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!
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!
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.
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.
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.
That is, if the TVS did it's job last time.
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.
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.
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.
Hi,
Those transient suppressor diodes should not be used directly across the mains. They can stand a lot of current and are rated in watts, not in joules. They can actually absorb not many joules, so they can take the transient current only for a very short time. These diodes can only be used AFTER a proper mains-filter that has also a proper transient protection varistor that takes the main bulk of the transient energy.
Probably there was originally a mains input filter in the apparatus your PSU came from.
Cheers
Those transient suppressor diodes should not be used directly across the mains. They can stand a lot of current and are rated in watts, not in joules. They can actually absorb not many joules, so they can take the transient current only for a very short time. These diodes can only be used AFTER a proper mains-filter that has also a proper transient protection varistor that takes the main bulk of the transient energy.
Probably there was originally a mains input filter in the apparatus your PSU came from.
Cheers
Pjotr, there is a pretty extensive filter section before the device in question and the main rectifier bridge. 2 large inductors with dual windings that the ac passes through, a large ac cap, and 3 devices that are either MOV's or caramic disk caps or a combination of the two. I think it might be PF corrected, but I'm not sure. This supply is from 1996 and I don't know if that was the norn then as it is today. Thanks!
the component in question is most likely a bidirectional transient suppressor diode. These are fast-acting, but their energy ratings are pretty poor. They tend to fail short, and this would account for blowing the fuse. You could substitue an appropriately rated ZnO varistor for the supressor diode for improved ruggedness. this approach is pretty much industry standard these days. If there is a substantial amount of filtering ahead of the suppressor element, you don't need to worry so much about having a fast-acting suppressor, as the filter will blunt the rising edge of the transient. A 14mm varistor should do the trick, and it will have a much better energy rating. It will eventually fail short, too, but they are easy to find and replace.
Thanks for all your help guys. I was gonna chuck this thing but I figured I'd throw what I knew up here and see what came back. This forum has saved me from spending considerable $- again. Wrenchone, is ZnO a certain type of MOV? I've been looking through data sheets but I don't see the juction material (I'm assuming thats what the metal oxide is) on them. I'm looking at littelfuses' UltraMOV line. Is this the type of MOV you were talking about? Thanks.
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