Torroid coil is humming.

[jwv3]

I have a coil that is humming in the SMPS of an automotive amplifier. This amp sustained some mechanical damage thanks to UPS, but I managed to fix everything (except this hum). I poured some epoxy over it, just enought to cover the top of the windings. That seemed to help for about a week or two. Now the hum is back on really cold mornings. Should I try some RTV between the coil and the PCB? More epoxy?

[jneutron]

Originally posted by jwv3
I have a coil that is humming in the SMPS of an automotive amplifier. This amp sustained some mechanical damage thanks to UPS, but I managed to fix everything (except this hum). I poured some epoxy over it, just enought to cover the top of the windings. That seemed to help for about a week or two. Now the hum is back on really cold mornings. Should I try some RTV between the coil and the PCB? More epoxy?

Epoxy has an expansion coeff of over 100 ppm/ degree C.

The mechanical stuff in your amp will run from 35 ppm (aluminum) to 12 (typical ferrites)

In english: the epoxy probably cracked due to the cold.

You need to fill it with something that will remain pliable cold.

John

[jwv3]

Thanks. I hadn't thought about thermal expansion. I'll try some silicon RTV perhaps.

[Eva]

The power supply of a car amplifier usually works at 30-60KHz so there should be no humming at all

LF noise from the transformer may be a sign of malfunction or unstability in the control circuit of the SMPS [This is the most usual case]

It may be also due to transformer saturation caused by a faulty regulator that allows the alternator to deliver 16V or more to the electrical system of the car [I've seen this problem but it's less usual]

You should check the voltage waveform that powers the amplifier and the switching waveforms on the transformer and switching MOSFET gates

Anyway, epoxy or hot glue are very useful to prevent transformer wires from scratching due to heavy car vibrations and shorting between them in the long term

I repair car amplifiers and sometimes I have to rewind toroids found to have intermitent shorts between turns or windings

[Symx]

[QUOTE] The power supply of a car amplifier usually works at 30-60KHz so there should be no humming at all.

This is true, do you mean it is resonating at perhaps a higher frequency than the usual 50 - 60 Hz that is implied ?

[jwv3]

I know the voltage is just a shade over 14 when the truck is running. I'll check out the switching supply. Thanks for the tip.

Originally posted by Eva
The power supply of a car amplifier usually works at 30-60KHz so there should be no humming at all

LF noise from the transformer may be a sign of malfunction or unstability in the control circuit of the SMPS [This is the most usual case]

It may be also due to transformer saturation caused by a faulty regulator that allows the alternator to deliver 16V or more to the electrical system of the car [I've seen this problem but it's less usual]

You should check the voltage waveform that powers the amplifier and the switching waveforms on the transformer and switching MOSFET gates

Anyway, epoxy or hot glue are very useful to prevent transformer wires from scratching due to heavy car vibrations and shorting between them in the long term

I repair car amplifiers and sometimes I have to rewind toroids found to have intermitent shorts between turns or windings