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I'm wanting to switch my hot water cylinder element based on the amount of solar i'm generating. The element is 3kw @ 230v = 13A. I need to switch the element ON for a short time every 15 minutes. This is equal to 4 times per hour. Solar would generate abort 10 hours per day at a guess. So the relay needs to switch 14,600 times per year. It my understanding relays are more likely to wear out and fail if switched at peak load. I'm wanting to make the system reliable and also reduce heat dissipation. So i'm thinking I switch a SSR first then a few ms later switch the mechanical relay. The mechanical relay will then take the load so the SSR effectively wont need to dissipate any power. I will then turn off the mechanical relay before turning off the SSR. I'm hoping that when the mechanical relay switches ON/OFF with this setup the load across it is effectively nothing so it will be able to handle ideally over 100,000 cycles before failure.

I could try to detect the zero cross and switch the relay. But this seems more simple.

Does this sound like a solution that will work?

TLDR; Where I live the export price for solar is very low. So you want to use it rather than export if possible. The power company calculates import/export as a net amount over a 15 minute interval. The plan is to export the power at the start of the 15 minute billing cycle. Tally up how much is exported and as the billing period draws to an end import the exported amount back to the hot water cylinder.

I know its common for this system to work off effectively a dimmer on the element and a PID. Although the dimmer gets really hot and I was thinking the on/off might be a better approach.

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  • \$\begingroup\$ Yes, I think you will find material on hybrid relay systems that work similarly. I'm doing it myself. However I use pairs of SCRs, rather than TRIACs/SSRs. \$\endgroup\$ Mar 12 at 8:18
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    \$\begingroup\$ I didn't research it, but personally I would try to find a SSR that was well rated for the load and just go with that. Less to go wrong. A quick search found plenty of them. They are slightly pricey but not that bad, and save you also a lot of time. \$\endgroup\$
    – danmcb
    Mar 12 at 8:36

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Hybrid relays are a very old concept. The relay only has to switch a volt or two so contact wear is minimized. There was a Motorola App note where they even suggested using a "well made" 5A relay at 50A (some relays were rather overdesigned in those days- most of the ones today are a bit iffy at full rated current).

The SSR will tend to fail 'on' so if that is an issue consider it. If the relay fails to close, the SSR soon will fail too (probably fail to open) unless it is adequately rated and has an appropriate heatsink.

15,000 operations per year is not a whole lot. I would also consider a beefy mechanical relay/contactor. It should not be too hard to get a predictable 10 years life by derating a bit. The hybrid might last 50 years or it might fail tomorrow in a thunderstorm.

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