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I am looking at an on-line schematic and struggling to understand the function of the fuse. The schematic is from https://turneraudio.com.au/leakampmods_files/schem-leaktl12-protect-2005.gif

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The single-ended power supply uses a centre-tapped transformer feeding a full-wave bridge rectifier, with two smoothing caps in series across the output. There is a fuse between the centre-tap and the junction of the two caps.

If the caps are perfectly matched, I can't see how there would be any current through the fuse. It seems to me that (i) there could be a fault in the main circuit causing an overload of the the transformer without blowing the fuse and (ii) if the fuse did blow, the main circuit would carry on with full power regardless.

The only way I can see for the fuse to blow would be if one (and only one) of the smoothing caps failed closed-circuit, in which case it would protect the transformer. However, moving the fuse into either of the other transformer secondary connections would protect against that just as well, while also offering protection against many possible failures in the rest of the main circuit.

Am I missing something or is this design flawed?

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2 Answers 2

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Looks like the designer decided to economize by not having two fuses, and ended up doing almost nothing to protect anything.

Electrolytic capacitors don't tend to go short (they increase ESR as they dry out), but diodes sure do, and a failure of any of the four diodes or a short across the power supply rails will kill the transformer.

I think it would be better to put the fuse as you suggest, and perhaps replace the present fuse with a resistor. It doesn't really have much purpose other than to balance the capacitor voltages.

Or, better, ignore the centre-tap entirely and use a single 470uF/50V capacitor.

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  • \$\begingroup\$ Thanks - that was what I was thinking. The situation is a little complicated by the possibility of high voltages appearing on the two external cathode connections but I still don't see the fuse helping in any way. I'd vote you up but I don't have enough points. Enjoy your day :-) \$\endgroup\$ Commented Feb 18, 2024 at 14:50
  • \$\begingroup\$ It's almost as if the supply design started out symmetrical, but evolved into a single-ended supply, and the caps and fuse never got updated. \$\endgroup\$ Commented Feb 18, 2024 at 15:19
  • \$\begingroup\$ @SimonFitch Yes, looks like that, but even with a symmetrical supply I don't think the CT fuse idea is very useful or good unless there's some particular situation that requires that (maybe shorts are possible from +V to common or -V to common but never from +V to -V). \$\endgroup\$ Commented Feb 18, 2024 at 15:32
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The whole idea of this seems somewhat flawed, the circuit is meant to sense the voltage from another supply and cut it off if it goes too high by activating a NC relay through an SCR. To do that it requires this protection circuit to have power. If power is lost here, the relay will remain closed and the protection is lost. It would have been better to have a NO relay that would only be closed if the protection circuit was operating properly and fail open if power to this circuit were lost.

As for the fuse, it doesn’t seem to have much use. The only way it will blow is if one of the filter caps shorts or a rectifier shorts putting AC on one of the caps. And then it would only blow if the transformer could supply at least the 2 A the fuse is rated for. The transformer is marked 7 VA so at 24 Vrms that’s 0.3 A, it might be able to blow a 2 A fuse under a short condition, maybe not.

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