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Re: Repairing PS, can't find this part or information... Litton varistor?


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Varistor is used for protection. What this means is that your power supply  must work without a varistor if it hasn't another problems.


Very true, I already have the replacement power MOSFETs on order since one of the old ones was cooked causing a short and popping the SG57. But more digging around and it looks like it's actually a Thermistor not a Varistor, hard to determine what a device is when it's blown up and the company is no longer and the part number isn't much help! :D

But yeah I still need to find something for a replacement, the SG57 was actually in series with the AC line of the bridge rectifier. I'm asuming once I get the FET's all replace I can throw in a decent sized resistor instead of the Thermister and try powering it up, at least that way if there is more to the short then I originally thought it shouldn't cook anything, and if things look good it should power up without any load attached
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It's an NTC (negative temperature coefficient) resistor.

When it's cold it has a fairly high resistance, as it heats up its resistance drops.

It's used a an inrush current limiter. When the power is first applied it's cold and its high resistance limits the current, as it warms up its resistance drops, allowing the full current to flow.

It's needed because the filter capacitor across the rectifier on the mains inlet draws a huge current when the power is applied.

The chances are other components have failed, I recommend checking the rectifier and filter capacitor (both value and ESR), if you don't have an ESR meter (value alone isnt' good enough) then replace the capacitor as a matter of course.

Failure of the filter capacitor, rectifier or chopper MOSFET(s) could cause the NTC resistor to fail. Additionally failure of the filter capacitor can cause the rectifier and MOSFET(s) to fail and failure of the MOSFET(s) can cause the rectifier and associated components to fail. Because of this, it can be pretty hard to find out what went wrong first.

There's also a good chance many other components have failed rendering it beyond economical repair.

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Very good information, and spot on!


The old guy here at work went digging through some old text books and found a couple articles on them including the SG series devices like the blown one, Surge-Gaurd circuit protection devices, NTC Thermistors just like you said.


I already had checked the rectifier and it's a pretty beefy SOB and checked out fine, ie removed, meter tested, then hooked up elsewhere and load tested. I didn't check the filter caps though, hadn't thought much about them since again pretty beefy but for sure I'll look into those since it'll be a couple days for my FETs anyway.

I understand there is a chance this PS might be a complete writeoff, it's for work though and I work for a power company so money isn't a huge deal and they've already ordered a new replacement PS to the tune of over $1K I believe... so this is kindof a play thing for me, if I can get it fixed/tested/working we'll throw it into stores in case of an extreme emergency we can use it. We use these power supplies for our data acquisition system at one of the plants, so really they would much rather replace with new then a reconditioned old unit... but again extreme cases have happened where a working rebuilt might be needed so it's a good thing to try and at least get it repaired

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Take a closer look at the filter capacitors:

Are any of them swollen?

Is there any sign of fluid leaking out of them?

Are the terminals corroded?

If the answer to all of the above is no, they might all right, although filter capacitors do go bad without showing any physical sings of doing so.

The risk yo take is a bad filter capacitor taking out the MOSFETs.

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