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Need assistance identifying this component

Joetsue

Oct 24, 2017
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Hello all,
I have an older switching power supply that I use on my bench for +- 12, 5, 24 volts dc. I recently had the main fuse blow and replaced it and it blows again when powering up. I found this component which is inline between the main fuse and what I think is an isolator before connecting to a four diode bridge rectifier. I have two questions, What is this device and could it cause the fuse to blow. Checking it with a meter reads a short circuit

component.jpg
 
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kellys_eye

Jun 25, 2010
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Looks like a simple insulated connecting post to me. It's unlikely to be the cause of fuse failure. Check the main bridge rectifier, the smoothing capacitor(s) and the main switching transistor (< the usual suspect).

Can you show a picture of where it came from? It just appears to have broken off the casing somewhere/somehow.
 

Joetsue

Oct 24, 2017
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Looks like a simple insulated connecting post to me. It's unlikely to be the cause of fuse failure. Check the main bridge rectifier, the smoothing capacitor(s) and the main switching transistor (< the usual suspect).

Can you show a picture of where it came from? It just appears to have broken off the casing somewhere/somehow.
power supply.jpg
 

Joetsue

Oct 24, 2017
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Thank you for your response. Here is a picture of the power supply. In the bottom right of the pictures is where the blown fuse was, to the left are the two yellow wires that connect to the device, labeled RT. The original location of the device is in the upper right corner where it is loosely screwed in place. The diodes in the rectifier check fine. I will check the switching transistors later today.
 

kellys_eye

Jun 25, 2010
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Potentially a thermistor (PTC input surge protector) but I have to admit I haven't seen the likes before.

Something of that age would need a full capacitor check - aged capacitors are a common problem in older equipment - but the fact that it's 'popped' (by the look of it) means there is certainly a short in the system.

Along the bottom, right-to-left, you have your input fuse, tap off to the PTC, common mode choke (filter), bridge rectifier (D1 to D4) and main smoothing capacitors.

Test for shorted diodes (they look ok) and the main smoothing capacitors for leaks - gunge on the bottom, possible visible coming from under them? The check across the capcitors for resistance - if you measure a short circuit then the main switching transistor would be the likely culprit.

What make/model is the PSU?
 

Joetsue

Oct 24, 2017
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Potentially a thermistor (PTC input surge protector) but I have to admit I haven't seen the likes before.

Something of that age would need a full capacitor check - aged capacitors are a common problem in older equipment - but the fact that it's 'popped' (by the look of it) means there is certainly a short in the system.

Along the bottom, right-to-left, you have your input fuse, tap off to the PTC, common mode choke (filter), bridge rectifier (D1 to D4) and main smoothing capacitors.

Test for shorted diodes (they look ok) and the main smoothing capacitors for leaks - gunge on the bottom, possible visible coming from under them? The check across the capcitors for resistance - if you measure a short circuit then the main switching transistor would be the likely culprit.

What make/model is the PSU?

Thanks for your quick responses and assistance. Still have not figured out what the device is but a quick check of the main switching transistor as you suggested it was found to be defective. Dead short Base to Collector, Base to Emitter and Collector to Emitter. Have to find a replacement, install new fuse and test. Discussions with some older guys at the electronics and surplus stores suggests it might be some sort of industrial thermistor. Their guess is as good as any. Thanks again.
 

Bluejets

Oct 5, 2014
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Looks like a section of an old selenium rectifier........
Kellys-eye, could it be part of a section such as below?
 

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kellys_eye

Jun 25, 2010
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Kellys-eye, could it be part of a section such as below?
Not by my reckoning. The rectification is clearly visible (D1 to D4).

Some kind of VDR is still my first guess - most such supplies have them to prevent switch-on surges but the modern devices are usually pcb-mounted and a lot smaller (shiny blue or mustardy-coloured).

Happy to see the OP has identified the cause of the problem. Fixing the dud switching transistor should get it going again and, probably, firing up regardless of the 'un-identified' part being shorted although I'd salvage a modern equivalent from a PC's SMPS and 'slap it in' as finding the right replacement will be nigh-on impossible!
 
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