Help protecting SSR? Dummy resistor & varistor? - Page 1
Forum section: electronics / projects

author: vv_eev
author_rank: Contributor
timestamp: June 15, 2023, 02:50:52 pm
content: I'm using an SSR to control a relay solenoid, and I need some help with protection circuitry. This is now holding up the entire design for a larger project.SSR: AQH3213AXhttps://api.pim.na.industrial.panasonic.com/file_stream/main/fileversion/2787Output current: 1.2 A ratedOutput peak = 10x rated = 12 AIntended output load: 24 VAC (RMS) which fires a relay solenoid, current = unknown,Voltage protection:I'm reading in various datasheets to use a varistor sized to (RMS*sqrt2)* 1.10^3 = (24*sqrt2)*1.1^3 = 45.18 V ratingIs this varistor selection what I need, using the "typical" voltage at just over 45V? Max 30 VAC makes me think so.https://www.digikey.com/short/5jn9wt9dCurrent protection:I'm reading to use a "dummy resistor" when powering an inductive load (solenoid), but I can't find any guidance on resistance or wattage ratings on any data sheets, forums, etc. What do I need to use?Thanks!

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 15, 2023, 03:49:14 pm
content: Honestly I doubt you even need any protection here.The voltage protection is for protecting the SSR. Your SSR rated voltage is much higher than your working voltage. Inductor back EMF also shouldn't be a problem with TRIAC switching AC. Therefore MOV/varistor isn't strictly needed. Sizing isn't critical either if you do want to put one, just pick anywhere from 45V to 100V max continuous voltage (read the datasheet and cross check the part number, don't rely on marketplace parametric values!)Do put snubber circuit tho, to tame off the switch on spike. Just the usual 100nF polyester in series with 100 ohm 1/2W resistor should be enoughDummy resistor isn't for protection either. It's to either bleed off the SSR (and MOV/snubber) leakage current so it won't inadvertently activate the solenoid when SSR is still in OFF state; and/or to provide additional load current for the SSR to operate reliably when in ON state if your solenoid current alone is too small. You should check your solenoid datasheet to figure out if you need it or not. Most of the time not needed unless your solenoid is extremely sensitive.

author: wraper
author_rank: Unknown
timestamp: June 15, 2023, 04:09:54 pm
content: Quote from: ArdWar on June 15, 2023, 03:49:14 pmHow it stops to be a problem? It's exactly the same problem as in DC circuits. Harder to fight actually as you cannot just put a diode and be done with it.

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 15, 2023, 05:00:10 pm
content: Quote from: wraper on June 15, 2023, 04:09:54 pmTRIAC switches off at zero current crossing, it therefore should not induce severe back EMF (unless I severely misunderstood something here).For AC, worst case scenario is when switching on right at peak voltage, and/or if the circuit have zero power factor where zero current crossing (switch off) happen at peak voltage. Both causes Vpeak stress which should be well within normal working range rating, nowhere near the magnitude of back EMF from severe di/dt collapse caused by transistors switching off DC coils.