Re: Understanding a split-mode power supply.

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Article #1613

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#1613

From: Sylvia Else
Date: Mon, 05 Jan 2009 11:37

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Ok, as I mentioned elsewhere in this thread, I managed to get the A/C 
working again by replacing Q1 and the 8.2 ohm resistor.

Actually, I'd misread it, and when I looked more carefully, it realised 
it was 6.2 ohm, as is the other one on the board with the same markings 
(i.e., a not blown one). Nice standard value that.

So now I have to replace the resistor with a fusible, but sourcing a 6.2 
ohm fusible is problematic.

The situation is not made any easier because if the switching transistor 
fails by shorting out, then after it blows the fusible will have 340V 
across it.

 From the suppliers I now know about (thanks, Trevor), I can get a 0.5 
watt 340V 4.7 ohm fusible. I propose to put it in series with a 1.5 ohm 
1 watt non-fusible. My reasoning is that the 1.5 ohm resistor will have 
only 1/3 the power dissipation, and will handle twice as much. Therefore 
the 4.7 ohm will go open circuit before the 1.5 ohm could get hot enough 
to be a problem.

Does this stand up?

Sylvia.

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Date: Mon, 05 Jan 2009 22:37:30 +1100
From: Sylvia Else <sylvia@not.at.this.address>
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