »
this is what I see when I look at your pictures - never seen this happen before on this forum.  So my guess you are doing something "funny"

Yeah, the pictures are acting up here as well. Can you please update them muhamadamru?  :)
Yes i will
 

Offline muhamadamru

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #17 on: September 18, 2015, 09:24:07 am »
this is what I see when I look at your pictures - never seen this happen before on this forum.  So my guess you are doing something "funny"
you can't see any of the pictures ?
 

Offline RJFreeman

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #18 on: September 18, 2015, 12:57:12 pm »
Quote
you can't see any of the pictures ?

I don't think any of us can
« Last Edit: September 18, 2015, 01:37:05 pm by RJFreeman »
 

Offline madires

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #19 on: September 18, 2015, 01:23:58 pm »
Your PSU looks like one of those Mastech clones. Please search for HY3003 or HY3005 schematics. The basic design is mostly the same, just minor changes for the different current ratings.
 

Offline muhamadamru

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #20 on: September 18, 2015, 02:53:25 pm »

I don't think any of us can

Now i uploaded them as attachment are they visible ?
 

Offline Len

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #21 on: September 18, 2015, 03:08:28 pm »
Now i uploaded them as attachment are they visible ?

Yes, thanks.
 

Online tooki

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #22 on: September 18, 2015, 03:17:27 pm »
Alright as I stated before, you can't do an amperage test without a resistive load. What you're doing when you put the meter on the contacts in amp measuring mode is shorting them out which is not what you should be doing.
How is it wrong? With that type of bench supply, you must short the output to set a current limit, as others have said.

Besides, how is the resistance of the test leads and the internal shunt resistor in the meter not a resistive load? (It's a low resistance to be sure, but it is one nonetheless.)
 

Offline muhamadamru

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #23 on: September 19, 2015, 12:24:39 am »
Today i wanted to do  a test to know the forward voltage for an LED though i connected a 1k resistor in series and  i also set the current limit to 0.8 A and try to test  :bullshit:
the power keeeeeps go down and up down and up and switch between taps
 

Offline jwm_

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Re: Repair Generic Power Supply that can't go beyond 10V
« Reply #24 on: September 19, 2015, 12:55:21 am »
Today i wanted to do  a test to know the forward voltage for an LED though i connected a 1k resistor in series and  i also set the current limit to 0.8 A and try to test  :bullshit:
the power keeeeeps go down and up down and up and switch between taps


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.


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