How to wire this AC SSR? - Page 1
Forum section: electronics / beginners

author: n4te
author_rank: Regular Contributor
timestamp: August 20, 2023, 09:14:32 pm
content: I want to use anAQH3213AZSSR. The schematic is:I'm good on the left side, but I'm not sure on the right side. Do I connect my 24VAC to pin 8, then my irrigation valve to pins 5 and 6, with pin 5 also connected to the other side of the 24VAC? My snubber RC would go between pins 5 and 6.TheSnapEDA componentshows only pins 8 and 5 -- it has pins 5 and 6 connected internally. Is that right? In that case do I connect 24VAC to pin 8, then my irrigation valve to pin 6, with the other side of the valve going to ground? My snubber RC would go between pins 8 and 6.

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2023, 09:32:54 pm
content: The data sheet shows pins 5 and 6 going in series with the load.https://api.pim.na.industrial.panasonic.com/file_stream/main/fileversion/2787Your drawing shows the zero crossing version, which isn't suitable for 24V AC, as the peak voltage is well-below that.I wouldn't recommend a TRIAC SSR relay for such a low voltage. MOSFET would have much lower losses. Obviously it needs to have two MOSFETs connected back-to-back, as it's AC.

author: n4te
author_rank: Regular Contributor
timestamp: August 20, 2023, 09:42:08 pm
content: QuoteTo be clear, you mean that 5 and 6 are indeed connected internally?The example shows only pin 6 connected, which seems to back that up:QuoteBy "that" I guess you mean where it says "Zero cross voltage, maximum, 50V" for zero crossing (PDF page 3)? Isn't that a maximum?I'm open to changing to something else, but I need 16 of these relays so ideally each one is a single component.

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2023, 10:07:51 pm
content: Sorry, I meant, the load should be in series with pin 6 and 8.The zero crossing voltage is the voltage which it has to be below, in order to fire. If the voltage is about 50V, it might not fire and the peak voltage of 24VAC is just 34V, so it's unlikely to work.It's possible to get MOSFET SSRs which are just one part.Here's an example. No doubt there are lower cost parts which can do the job. I just did a quick Google search for MOSFET solid state relay data sheets.https://docs.rs-online.com/6a0b/0900766b80db8698.pdf

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 20, 2023, 10:10:20 pm
content: The SSR is rated at 200VAC and thus I don't think it would work for 24VAC.

author: n4te
author_rank: Regular Contributor
timestamp: August 20, 2023, 11:44:06 pm
content: Quote from: Zero999 on August 20, 2023, 10:07:51 pmAh, thanks. I didn't realize how important that characteristic is. However, it looks like the zero crossing should work at about 30V:I guess if you guys still think it's not a good choice 24VAC irrigation valves (solenoid, 350 mA inrush, 190 mA holding, 60 Hz) then I should find something else. I tried Digikey withthis searchbut it comes up with basically just 2 options: the AQH3213 or CPC1964B (SCR). Are my search parameters wrong? I choose AC, zero crossing, and 1A+ (in stock only).The ASSR-1611 looks OK (though I'm clearly not the best judge), except it doesn't have zero crossing. Do I need that? Maybe that's why my search returns so few results. Is it rarely needed?I need to read more about MOSFET vs SCR vs TRIAC SSRs.

author: Zero999
author_rank: Super Contributor
timestamp: August 21, 2023, 01:01:21 pm
content: Quote from: n4te on August 20, 2023, 11:44:06 pmNot properly. It'll switch on when the voltage is over 30V, which means the output will look like a lamp dimmer set to just over 50% power level.

author: wraper
author_rank: Unknown
timestamp: August 21, 2023, 01:25:14 pm
content: Quote from: Zero999 on August 21, 2023, 01:01:21 pmWhat? Zero-cross means that SSR will turn ON only when output is at below certain voltage (as said in your previous post). The problem with zero-coss at low voltages is that feature becomes nearly useless and SSR becomes nearly instant ON type. 50V max (no min) spec is quite high and it means that at 24 VAC zero-cross feature probably won't ever kick in at all and SSR will be able to turn ON at any time. How it becomes a dimmer when it will just instantly turn ON regardless of at what part of output voltage cycle input signal arrives?

author: n4te
author_rank: Regular Contributor
timestamp: August 21, 2023, 02:58:54 pm
content: In that case it seems like the AQH3213 should work, just the zero cross feature will do nothing useful. I have some AQH3213 parts, so using them would be convenient.I setup this circuit:R16 should draw 44mA. It gets warm when connected directly to 24VAC, so I expect the same from this circuit, but it doesn't get warm. Any ideas why?I tried probing with a DMM and scope, but it didn't seem useful. I know probing an SSR is not the same as probing a mechanical switch/relay. Where should I probe to debug further?

author: Zero999
author_rank: Super Contributor
timestamp: August 21, 2023, 04:57:24 pm
content: Quote from: wraper on August 21, 2023, 01:25:14 pmYes. I was talking bollocks. It will work of course, but the zero crossing won't work properly. Not that it matters, as the original poster wants to control an inductive load, which is better without zero crossing.Quote from: n4te on August 21, 2023, 02:58:54 pmThe load should be connected to pin 5, not pin 6.The resistor doesn't get hot, because it's in series with a capacitor. It's a snubber network, which damps the high voltage pulses generated by the coil.

author: n4te
author_rank: Regular Contributor
timestamp: August 21, 2023, 05:13:56 pm
content: QuoteI think you mean pin 6 (shown in the datasheet) not 5 (shown in my schematic). The schematic symbol (from SnapEDA) only shows pin 5 and the pattern has pins 5 and 6 connected internally. I actually wired it to pin 6.QuoteNo coil yet, just the 546 ohm resistor? What does the snubber do to the resistor? How many mA should I expect to see? If it's expected to be unable to draw the 44mA, maybe it's actually working correctly. What would be a better test? I don't have an irrigation valve on hand.

author: Zero999
author_rank: Super Contributor
timestamp: August 21, 2023, 05:27:50 pm
content: https://www.st.com/resource/en/application_note/an437-rc-snubber-circuit-design-for-triacs-stmicroelectronics.pdf

author: n4te
author_rank: Regular Contributor
timestamp: August 21, 2023, 05:40:14 pm
content: Thanks. I read that and absorbed some of it, though my understanding of TRIACs is still pretty weak.Right now I'm just trying to test my relay without an irrigation valve. If I understand correctly, the snubber's impedance at 60Hz is ~27.5K. That's quite a bit higher than the 546 ohm resistor and so should have little affect on the current draw -- I should still see ~44mA and the resistor should get hot. It doesn't though, why?

author: Zero999
author_rank: Super Contributor
timestamp: August 21, 2023, 06:50:52 pm
content: The resistor has a 100nF capacitor in series with it, which dominates the impedance, so hardly any current will flow and the power dissipation in the resistor will be minimal.

author: n4te
author_rank: Regular Contributor
timestamp: August 21, 2023, 07:36:51 pm
content: I assume you mean the high impedance due to the capacitor causes the snubber circuit to draw very little current, so power dissipation of thesnubberresistor will be minimal. That makes sense to me, essentially the snubber barely affects the rest of the circuit, meaning I should see my load (the 546 ohm resistor) heat up just as it does when connected directly to 24VAC.The pin I thought would sink current to turn on the LED wasn't doing so, as some other code was surprise changing the pin. I hate it when it's my own stupid fault! Sort of, better this than a bum component I suppose. Anyway, my relays are working and my resistor is heating up! Great success!Thanks everybody! We did it! \o/