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replacement transformer issue for game console

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eobard

Give me a break Senior
Joined
Jul 12, 2001
So I bought a transformer to replace a dead one in an old game console and I've some issues. I wasn't able to get an exact replacement but found one only about 10% out of spec. What I was replacing was a transformer that put out 18v and a separate 18.5v with a center tap (2*9.25v) on the secondary side. What I was able to order from this one place was a transformer with a 20v feed and a center tap (2*10v). When I received it I found it was 20% beyond what it was rated for (12v on the 10v secondary, 24v on the 20v secondary). I got the company to send me a replacement and it too was labeled as a 20v/10v but outputs 24v/12v instead. But I wired it up instead to see if it was close enough. I wired the transformer to the power circuit board and separated the power circuit board from the rest of the system. That way if something blows it only takes out the power reg circuitry and I've already got a contingency if that happens. I then tested the outgoing DC voltages from the circuit board and the were within spec so I suspect the 24v-18v difference is not too much to cause problems nor is the 12v-9.25v difference. It seems like there isn't a problem but the one thing that concerns me is that the transformer get hot. VERY HOT! It's only been powered up for a few minutes at a time and it's making a bit of noise as the insulating tape wrapped around it heats up and melts to it. Also the transformer vibrates when it's plugged into the circuit board and the transformer is plugged into the wall, regardless of whether or not the system is powered on! So not withstanding the voltage differences I think I have one other concern but I'm not if it's the reason for the heat. The original transformer had one secondary line at 18v and then a second, separate secondary line at 18.5v with a center tap. The replacement has two secondary lines that can be bridged to make a single with double voltage, in other words basically a center tap. But I'm running the lines expecting the separate 18v and 18.5v(ct) off of only one secondary winding set. If my descriptions aren't clear enough, I've attached a paint image to make things worse. So tell me what you all think, is it the difference in layout of secondary windings generating the heat? Differing voltages? Bad transformer?
 

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  • transform.bmp
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Quite possibly the wrong inductance for the switching frequency. If the inductance is too low for a given frequency (or if the frequency is to low for a given inductance) then for part of each switching cycle the transformer primarily coil acts as a dead short and gets very very hot.

As to the voltages, a non related (or barely related) linear psu (not so much SMPS, though they do it too) with no load or too little load will put out a higher voltage than it is rated at.
If you have a 1a 20v wall wart, that means that worth a 1a load it will be 20v, at a lower load the voltage will be higher.

This is a typed out on my phone, please excuse any DYAC issues I missed.
 
^^^^^^^^^^^^^^^

I understand the above well enough to know I would have never typed any statement nearly as correct involving this subject matter.

Necron out.
 
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I've found that transformers can vary a huge amount in output voltages, between no-load and full load, even 20%, especially small ones, and the ratings for surplus transformers may be done by the seller, without following any industry standard.
 
If you were checking the transformer voltages with no load, they'll be higher than spec. They should go down a bit once you've connected a load.

As for the heat, that doesn't sound so good. Not sure about that.
 
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