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Solid State Relay experts - why am I cooking my SSRs??

crossy

crossy

Solar Addict
Joined
Apr 27, 2021
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758
Location
Thailand, just north of Bangkok.
I have an array of 3S2P x 380W panels, Voc 48.71V Isc 10.05A

So worst case should be Voc 146.13V and 20.1A.

This array is being switched by an SSR - DD 220VDC x 100A this "should" have no issues with the energy involved. SSR is mounted on the recommended "100A" heatsink using heat-transfer gloop and securely bolted down.

BUT

I've popped two SSRs, both have failed short-circuit.

The other side is feeding the input to a PowMr 60A MPPT which tends to over-charge the 48V pack hence the SSR to kill the panels.

1713953314563.png1713953339362.png

Any bright ideas? What am I doing to kill these SSRs and what can I do to prevent it?
 
Last edited:
It's driving the input of an MPPT so it could possibly be a kick since these things have inductors (ok not very big ones), maybe an inverse diode would help.

I have suitable diodes in stock so I could bung one across and see if it still dies.
 
A friend had trouble with big HVAC system he had to keep replacing contractors on. He rigged several solid state (AC) relays in parallel to get the current carrying capability but kept the contractor also in parallel. Since the solid state switched faster than the mechanical device they took the initial inrush. While running the contactor carried the brunt of the load because of the voltage loss of the solid state device hence the devices didn’t need much heat sink. This also made it more efficient. He used a two second delay off on the solid state relays so they did most of the work on shut off. To my knowledge I think he hasn’t been back to work on it since.
 
Send me an old one to take apart. I've taken AC models apart and it is shameful what they use inside. One 40A unit used the same triac that is in my kitchen stand mixer, and that was one of the better ones. Really, do you think those terminals can take 100A. I use a simple relay to open the input of my charge controller. The input caps of the controller should keep the voltage differential low enough to prevent arcing.
 
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