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Power Amplifier safety modifications
Power Amplifier safety modifications
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Old 11th March 2017, 06:22 PM   #1
diy didi is offline diy didi  South Africa
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Default Power Amplifier safety modifications

I have a Large power amplifier that needs repairing for a client of mine.
It's a no-name brand stereo power amplifier and uses TO-3 lateral mosfets.
It has a large 500VA toroidal power transformer as well as a smaller 60VA toroidal transformer for the relay speaker protect circuits.
The builder didn't add any fusing except for a large 20A trip-switch wich doubles as the power-on switch too. I'm afraid the switch is too big in my opinion and will not trip during a fault situation. Removing it is not an option. Too much metal work.
Amp at least has a soft start circuit built in for the large transformer.
I would like to make it safer by adding a fuse on the mains side where the power enters.
My question is, do each of the transformers need its own primary fuse, or will one suffice.
Also how does one go about sizing the fuse/s? I assume they will all be slow blow.
Mains where I am is 230V.
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Old 12th March 2017, 04:12 AM   #2
MAAC0 is offline MAAC0  Portugal
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Didi

What are You trying to protect ? Mosfets ?

A fuse is too slow for that, at least you could add some auxiliary fuse protection to the power rails. The input fuse protects when there is a catastrophic failure to prevent catching fire for example when the transformer or mosfets go short.

Given the 500VA rating you could try a 2 or 2.5 A fuse on the 230V input.

Since you have two transformers you must make sure when using (I recommend say a 200 mA) a slower rating fuse for this auxiliary transformer / board it disconnects the speakers when the fuse blows or the protection isn't going to do its job if the board isn't powered.
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Old 12th March 2017, 05:58 AM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by diy didi View Post
do each of the transformers need its own primary fuse, or will one suffice.
Safety requires a separate, proper size fuse for EACH transformer primary.
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Old 12th March 2017, 11:21 AM   #4
diy didi is offline diy didi  South Africa
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Quote:
Originally Posted by rayma View Post
Safety requires a separate, proper size fuse for EACH transformer primary.
I thought as much. Thank you for confirming this.
How do I scale the fuse size on the primary. Any papers online for this?
Maybe there is an empirical way of doing this?
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Old 12th March 2017, 11:53 AM   #5
scottjoplin is offline scottjoplin  Wales
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Quote:
how does one go about sizing the fuse/s? I assume they will all be slow blow.
Mains where I am is 230V.
Since the amp has soft start you may not need slow blow fuse. A=VA/V
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Old 12th March 2017, 12:46 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by rayma View Post
Safety requires a separate, proper size fuse for EACH transformer primary.
I agree.
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Old 12th March 2017, 12:50 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by scottjoplin View Post
Since the amp has soft start you may not need slow blow fuse. A=VA/V
Yes.
It is simply the transformer VA divided by the supply voltage.

500VA/ 230Vac = 2.2Aac
Use either T2A, it might blow on start up from cold, or T2.5A
The added resistance in series with the primary will need to be ~230/{2*2A} = 52r
You may find that 40r is enough to prevent the T2A fuse from blowing at start up.
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