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Instant fuse blow on lm3886
Instant fuse blow on lm3886
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Old 29th May 2015, 04:29 PM   #1
LandIsland is offline LandIsland  Norway
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Default Instant fuse blow on lm3886

Hello! Have had a problem with my lm3886 chip amp since I built it. The problem is just that the fuse (2A) blows instantly when turning on the amp. But this just happens like every 15 times I use it. Thats why it is so weird. And the fuse for the amplifier is not the only one that blows. My house's fuse box also blows for the floor where the amp is placed. Adding a drawing of the the circuit before the transformer, and a picture of the amplifier for you to see.
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Old 29th May 2015, 04:49 PM   #2
playOvation is offline playOvation  Germany
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Hi!

Sometimes the inrush current of big transformers (especially toroidal) can cause the fuse to blow.
I cannot estimate it by picture, how is the wattage of your transformer?

Best regards
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Old 29th May 2015, 04:50 PM   #3
djoffe is offline djoffe  United States
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Perhaps there are two different problems going on...

With a 2 Amp fuse, you shouldn't blow the house fuse, unless the circuit is so loaded to near the limit that the starting surge for the amp puts the house fuse over the limit. Of course, there might be something even worse going on, which is a little scary, so please be careful.

As to the 2 Amp-fuse itself blowing every 15 times...What timing is the fuse?
A fast blow fuse will typically have the problem you cite...the issue is charging the capacitors, and the phase of the AC line when you turn on the power.

Medium or Slow blow fuses are probably more appropriate for this fuse position.

Here's a place where I learned the hard way about this:

GT-101 Troubleshooting

Search the page for blown fuses 1...
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Old 29th May 2015, 04:52 PM   #4
LandIsland is offline LandIsland  Norway
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This is where I bought it, a datasheet is provided on the right side of the page, but I couldn't see anything about the wattage..

88016 - Toroidal power transformer 2 x 25V 10A 500VA - Europe Audio

Edit: The fuse I bought was supposed to be slow blow, but when I now look at the package it says: "type: FST" don't now if this means fast blow?

Last edited by LandIsland; 29th May 2015 at 04:56 PM.
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Old 29th May 2015, 04:55 PM   #5
AndrewT is offline AndrewT  Scotland
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Using a Mains Bulb Tester prevents the mains fuse blowing even when you have completely messed up the mains wiring.

A 500VA transformer fed from 220Vac needs a T6A fuse if you start it directly on line.
If you use a soft start you can use a T2A fuse. The soft start will need a resistance of aprox 50r and that will need to be bypassed after about 100ms to 300ms
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Last edited by AndrewT; 29th May 2015 at 04:59 PM.
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Old 29th May 2015, 06:02 PM   #6
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by AndrewT View Post
Using a Mains Bulb Tester prevents the mains fuse blowing even when you have completely messed up the mains wiring.

A 500VA transformer fed from 220Vac needs a T6A fuse if you start it directly on line.
If you use a soft start you can use a T2A fuse. The soft start will need a resistance of aprox 50r and that will need to be bypassed after about 100ms to 300ms
A 5-6 A slow-blow fuse may solve the problem with the fuse within the amp blowing, but it will do nothing to address the house fuse blowing.

It's rather surprising that the house fuse blows, actually. Usually they can withstand 100 % overload for over an hour. If the circuit is already heavily loaded, the inrush of the transformer may be enough to push it over the edge.

I suggest measuring the amount of current drawn by the transformer when the amp is running. When the amp is idle, there should hardly be any current draw. A few mA plus the magnetizing current of the transformer. If you see more than that, there's something wrong.

If nothing is wrong with the transformer or the rest of the circuit, a soft-start may be the way to go. Those are not trivial to design. The components need to be able to survive start-up, the soft-start needs to get out of the way once it's done its job, and it needs to re-engage if the power glitches.
I'd be surprised if you need a soft start circuit, though. I used to have two 500 VA transformers turn on in my power amp without a soft start. Even on 230 V, I never popped a fuse.

Tom
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Old 29th May 2015, 06:09 PM   #7
sreten is offline sreten  United Kingdom
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Hi,

A 500VA transformer ? How many lm3886's are you running off it ?

rgds, sreten.
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Old 29th May 2015, 06:17 PM   #8
LandIsland is offline LandIsland  Norway
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Thank you for many good suggestions, I will look into it and report back when/if I find the problem
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Old 9th June 2015, 11:43 AM   #9
LandIsland is offline LandIsland  Norway
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Hi again, i luckily figured out the problem. After a simple check the amp fuse actually doesn't blow, I only thought it did. But the house fuse blows, and this is because it is only rated 10A. With some computers and other stuff connected it's not that strange it blows some times
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Old 9th June 2015, 06:51 PM   #10
tomchr is offline tomchr  Canada
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Yeah, but it's a 10 A, extremely slow blow fuse. You're either running right at 10 A on that circuit and the amp happens to pop the fuse or something is wrong in the amp. Many European countries now use 16 A fuses. You may want to talk to an electrician to see if you can upgrade the circuit to 16 A.

Do you have the option for testing the amp on a dedicated circuit? Maybe unplug your fridge for a few minutes so you can use that circuit...

Another option would be to build a Light Bulb Tester. The light bulb will effectively work as an inrush current limiter. It should blip bright when you turn on the amp and extinguish after maybe a second. If it remains lit, you have a problem in the amp.

Another question for you: What's the capacitance of your supply caps?

Tom
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