Why does my PSU KEEP blowing fuses?!

[Rarkov]

Thanks for all your help!

I think I may have sorted it though. It seems that not all ceramic fuses are slow blow. In fact these were positively quick blow!

I am buying some tonight. Along with some decent solder. I bought some the other day and I had about as much success melting it as trying to melt a spanner in your mouth!

I'll let you know...
Gaz

 

[Rarkov]

Hi,
Well I got the new fuses. I put them in with high hopes...

The fuse came out as though a bomb had been let off in it! There was molten balls of metal all over the glass. The glass was cracked and all charred! What the hell is wrong with this thing?!

I'm going to try changing the wiring...

Gaz

 

[jleaman]

take off every thing yuo have atached to the transformer that means cap resisters brge rectifiers. ect ect then test it. go to the manufactures website about the wireing.

Ceriouse what kinda to you have..?

 

[pinkmouse]

Sounds like the transfomer has internal shorts, and so is probably FUBAR!!

Check the resistance of the taps on the input side, (and output side) and see if there are any relative differences, if so, see the above!

 

[jleaman]

I was thinking the same thing if it so you should contact the manufacture an tell them you have a brand new ( if it is brandnew ) psu and it is shorted mabey buy shipping..

 

[Rarkov]

I've located the problem...We're all wrong!

It was the bridge rectifier. I got a continuity test on it. There was contact from + to AC on the knackered one and no contact between any pins on the good one. Seems that the diodes blew up...(reverse voltage?!)

So, why isn't my BR working?!


Edit:
I checked the output from the Xformer with the secondary taps disconnected and without the BR...44V RMS on each

Thanks,
Gaz

 

[phase_accurate]

Hi Rarkov

First of all disconnect everything on the secondary just to make sure that the problem isn't caused there - as Jason suggests - before making further tests.

Are you sure that you wired the primary windings with the right phase (as others mentioned above) ? Otherwise the transformer will indeed act like a short-circuit. Maybe you should try to connect one of the primaries the other way around. Before firing it up connect a light-bulb in series with the primary (40 to 60 Watts). If everything is O.K. you will measure almost the nominal output voltage (under idle condition !!) with the lightbulb glowing just a little. If the lightbulb is illuminated then there is still a short somewhere. Maybe you killed your transformer, but I don't think so since transformers belong to the sturdiest components and can bear some abuse.

Regards

Charles

 

[jleaman]

i new it was the bridge rectifier. it is like a dead short. i've been there done that many times.. like my dad alsways tells me start at the simplest point work your way up remove all the things and test one at a time. ex


tx > bridge > then caps > then wire >

 

[Rarkov]

Success!

I have found the problem. The diode between +ve and AC on the bridge rectifier was fried (but internal so was not noticed). The must have short circuited it an overloaded the fuse. Even after replacing the BR though (and wiring up the Caps) the fuse would still blow. I replaced the Slow Blow 2A fuse with a SW 5A fuse and all is well. I have a very stable +- 61.3V!!! Thanks so much!

You've also helped almost clear my fobia of PSUs...One thing though...One of the solder contacts came off the cap whilst it was charged...When it came into contact there was an almighty spark...I want to drain the charge when not in use. What resistors do I need?!

Thanks,
Gaz

 

[CryingDragon]

I'd say 10K 1 watt for bleeding.