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Relay in series with SSR as high current switch for longetivity ?
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BravoV:
Please set aside on cost and complexity matter in this discussion, as this is probably just for one off personal project.

With topic Relay vs SSR, each as it's own pro & con, and I was thinking why not combine both.

Circuit concept attached below.

The objectives are :

1. Significantly increase the relay's contacts life, as the most tear & wear was happened during serving high current when the contacts close, and also when opened as it was arc-ing, reference -> Contact protection

2. No leakage current during off state exactly like relay does, as even the best SSR still have it.


The idea is, when turning on, energize the relay 1st, delay abit, and then turn on/close the mosfets switch, and also when turning off, open the Mosfets 1st, add delay and then power off the the relay to release the contacts.

Questions :

- Will this work ?

- If it works as I'm expecting, will it work at both either in AC or DC ?

- Appreciate any other insight/critics/after thought and etc.

TIA

Edit :
Typo in the illustration, Phovoltaic .. should be PhotoVoltaic.
hagster:
Looks like it should work, but you do loose the benefit of very low on state resistance that the relay provides.

The other trick I have seen done to extend the life of relays is to use a MOSFET SSR in parallel with the relay. This can be turned on briefly during switching to prevent arcing. As you are not reliant on this SSR for much current handling for very long you can pick one with high off resistance.
floobydust:
I'm used to them being in parallel - a TRIAC/SSR and mechanical relay. This is for extra long life, very high number of cycles for industrial heaters over 100,000.

To turn on the load, first activate the SSR, then the relay to short the SSR and eliminate that heat dissipation. In reality you active both at the same time and the relay pulls in around 20msec later, covering even a possible SSR zero-cross 1/2 cycle delay.
To turn off the load, turn on the SSR then open the relay contacts then turn off the SSR.
This approach also uses next to nothing for SSR heatsinking because it's never seeing current for more than a few cycles. For small size and low cost, getting rid of the heatsink is best.
The relay never switches (only holds) current so very long contact life.
But there is leakage current present.
jbb:
Well, if you really really need to, you could conceivably do a three switch solution...

Parallel a SSR and a mechanical relay together for low arcing and low conduction loss. Add a second mechanical relay in series with the both of them to eliminate leakage current.  Seems very complicated...
NiHaoMike:

--- Quote from: jbb on February 16, 2020, 09:42:19 am ---Well, if you really really need to, you could conceivably do a three switch solution...

Parallel a SSR and a mechanical relay together for low arcing and low conduction loss. Add a second mechanical relay in series with the both of them to eliminate leakage current.  Seems very complicated...

--- End quote ---
A simpler solution is to replace the SSR with a bridge rectifier, capacitor, and resistor. The idea being that the combo acts like a short circuit for a very brief moment. It might also be possible to use a PTC element of the kind used in motor starters.
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