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Power Amplifier safety modifications.

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diydidi

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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.
 

Hi,

Toroidal transformers may have very high inrush current caused by it's extreme saturation bahaviour.
I just tested a 250VA 230V rated toroidal teansformer...
Worst case I saw more than 75A peak over 1ms...(while no load current was less tan 0.04A)

Klaus
 

Hello diydidi,
According to Altronics here in Australia, their 500VA Toroidals require a 7.5Amp Slow Blow (Time Delay) fuse for the active primary windiing.
I hope the above helps.
Regards,
Relayer
 

Hello,
you can calculate the current by ohm' law.
For the primary fuse use the mainvoltage in worst case (-5%).
According to the soft start circuit, the 500 VA transformer needs I = P / U = 500 VA / 215 V = 2,32 A. Use 2,5 A very slow blow up or 3,15 A slow blow up Type.

The 60 VA transformer needs I = P/ U = 60 VA / 215 V = 0,28 A. Use herefor 0,315 A slow blow upType.

To be save in all events of failure you can additional use fuse on the secundary side of the transformers. These fuses must be normal blow up types.

I hope it will help

regards

Rainer
 

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