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Old 27th June 2004, 04:46 PM   #1
imix500 is offline imix500  United States
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Default Is it a Diode?

Hey guys, I'm trying to repair a hefty 12v 30A switcher. There is a visually damaged part (it's split in two) that lies after ac input voltage filtering and protection and right before the full wave bridge. It is between the neutral side and the negative rails of the main filter caps. It's called "CR26" on the pcb so I'm thinking it's a diode but I'm not sure. The markings on the package are:
G1 then Backwards "R" and "U" symbol (it's familiar, the manufacturer?)
9432
1 (dot) 5KE
220 (0?) A Second zero is damaged, cant be sure if it's a zero
Btw, it's a Todd SC12-30C supply
Thanks!
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Old 27th June 2004, 04:59 PM   #2
dhaen is offline dhaen  Europe
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The R L symbol is the recognised component marking for Underwriters Laboratories. It's a US safety certification system. How they came to use such a strange mark, I don't know, but it's certainly distinctive.

Not sure about your blown component.
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Old 27th June 2004, 05:03 PM   #3
imix500 is offline imix500  United States
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Correction- it looks like it lies between the hot side of the ac in and a center tap between the two main filter caps in series.
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Old 27th June 2004, 05:11 PM   #4
dhaen is offline dhaen  Europe
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In that case I think it is a thermistor or similar device to limit the inrush current. No doubt the other pole of the mains connects to the centre of 2 diodes, thus forming a voltage doubler.
They can fail of their own accord, but it's more likely that something is blown downstream.
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Old 27th June 2004, 05:23 PM   #5
imix500 is offline imix500  United States
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Thanks John, I think you're right about it being a thermistor or such. It had shorted and appears to be what was blowing the main fuse instantly. The only thing right after the divice is the bridge rect. (checks ok) that feeds either side of the 2 series caps (check ok), with the thermistor tied to the center of them. Looks like the caps feed right into the switching mosfets, so I'll start there. Thanks again.
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Old 27th June 2004, 05:31 PM   #6
johnnyx is offline johnnyx  United Kingdom
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1.5KE 220 is a transient suppression diodeLook Here

These are common on psu and inverter boards. They act like a zenner, clamping high voltage transients. Some are unidirectional like a diode, and some are bi - directional like back - to - back diodes. They are rated in Joules not Amps, so if the transient is long enough they blow. Similar things are Varistors, but Transorbs act faster.

Hope this helps.
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Old 27th June 2004, 05:39 PM   #7
dhaen is offline dhaen  Europe
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Quote:
Hope this helps
It does
So this device is in parallel, not series with the supply.
It may be that once removed, the PS works.... but only 'till the next spike comes along That is, if the TVS did it's job last time.
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Old 27th June 2004, 05:42 PM   #8
imix500 is offline imix500  United States
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Very cool, thanks johnnyx. I'll get me a few of those. What is the usual cause of failure for these? Voltage spikes? Incorrect input voltage? This supply is switchable for 115/230. Would the device clamp if 230v was applied while set to 115v? I bought this used so I'm trying to figure out it's history.
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Old 27th June 2004, 05:45 PM   #9
imix500 is offline imix500  United States
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dhaen, it is indeed in parralel- eventually, after all the filtering and protection so it was hard to tell. So the supply should work without it? Is it just a bad idea to do so? Thanks.
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Old 27th June 2004, 06:11 PM   #10
johnnyx is offline johnnyx  United Kingdom
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They could be in parallel with each cap if there's two of them, or in parallel with the bus volts if there's one. It could be they blew if the voltage setting was wrong. Often for 110v, the caps act as a voltage doubler, so they save an explosion
It would probably work without, but without the protection it offered. Check that nothing else has suffered first though.
Connecting the wrong voltage happens all the time; done it myself.
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