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I am giving power supply to a 24 V DC solenoid coil plug with an LED; positive from a PNP transistor board and negative directly from the SMPS.

When the PNP board energises the output terminal, the coil plug gets short-circuited and also it makes the transistor fail.

I think it happens when I am giving reverse polarity, but it does not happen to all the coil plugs. Some other coil plugs with LEDs are also reverse-polarised but are working well.

Can someone help me understand what makes the LED fail?

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  • \$\begingroup\$ avoid exceeding LED Vr=-5V to avoid failures with protection diode rated for same current. \$\endgroup\$ Commented Dec 8, 2016 at 19:41

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Usually there is surge protection on solenoid plugs with LED, but that protection could be a zener diode or a MOV.

If it is a MOV the solenoid will work in either direction but the LED will be reverse biased and will not work in the reverse direction. The LED will probably not be destroyed by this but it will typically exceed the reverse voltage rating.

If it is a Zener then the diode will be forward biased in the reverse direction and the driver will see a short circuit. This can destroy the Zener and/or the driver.

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