Can AC Solid State Relay be used for DC applications? - Page 1
Forum section: electronics / beginners

author: Swapnil
author_rank: Contributor
timestamp: June 30, 2022, 04:56:48 am
content: Hello, everyone! I am having an AC solid-state relay rated for 10A but I have read somewhere that it can't be used for 10A DC current. I am not able to find the exact reason why it can't be used for DC applications other than the arching effect. So, is the arching effect the only reason why an AC solid-state relay can't be used for DC applications rated for the same current rating? Are there more reasons behind this?

author: Per Hansson
author_rank: Unknown
timestamp: June 30, 2022, 06:06:56 am
content: Most solid state relays switch during the zero crossing, that will never happen with DC so it will not switch.Do note that there are solid state relays that can switch even with a load, they are then called "instantaneous switching" instead of "zero cross switching" but they are generally more expensive.

author: Ian.M
author_rank: Super Contributor
timestamp: June 30, 2022, 06:50:58 am
content: AC only SSRs use aTRIACto switch the load, whereas DC rated ones use a power MOSFET. Assuming the SSR doesn't have zero volt switching, which as Per points out, would be a game-stopper for DC, you could trigger the TRIAC, which would turn on, but it wouldn't turn off again till you removed power. There's also the gate drive circuit to consider. It may well be incompatible with a DC supply, with potential consequences anywhere between simply failing to power up on DC to burning out parts!

author: Siwastaja
author_rank: Super Contributor
timestamp: June 30, 2022, 07:22:31 am
content: As others have said, AC solid state relays use TRIACs. TRIAC is a semiconductor which is triggered on by applying voltage on GATE. This is what AC SSRs internally do. But, TRIAC conducts as long as current flows through it. There must be some external way for current to stop. Sinusoidal AC supply voltage goes to zero twice every cycle, and so resistive load current also goes to zero, creating an opportunity for TRIAC to turn off. But TRIAC-based SSRs keep always on if there is no external way of getting current to zero. Examples are DC circuits, or even AC circuits with poor power factor.

author: Swapnil
author_rank: Contributor
timestamp: June 30, 2022, 11:47:57 am
content: Quote from: Ian.M on June 30, 2022, 06:50:58 amThank you for your response. So, if we connect power MOSFETs at the terminal of AC Solid State Relay, will it work for DC applications?

author: Siwastaja
author_rank: Super Contributor
timestamp: June 30, 2022, 12:19:06 pm
content: Quote from: Swapnil on June 30, 2022, 11:47:57 amOf course not.

author: Zero999
author_rank: Super Contributor
timestamp: June 30, 2022, 12:20:10 pm
content: Quote from: Swapnil on June 30, 2022, 11:47:57 amNo that won't work. The solid state relay needs to be replaced with one with a MOSFET output which can switch DC.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 30, 2022, 05:37:38 pm
content: I once had a vendor send me a piece of equipment with a AC SSR in a DC application. It would not turn off until power was removed. You need a DC SSR.

author: Swapnil
author_rank: Contributor
timestamp: July 01, 2022, 05:19:41 am
content: Quote from: Zero999 on June 30, 2022, 12:20:10 pmThat means the AC SSR will turn on but won't turn off until we cut off the input supply. Just consider if we are using a switch between the AC SSR and the supply, then will it be able to turn on and off using those switches?

author: agehall
author_rank: Frequent Contributor
timestamp: July 01, 2022, 05:32:37 am
content: NO, NO and NO.Read the answers you have gotten - AC SSRs typically require a zero-crossing on the load side or they will never turn off. No MOSFET will solve this for you without making the SSR redundant.

author: Zero999
author_rank: Super Contributor
timestamp: July 01, 2022, 07:04:34 am
content: Quote from: Swapnil on July 01, 2022, 05:19:41 amI think there may be a language barrier.The AC SSR is no good for DC.For DC, use a DC SSR.Nothing else will do.Don't use the AC SSR for DC. Only use a DC SSR for DC.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 01, 2022, 08:28:03 am
content: Quote from: Swapnil on July 01, 2022, 05:19:41 amWell if you already have "those switches" able to switch on/off your load, then why do you need to add anything more.

author: Seekonk
author_rank: Super Contributor
timestamp: July 02, 2022, 08:28:46 pm
content: This falls under stupid engineer tricks. I had a bunch of CRYDOM back to back SCR AC bricks which had 120V AC for the control voltage instead of the usual 3-30V DC. I thought they would just have a full wave bridge in front of the opto isolator LED. I had an application where I only had a DC control signal. So, I tried it and it would rectify the output in one direction. Reverse the control voltage and it would rectify in the opposite direction. I ended up putting it in a reversing DC motor control application.