power supply transformer noise

[thefamilyman]

Hi everyone

I've got a pit of a problem with a power supply i have (it's a clone of the HP E3632A)
the transformer is very loud under load and becomes quite hot.
I'm an electrical engineer and only a hobby electronics-ist, so your expert help could be grateful to me.

from what i understand the transformer should be matched for the loading, so i dont feel it's being overloaded.
the inductor before the rectifier has been omitted.

Images below are measurements taken from P9 and P8 (the main input from the transformer) and with a current transformer on P9.
Sorry that the scematics is all crazy, it's what the manufacturer gave me.

[Simon]

Is this a transformer supply or a switchmode. looks a bit complex for a normal transformer. Could be poor design switchmode.

[mij59]

The inductor L1 can't be omitted. Did the power supply break down, or did you get this way ?
I think the inductor L1 should be connected to P9.
What kind of current transformer is used, whats the scale in the second picture ?

The loud noise could be caused by asymmetric loading of the transformer.
 

[thefamilyman]

Thanks for the reply.

The inductor overheated at high loads (around 8-10A), so the fix from the factory was to omit it and bridge it with a thick copper wire.
honestly i dont know the ratio of the current transformer, it's one i had lying around.

[mij59]

The provided schematic makes no sense, i have attached the E3632 schematic its uses multiple transformer taps.
Pictures  of the power supply may be helpful.

[Dave]

It doesn't surprise me that the transformer started overheating after you replaced the inductor with a short. That inductor could significantly reduces the RMS current in the transformer coils, therefore reducing the heat dissipation of the transformer. Then again, if the transformer was properly rated for the job...

I'd suggest that you save yourself the trouble, ditch this turd copy and get a real E3632A. They are not that expensive if you buy them second hand.

Is this a transformer supply or a switchmode. looks a bit complex for a normal transformer. Could be poor design switchmode.

Please refrain from commenting if you don't know what you are looking at.

[thefamilyman]

Thanks Dave, I had this assumption too.

The inductor removal was done by the manufacturer after I complained to them that it was undersized and over heating.
So they sent me another unit that had been modified.
I've got two of these now but with two different MOSFETs installed.

I was looking at a secondhand HP unit but they are still quite pricey and especially with shipping on top. These Chinese ones only cost me US$380 with free shipping.

I might keep the inductor in there from the old unit and derate it and make up a new inductor for the second inductor-less unit.

Apart from these perhaps quite bad design issues its a very accurate and clean power supply with sense which is a big plus for me.

[thefamilyman]

The provided schematic makes no sense, i have attached the E3632 schematic its uses multiple transformer taps.
Pictures  of the power supply may be helpful.

do you have the full schematics that i may see?

[mij59]

The provided schematic makes no sense, i have attached the E3632 schematic its uses multiple transformer taps.
Pictures  of the power supply may be helpful.

do you have the full schematics that i may see?

Sorry do not have it, just google for E3632 service manual.

[thefamilyman]

I've replaced the inductor with something i had lying around; a 2uH rated at 18A



This is the one that was over heating anything over 6A running through it. i measured it at 0.55mH


after installing it i didn't really expect it to do much as it's a very low value but i thought it maybe better than nothing and it can handle the current too.


the results wasn't what i was expecting.
just from my very crude testing it seems that the smaller inductor performs worse than no inductor at all.
all at 12V@4A

No inductor - yellow is voltage, blue is current


new inductor


old inductor

[mij59]

The current waveform looks strange, is the current transformer terminated with a resistor ?
The old inductor may be driven in to saturation, probably need a much bigger core with thicker wire, what is  the color(s) on the core ?