Can't find the resistor value.

Thread Starter

rahul411

Joined Feb 19, 2018
209
I'm attempting to repair a 12V 10A switching power supply. Although I have bought a new replacement but i think it's good to have such beefy supply handy.

During the initial stage of repair the board was having fuse and switching mosfet blown out, have replaced that.
But when connected to mains there's clicking (ticking) noise coming from the transformer. Then i googled it and found similar problems people having. According to those references i replaced all electrolytic capacitors and problem still persist.

I read on another forum that, this is because the board is getting its over current triggered and that is causing ticking. There it was advised to check for short on load side and then check shorted components on board. I found a resistor thats shorted and unfortunately the color code of that resistor is rubbed off and it's 8th of a watt resistor.

So how to find the unknown value of a resistor. Only black band is visible. And its the second band.

Datasheet for the ic is attached and the resistor is connected to pin1 and it is in series with 10nf cap which goes to ground.
 

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Hymie

Joined Mar 30, 2018
702
I would recommend you measure the resistor value while in circuit (swapping the meter polarity to observe any difference), and then do the same with the meter in diode mode measuring the voltage drop. These 4 values should give a good idea of the resistor value.

Failing that, cut one end of the resistor out of circuit and measure the resistor.
 

bwilliams60

Joined Nov 18, 2012
1,156
What make and model of power supply? Can you find a schematic for it? Some manufacturers will give you the diagram if you ask for it.
If not can you measure the rsistor as a whole or measure the two halves of it if burnt through? May be able to get you close.
Any picture online showing the board?
Can you provide any pictures of the board and resistor?
 

MisterBill2

Joined Jan 23, 2018
3,718
Just like AH, I have never come across a resistor failing short circuited. There are small inductors, called chokes, that look a lot like resistors but just have a coil of copper wire inside, which measure close to a short on many meters. There are also ferrite bead chokes that just have a wire going through a ferrite bead. And there are zero-ohm things that merely serve as jumpers across a section of circuit board.
One caution about servicing electronic devices of any kind, which is that if some part has failed and burned up, most of the time that burn up is caused by another part that has no appearance of having failed. So just replacing the burned up part will usually not solve the problem. The one exception is those audio power amplifier modules, which sometimes just overheat and burn out. But even there, often there is a leaky capacitor helping it overheat. So there you have some more information. AND, can you give us the model number and manufacturer of the power supply? There are several brands that are simply not worth repairing, and other brands that are always worth fixing.
 

Thread Starter

rahul411

Joined Feb 19, 2018
209
I have never found a resistor that failed short circuit. I you sure it isn't just a very low value resistor, perhaps less than 0.5Ω, which are not uncommon in such supplies.
I can measure resistances as low as 0.1ohm with me multimeter. So if that's the case, multimeter would have shown 0.5ohm.
 

Thread Starter

rahul411

Joined Feb 19, 2018
209
What make and model of power supply? Can you find a schematic for it? Some manufacturers will give you the diagram if you ask for it.
If not can you measure the rsistor as a whole or measure the two halves of it if burnt through? May be able to get you close.
Any picture online showing the board?
Can you provide any pictures of the board and resistor?
Its a Chinese power supply.
YL338-03 is only thing written on board, but i cannot find any data related to it online
 

Thread Starter

rahul411

Joined Feb 19, 2018
209
Just like AH, I have never come across a resistor failing short circuited. There are small inductors, called chokes, that look a lot like resistors but just have a coil of copper wire inside, which measure close to a short on many meters. There are also ferrite bead chokes that just have a wire going through a ferrite bead. And there are zero-ohm things that merely serve as jumpers across a section of circuit board.
One caution about servicing electronic devices of any kind, which is that if some part has failed and burned up, most of the time that burn up is caused by another part that has no appearance of having failed. So just replacing the burned up part will usually not solve the problem. The one exception is those audio power amplifier modules, which sometimes just overheat and burn out. But even there, often there is a leaky capacitor helping it overheat. So there you have some more information. AND, can you give us the model number and manufacturer of the power supply? There are several brands that are simply not worth repairing, and other brands that are always worth fixing.
Yes i know about that inductor thing but here its resistor because on the silkscreen it's written R14 corresponding to the failed resistor.
 

Thread Starter

rahul411

Joined Feb 19, 2018
209
Check for shorted rectifiers as this often shows up as a short circuit on the output.
If that was the problem then i wouldn't get any output. And the fuse will blow Everytime i connect it to mains.
I'm getting like 7.23 to 7.01 volts.
 

Thread Starter

rahul411

Joined Feb 19, 2018
209
Check for shorted rectifiers as this often shows up as a short circuit on the output.
The clicking is probably an indication that the power supply is in current limit and it periodicallycoming out of current limit in case the short is removed.

If possible, disconnect the load from the power supply and check the load at the power supply connector to see whether it has a very low resistance of shot. If that looks ok, move on to checking the rectifiers on the power supply's secondary. If those are ok keep digging until you find a short.

Yes, it could be something other than a short, but that is the most likely cause.



These were your words from 2014, and I followed it and found a short resistor whose value is unknown and the bands are rubbed off.
 

Dodgydave

Joined Jun 22, 2012
8,288
If it's a UC3844, pin 2 is Voltage sense adjustable, pin 3 is Current sense, which is picked up from the mosfet Source resistor. If the output is shorted or loaded too much the mosfet takes too much current and the chip shuts down ( ticking).

Pin 1 is the Error amp, minimum resistor value is 10K ohms.

Disconnect the output diode or test it for shorts...
 

AlbertHall

Joined Jun 4, 2014
8,184
The clicking is probably an indication that the power supply is in current limit and it periodicallycoming out of current limit in case the short is removed.
It isn't only overcurrent which will cause that but also under or overvoltage output.
 

Thread Starter

rahul411

Joined Feb 19, 2018
209
If it's a UC3844, pin 2 is Voltage sense adjustable, pin 3 is Current sense, which is picked up from the mosfet Source resistor. If the output is shorted or loaded too much the mosfet takes too much current and the chip shuts down ( ticking).

Pin 1 is the Error amp, minimum resistor value is 10K ohms.

Disconnect the output diode or test it for shorts...
What does it mean, output shorted or loaded?
 

Dodgydave

Joined Jun 22, 2012
8,288
It can be the output diode is shorted or the output is loaded too much, so the transformer takes more current and the chip goes into shutdown. Check the output for shorts and check the rectifier diodes on the output.

Can you post pictures of your psu both sides.
 
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Thread Starter

rahul411

Joined Feb 19, 2018
209
It can be the diode is shorted or the output is loaded too much, so the transformer takes more current and the chip goes into shutdown. Check the output for shorts and check the rectifier diodes on the output.

Can you post pictures of your psu both sides.
Yes i checked continuity of everything on the board and found one resistor to be shorted.
At the output I'm getting 173.2 ohms between positive and negative terminals.
Picture of faulty board is posted above and the good working one is in it's enclosure and it's for CCTV cameras so i can't turn it off.
 
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