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I have some outdoor lights controlled by two PIR sensors at the top and bottom of the driveway. They are 24 V lights and will pull up to 10 A in total.

I initially had some trouble with the PIR sensors failing and rectified this to some degree by adding in two solid state relays so that the PIR sensors trigger the relays to turn the lights on.

One of the SSRs keeps failing (no longer switching), usually after a couple of months of use. The DC to DC relays are rated up to 60 A and 60 V but I'm wondering if I'm getting some voltage spiking on switch-off which is overloading something. I've read a bit about flyback diodes, is this what I need?

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    \$\begingroup\$ Add datasheet links for the SSRs and LEDs. It is 24 V DC isn't it? \$\endgroup\$
    – Transistor
    Oct 24, 2021 at 21:48
  • \$\begingroup\$ What power supply are you using to drive the LEDs? \$\endgroup\$ Oct 24, 2021 at 23:28
  • \$\begingroup\$ @Bruce Abbott It is a generic switching power supply: S-500-24 500W DC 24V 20A output AC 220V input single group switching power supply. \$\endgroup\$
    – GWiz
    Oct 24, 2021 at 23:36
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    \$\begingroup\$ How hot do the SSRs get when running at 10A? They may require a heatsink. Could lightning be an issue? When to the SSRs die? When turned off or on? \$\endgroup\$
    – Kartman
    Oct 25, 2021 at 1:25
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    \$\begingroup\$ Since it's in outdoor use, does it have a long wire run? If there are large surges, such as lightning strikes (not necessarily direct, but even a few hundred meters away), there could be high voltages induced in the mains line. Do you have heavy-duty surge-protection devices, e.g. MOV + gas tube? See littelfuse.com/~/media/electronics_technical/application_notes/… \$\endgroup\$ Oct 25, 2021 at 1:46

1 Answer 1

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With dry contact relay switching, HV flyback voltages must be suppressed with some snubber or diode clamp from switch output to the opposite rail for turn off.

As dry contacts switch towards 0 time, V=LdI/dt is a very high voltage even for small L lines. The same is true to a lesser extend for fast SSR's are not as fast turning off.

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