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I'm having a little trouble with a PNP based blown fuse circuit. I'm using a similar circuit in other boards for blown fuse indications for 12VDC and 24VDC supplies without issue. I'm trying to repeat the same for a 300VDC system. Naturally I have had to up the specifications of the diode and PNP for the higher voltage, but I've clearly chosen something wrong, but can't for the life of me work out what.

When the fuse blows, the red LED lights up fine, but when the fuse is intact, it's still lit. I know that the PNP should be off if the base and emitter are the same potential, so I thought that maybe the voltage drop over the S1M was too low and changed it for a green LED with Vf of 3.2V. But the results was I then had two led's lit! I've got my creepage and clearances all sorted, it's just the LED that's the issue. Anyone got any pointers? enter image description here

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  • \$\begingroup\$ Put the transistor into a tester, see if it is still working \$\endgroup\$
    – Ferrybig
    Commented Jun 22, 2022 at 6:56
  • \$\begingroup\$ Have you measured (carefully!) the voltage drop between emitter and base? With the fuse intact, this circuit is very sensitive to the difference in voltage drops across D and DS. As the Vbe approaches the knee it can conduct enough to turn the LED on. \$\endgroup\$
    – Buck8pe
    Commented Jun 22, 2022 at 7:31
  • \$\begingroup\$ Consider using a higher voltage led (eg 20V), or even an NE2 so that you can use lower currents. \$\endgroup\$
    – Jasen Слава Україні
    Commented Jun 22, 2022 at 11:14
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    \$\begingroup\$ I think this will work without the transistor. For a test just connect base and emitter to disable it. \$\endgroup\$
    – Jens
    Commented Jun 22, 2022 at 13:59
  • \$\begingroup\$ Thanks all, I haven't put the transistor in a tester, nor done any direct measurements as we (the company) are quite conservative on the safety side. Not a bad thing in general, but it does make just getting this thing run up on a bench quite a mission! I'll swap the transistor out for another and make sure it's not that (and then can test the first on the bench). Thanks @Jens for the suggestion, I'll see if I can try that. \$\endgroup\$
    – Sylvester
    Commented Jun 22, 2022 at 22:02

2 Answers 2

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The function of the transistor in such a circuit is to reduce the current consumtion while the fuse is intact. A small current flows all the time and only with a blown fuse the LED current is drawn.

This feature is not used in the original circuit, so the transistor is obsolete.

schematic

simulate this circuit – Schematic created using CircuitLab

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From my point of view, I would change a little the "circuit".

Something as this.
If diode current too "high", you can change 27k -> 47k ...

enter image description here

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  • \$\begingroup\$ Thanks for that! Due to time contraints, what I might do it pad up the next revision of the board to be able to encompass both your and @Jen's suggestions. Then once they are in, I can try them out to see configuration works best. I'll report back when I get the testing done, thanks! \$\endgroup\$
    – Sylvester
    Commented Jun 22, 2022 at 22:04

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