LM3886 amp started blowing fuses. Where do I start? - diyAudio
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Old 2nd February 2009, 02:40 PM   #1
bluegti is offline bluegti  United States
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Default LM3886 amp started blowing fuses. Where do I start?

I finished my dual-mono LM3886 amp based upon the boards from chipamp.com. Everything was working and sounding great for the last 2 weeks.

Last night I went to turn it on and no sound. I noticed the LED's were not on so I checked the fuse and it was blown. So I disconnected everything, replaced the fuse and everything came on.

So, I reconnected the pre-amp only and the LED's lit up when I turned it on. I turned it off, connected the speakers, turned it on and the fuse blew.

So, I disconnected the speakers, replaced the fuse and it blew the fuse again.

Then I disconnected everything, replaced the fuse and it blew again.

How do I start troubleshooting this?
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Old 2nd February 2009, 04:02 PM   #2
Nuuk is offline Nuuk  United Kingdom
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Start by telling us what value fuse you are using - and the type!
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Old 2nd February 2009, 07:01 PM   #3
bluegti is offline bluegti  United States
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3 amp fast blow
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Old 2nd February 2009, 07:09 PM   #4
Nuuk is offline Nuuk  United Kingdom
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On the primary side? If so change it to slow blow.
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Old 3rd February 2009, 05:16 PM   #5
AndrewT is offline AndrewT  Scotland
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a T3A fuse on 110/120Vac will start up a 100VA transformer.
What size is your transformer?
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Old 3rd February 2009, 08:18 PM   #6
bluegti is offline bluegti  United States
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I'm using Avel Y236503 160VA 22V+22V Toroidal Transformer
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Old 4th February 2009, 08:22 AM   #7
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Fuse blow even if secondary winding connect to amp?
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Old 4th February 2009, 10:22 AM   #8
AndrewT is offline AndrewT  Scotland
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110Vac 160VA will require a T4A or T5A to start without a soft start circuit.
With a soft start circuit it will run on a T1A or T1.6A fuse
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Old 5th February 2009, 01:57 AM   #9
stpeter is offline stpeter  United States
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Default Sometimes its better to be slow....

I agree with Nuuk.... If the fuse is on the primary side, it has to be slow blow

"Some notes on fuses: (a) It is best to use a "slow-blow" type in the power-line circuit, because there is invariably a large current transient at turn-on (caused mostly by rapid charging of the power-supply filter capacitors)." - The Art of Electronics ; Horowitz & Hill
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Old 5th February 2009, 08:35 AM   #10
AndrewT is offline AndrewT  Scotland
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Default Re: Sometimes its better to be slow....

Quote:
Originally posted by stpeter
because there is invariably a large current transient at turn-on (caused mostly by rapid charging of the power-supply filter capacitors)." - The Art of Electronics ; Horowitz & Hill
I think Horowitz and Hill are wrong here.
The purpose of the soft start resistor and/or thermister in the primary circuit is to reduce the start up current that establishes the flux in the transformer. Prior to this flux being established the transformer might try to operate as an air cored transformer and that produces very little if any output at the secondary

Horowitz and Hill are referring to the charging current and that can only flow after that start up flux has been established.
The best place for slowing down the smoothing capacitor charging is in the secondary circuit and requires a much longer time delay than the soft start.

Try an experiment:
use a >300VA transformer and find the largest fuse that blows when direct on line starting is used.
Now fit a timed delayed bypass across a soft start resistor. Set the relay time delay to 100ms. Does the charging of the downstream smoothing caps blow the soft started fuse? Can the fuse be made even lower and yet still not blow on start up? Remember to discharge the smoothing caps before doing these before and after experiments.
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