Power supply noisy inductor

[sivolc73]

Hi, I'm working on a power supply from a cheap TV I want to repair.

The power supply work fine but most of the time, after few minutes, a ferrite core radial inductor start to make a loud and highly pitched noise. I already replaced the main capacitor on this power supply, the TV wasn't working at all when I got it but now with the new capacitor it work fine except for the noise. I tried to crazy-glue the coil to reduce the sound but it didn't worked. The inductor is filtering the 24V power line that supply the backlight and the motherboard.

My questions are:

- Is it possible that the problem come from capacitors? Or I have to try to identify and replace the inductor?
- Is it possible that any other faulty part cause the coil to resonate like this?


I also noticed that the two capacitors close to the inductor (capacitors connected between the 24V line and the ground) became hot. I have a multimeter if I need to test any voltages.


I hope this is clear enough, English is not my first language.
Thank you!

 

[cjdelphi]

While working on a circuit with an inductor the other day i noticed a inductor begin to whine when it received a lower voltage/current maybe the voltage has dipped?

 

[Harald Kapp]

Coil noise or inductor hum comes from the forces of the magnetic field induced by the current in the wire. It can be fixed by using a low noise inductor. You can also try to stabilize the coil by covering it with some elastic glue (e.g. hot glue) - without any guaranteee that this works in your case.

The capacitors becoming hot may have several causes:

1. The capacitors are close to the heatsink, so any heat radiated by the heatsink will warm up the components around it.
2. The capacitors may see excessive ripple current (which could be a design failure or a failure of another component in the circuit.
3. The capacitors may exhibit excessive ESR. This may be due to aging or bad component quality.

In cases 2 and 3 you can replace the capcitors with special low-ESR types of the same rating (capacitance, voltage) but less ESR.

 

[sivolc73]

Thank you for these fast answers

While working on a circuit with an inductor the other day i noticed a inductor begin to whine when it received a lower voltage/current maybe the voltage has dipped?

I measured the output voltage before and after it start to make this annoying noise but it didn't change, 24.3V steady. I removed the load by unplugging the inverter and the sound became a bit less highly pitched but still present. Even when I start the PSU alone by jumping the 5Vsb and PSON I can ear the sound. If I'm right this kind on PSU change the frequency of the HV transformer to regulate the output voltage, maybe the voltage is a bit too low even before the filtering circuit? so the IC boost the frequency to keep a correct output voltage.

Coil noise or inductor hum comes from the forces of the magnetic field induced by the current in the wire. It can be fixed by using a low noise inductor. You can also try to stabilize the coil by covering it with some elastic glue (e.g. hot glue) - without any guaranteee that this works in your case.

The capacitors becoming hot may have several causes:

1. The capacitors are close to the heatsink, so any heat radiated by the heatsink will warm up the components around it.
2. The capacitors may see excessive ripple current (which could be a design failure or a failure of another component in the circuit.
3. The capacitors may exhibit excessive ESR. This may be due to aging or bad component quality.

In cases 2 and 3 you can replace the capcitors with special low-ESR types of the same rating (capacitance, voltage) but less ESR.

If excessive ripple cause capacitor to heat can the inductor be a little bit too weak to regulate the current? I don't even know if an inductor can "broke"

During my tests I noticed that the capacitors became hot even before the heatsink heat-up. I will try to replace the two capacitor by low ESR according to your advice. I don't know if a capacitor can alone cause an inductor to whine but I linked the schematic I draw from what I see on the board (only the 24V filtering circuit, I removed the control IC and all this stuff)

 

[davenn]

I don't know if a capacitor can alone cause an inductor to whine but I linked the schematic I draw from what I see on the board (only the 24V filtering circuit,

I assume for EC 13 and 14 you really mean 1000 uF as is what is written on them ???


Dave

 

[sivolc73]

I assume for EC 13 and 14 you really mean 1000 uF as is what is written on them ???


Dave

Yes it's 1000μF capacitors, but Fritzing have an anoying bug that doesn't let me write a value with more than 3 digit, I should upgrade it to the last version


I ordered some low ESR capacitors I will be back as soon as I received it!

 

[sivolc73]

Okay, I received and installed my brand new Low ESR caps and... The noise start immediately instead of after a moment when I powered up the board, with or without any load... I removed the coil, inspected it, unwind it, cleaned it, re-winded and super-glued it back on his ferrite core and added a huge layer of hot glue on it.

Then I had a shitty idea (I know, that's obviously stupid when you think about if more than 3 seconds) I think "hey, if the coil is not soldered on the board I could power it up and listen if any sound came from anywhere else on the board" Genius! I blew-up my new capacitors (obviously, the two caps received highly unregulated current), that was wonderful with explosion, a gigantic "pop" sound and electrolyte everywhere on the sofa XD At least I learn with this board Whatever, The fuse protecting this part of the power board blew too, I replaced the two caps with new one, solder back the coil, unsolder the fuse and now I will find new one.

I am wondering if I should just test the power board with a wire instead of the fuse just in case I just destroyed the 24V circuit before buy new fuse...

And what are you thinking about the noise even after caps replacement? Anywhere else to look on the circuit? The Schottky diode? The coil itself? before the 24V stage maybe?

Thank you for your help!

 

[cjdelphi]

With those kinds of voltages and current, it could have damaged more than the caps..

 

[elebish]

Check C28