transformer and resistance

[svtiss]

Hi all
I try to fix a power supply and don’t have any documentation for it.
It looks like switching power supply , I checked everything ,what I could and all look good , but I am not sure about transformer , the output coil has 0.00 resistance, Is that normal ?
Thanks
P.S. the power supply not working

 

[dknguyen]

Transformers are coils of wire and wire has very low resistance - your ohmeter isn't sensitive enough.

You are going to have to give more information...the FETs could have burned out or something...but honestly I have no idea.

 

[svtiss]

well , I have a good metter , I even checked empedance of the coil , shows 0 at
1000 hz

 

[dknguyen]

It's probably not the inductor since it would probably be up in smoke if anything happened. I doubt the insulation on the transformer broke down since you said everything looks good.

What size is the power supply? Maybe it's just a fuse you don't know about?

 

[Hero999]

How did you check the impedance?

What sort of power supply is it?

 

[Andy1845c]

How did you check the impedance?

I was wondering that too. It isn't somthing you can check with a meter is it?

 

[dknguyen]

I assumed his meter can somehow drive a 1000khz through a transformer...but now that I give it a few seconds though, it doesn't make much sense.

 

[svtiss]

Well , I checked the fuses ...
And the meter is good too , I used 3 feet of 22 awg wire and the meter showed somethig like 0.03 ohms , and for impedance , I used a special meter , when power supply was off ... I giving up on it , still think the transformer is no good ..., thanks anyway.

 

[dknguyen]

I'm just aiming in the dark here, but perhaps a voltage spike (or static electricity) blew the transistors. Why don't you try testing across the source, drain, and gate of the switching transistors and see if they read as a short?

 

[svtiss]

SURE , i did check everything ,what i could, seem fine.
it is big power supply from RV , no info , only what i can see is
Todd engeeniring inc, thats all.

 

[ericgibbs]

hi,
Re checking the impedance of a transformer [inductor].
Some 'smart' meters output a 1kHz sine wave of a known amplitude, say 1volt rms,

The meter measures the rms values of the applied ac voltage and the ac current
flowing in the inductor.

So using the equation

Zl= 2 * pi * 1kHz * L, Zl= 6280 * L

Knowing the Vac and Iac gives the impedance Zl

Vac/Iac = 6280 * L

So, L= Vac / (Iac * 6280) mHenry ; [ correction for units required]

The impedance values when you measure a transformer will be depend upon
on how the windings on the rest the transformer are connected.
ie; open, shorted or connected to a matching load.


If its a high output current transformer the secondary winding resistance
will be very low.

A couple of questions.
Did you do the test meter measurements with the transformer still wired up ?.

Is it easy for you to disconnect the input/output cables from the transformer ?

Let me know, I may be able to assist.


Regards

EricG

 

[Nigel Goodwin]

If its a high output current transformer the secondary winding resistance
will be very low.

He specified it's a switchmode supply, so the transformer windings will be VERY, VERY low impedance - particularly if you only measure them at 1KHz.

 

[svtiss]

Nothing connecting to the power supply , its just a module ,
And I dont know what current and voltage , maybe 24v , and heatsink 6" by 8"
, so lots of power , as i said early its from RV.
I understand that the resistanse is low and very low , but if i tried 2 different meters and both showed 0, is that transformer problem ?
it has feedback and auto shut down , that why no burns on the transformer and on the PCB... anyways , i gave up on it , no chance to find new parts , google doesnt help . thanks everybody

 

[Nigel Goodwin]

but if i tried 2 different meters and both showed 0, is that transformer problem ?

I wouldn't have thought so, if it's a switchmode transformer I wouldn't expect it to read any different. If the transformer was shorted turns (which would be EXTREMELY rare) you would find lots of components blown up!.