Relay in series with SSR as high current switch for longetivity ? - Page 1
Forum section: electronics / projects

author: BravoV
author_rank: Super Contributor
timestamp: February 16, 2020, 06:57:59 am
content: 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 protection2. 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.TIAEdit :Typo in the illustration, Phovoltaic .. should be PhotoVoltaic.

author: hagster
author_rank: Frequent Contributor
timestamp: February 16, 2020, 07:47:52 am
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: February 16, 2020, 08:15:28 am
content: I'm used to them being inparallel- 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.

author: jbb
author_rank: Super Contributor
timestamp: February 16, 2020, 09:42:19 am
content: 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...

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 16, 2020, 01:46:03 pm
content: Quote from: jbb on February 16, 2020, 09:42:19 amA 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.

author: fcb
author_rank: Super Contributor
timestamp: February 16, 2020, 02:07:27 pm
content: What about a half-way house. Sample the mains and switch the mechanical relay at a known posistion on the phase.The variation relay switching times shouldn't vary by more than few milliseconds (e.g. between 17-20ms), it should be at least possible to ensure you avoid switching at the peak of a wave or at least (depending on load type) between two phase angles of your choosing. So cheap to do and likely free in many applications, will likely extend the life of a relay by some measurable amount.

author: BravoV
author_rank: Super Contributor
timestamp: February 16, 2020, 02:18:19 pm
content: For AC mains and specifically for heavy inductive load like big ass transformer, toroid or high current rotary power tools, I use this soft starter circuit.The relay is parallel with SCR, and will energize "near" at the AC at V-Peak to reduce signicantly the inductive load inrush current, as sometimes can be 20 to 100 times the rated current, and trip the AC circuit breaker which is annoying.Example for 220V AC mains only, for other different AC voltage, adjust accordingly all other components.

author: BravoV
author_rank: Super Contributor
timestamp: February 16, 2020, 04:53:17 pm
content: Quote from: hagster on February 16, 2020, 07:47:52 amYes, I'm fully aware of it, but with today's MOSFET N-type, which some has crazy low RDS-on, especially for relatively low voltage ones, I think I can live with that.Quote from: hagster on February 16, 2020, 07:47:52 amStill, there is a leakage current while off, which one of the primary advantage of mechanical relay over SSR.

author: BravoV
author_rank: Super Contributor
timestamp: February 16, 2020, 04:55:32 pm
content: Quote from: jbb on February 16, 2020, 09:42:19 amI thought about that too, yeah, way too complicated for my need, but I may build it should the FET's dissipation is becoming a problem.Like this below ?

author: 741
author_rank: Frequent Contributor
timestamp: February 16, 2020, 06:53:32 pm
content: Presumably in some applications it would be a safety requirement the relay contacts are not bypassed? However, contacts can weld shut, and if the answer to that is to monitor for that situation (and then do what???), that covers a failed parallel SSR too.I am interested in this discussion in the context ofhttps://www.eevblog.com/forum/projects/how-do-i-rate-a-ct-for-leakage-detection-use-also-a-dc-leak-detector-coil/post #16. The video link's narrators suggests when contact weld is seen the "Zappi 2" PCB's parts step in to cut the current. I wondered how this could be, maybe the PCB just has another 'rarely opened' emergency series relay, or maybe an SSR?

author: duak
author_rank: Super Contributor
timestamp: February 16, 2020, 07:46:45 pm
content: To respond to 741:If safety aspects have to be considered to get agency approval, it's far easier if there's a relay or switch in series. When I worked on getting a laser based product approved in the last century we followed IEC-950. Apparently somewhere in it is something about fail safe design, single point failures and semiconductors failing shorted but I relied on our in house safety experts to guide me on the details. I went back to R&D before retiring so I don't know where it's all got to these days. I recall our experts were talking about how the regulators were trying to integrate firmware into IEC-950.I've got a few microwave oven repair stories. If there's one product or device where fail safe design is important this is it. Two of them had failures that were fail safe that inherently prevented hazardous operation. In one, the magnetron simply died. The second had disintegrated its HV rectifier so it also simply stopped working. (I made it work again by transplanting a replacement from the first oven.) Two more ovens had failures where hazard was prevented by a safety interlock. The first had its main switching thyristor short out so the magnetron ran as long as the door was closed. Fortunately, regulations require two door interlock switches to prevent hazardous radiation, although by running without a load, the oven may destroy itself. Another oven had a similar failure. It has a relay that seems to occasionally stick on but so far the door switches are fine. Neither of the last two ovens detected that there was a possible hazard - that was left to an alert operator.

author: fcb
author_rank: Super Contributor
timestamp: February 16, 2020, 07:54:55 pm
content: Quote from: 741 on February 16, 2020, 06:53:32 pmCorrect. There is a DPST relay just up from the LNE input connector. This will be in series with the contactors. You can see the footprint on the video for the second relay for L2/L3 when the PCB is populated for 3phase.