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Old 1st June 2005, 06:14 PM   #1
StalfoS is offline StalfoS  Canada
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Default Blown fuse!!!

So, I finally constructed my brian gt lm3875 chip amp. I made sure that my power supply was working properly first. Sure enough, +30VDC on the +'ve rail, -30VDC on the -'ve rail. I wired up the amp boards, and finally plugged the thing in. The fuse blew in less than 5 seconds.

I double checked my wiring and every thing seemed to be in order. There were no obvious shorts, and I poked around with my meter on circuit test mode to see if i could find anything unusual. I couldn't.

A few thoughts:
-i have a 24V (5A) lamp connected in parallel with my 22V-0V-22V toroid's secondaries (beween 0 and one 22 wire). Could this have pushed the current limits of my 2A slow blow fuse?
-is a 2A fuse appropriate from my setup?
-my grounding arangement might be a bit wonky.. I dont see how this would cause a problem (other than maybe hum), but my inputs are grounded through the chasis, due to the way they are attached to the case.
-My heatsink, as well as the screw that holds the chip on to it, are grounded. I have the insullated vesion of the chip, so i dont think that should be a problem. Maybe the threads of the screw cut through the insulation?

Can you guys suggest anything that I should check for, before I pop another fuse in there and try again? Thanks for your help.
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Old 1st June 2005, 06:35 PM   #2
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Default Re: Blown fuse!!!

Quote:
Originally posted by StalfoS

-i have a 24V (5A) lamp connected in parallel with my 22V-0V-22V toroid's secondaries...
Why? That's over 100W!
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Old 1st June 2005, 07:02 PM   #3
StalfoS is offline StalfoS  Canada
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It's a little red panel mount status lamp. It cant possibly be 100W...
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Old 1st June 2005, 07:40 PM   #4
Netlist is offline Netlist  Belgium
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On the IC board, the V+ and PG+ connections are rather close to each other, just as the V- and PG- .
Perhaps the wires are touching each other there.
Perhaps one or more rectifier failed. Are they mounted correctly?

/Hugo
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Old 1st June 2005, 07:45 PM   #5
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How big is your toroid?

If it's abotu 300VA or more, you should be using at least 4A fuse, as toroid's inrush current is (probably) responsible for blown fuse.

Presently, I'm not going for less than 5A fuses.
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Old 1st June 2005, 07:57 PM   #6
Netlist is offline Netlist  Belgium
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Peter, you are hopefully right but in such a case I'd also recommend to use a variac or if StalfoS has variable power supplies, he could slowly increase voltage and watch the current.
No inrush currents that way and if the fuse still blows…

/Hugo
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Old 1st June 2005, 08:46 PM   #7
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When using variac, it's worth mentioning that DC offset goes initially pretty high (to about 2V or so) and then its value settles down to almost 0V when supply voltage increases.

Sure, variac is the best in such cases, but don't get discouraged when it's not available

And don't forget to wear glasses, when running new circuits for the first time
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Old 1st June 2005, 09:20 PM   #8
StalfoS is offline StalfoS  Canada
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Thanks fro the responses guys...

Im pretty sure that PG+ an V+ are not shorted, nor PG- and V- for that matter.

I do not have acces to much in the way of test equipment, so use of any sort of variable power supply is out the window..

I picked up my toroid a while ago, so i can't imediately remeber how big it is. It is probably beween 200-300 VA, definately no more than 300. I will try to find out.

Perhaps I will try using a 4A fuse and see if I have any more luck.
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Old 3rd June 2005, 05:52 AM   #9
StalfoS is offline StalfoS  Canada
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well... i tried a 5A fuse, and it blew just as quickly...

any other ideas?
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Old 3rd June 2005, 05:55 AM   #10
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check your rectifier board, and everything after that
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