High current relay switching very low current - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: October 22, 2024, 10:28:11 am
content: I have a project where I have to sense whether a 240VAC heating element is powered on or not and indicate this to a micro. To keep things dirt simple I want to use a small 240VAC relay. The thing is, the smallest relay I can find has a contact rating of 10 amps. I know relays like to have a certain minimum current through their contacts to guarantee reliable switching whereas I am only pulling down a 5K1 resistor from 5 volts...Would it help to have a small capacitor, say 1uF, that gets shorted every time the contacts close and it discharges it's 5 volts and makes a *very* tiny arc at the contacts? Would this improve contact reliability at an otherwise switched current of 1mA?

author: Gyro
author_rank: Super Contributor
timestamp: October 22, 2024, 11:43:00 am
content: 5V is probably not going to be sufficient (at whatever current) to break down the insulating film on most high current contact alloys reliably (I suppose it might with the contact shuffle you that get with AC relay coils but I wouldn't bet on it being reliable). The exception might be ones with gold flashed contacts but only until the flashing wears away. I don't think you are likely to find many of them with mains voltage coils though, as their application makes it very likely that they will be switching mains on the contacts too.The capacitor approach can work (I used it when converting a Weller TCP iron to SSR switching - it refused to work without the contact suppression capacitor), but not necessarily as low as 5V.My choice would be a miniature low wattage mains transformer and then condition the output as needed. [Edit: It could yield zero crossing information too].

author: Circlotron
author_rank: Super Contributor
timestamp: October 22, 2024, 12:11:39 pm
content: A transformer would definitely work but take up way to much space. I might try a small signal relay and a capacitive dropper and rectifier. And something to take care of things if the AC comes on at the peak of the sine wave. Or maybe a capacitive dropper and opto.

author: Henrik_V
author_rank: Regular Contributor
timestamp: October 22, 2024, 12:45:06 pm
content: Capacitor , resistor, opto-couplersize will be only limited by isolation constrainsor use a hall element with a little iron field guide to sense the current in one line ..

author: wraper
author_rank: Unknown
timestamp: October 22, 2024, 12:48:58 pm
content: Relay is a wrong part for this. I'd use optocoupler (either with bidirectional emitter or added reverse voltage protective diode) with series resistance selected in a way that it's reliably triggered at like 1/3-1/2 of peak voltage to reduce heat, reduce optocoupler wear. Or add series capacitor to reduce power waste even more. If galvanic isolation is not needed, it's even simpler.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 22, 2024, 02:41:48 pm
content: Quote from: Circlotron on October 22, 2024, 10:28:11 amThe state of a relay does not indicate the heater ispowered. The current does. Measure the current?(this can literally be a small CT (eg TALEMA ASM-010) into a bridge rectifier with capacitor and a comparator)

author: valcher
author_rank: Contributor
timestamp: October 22, 2024, 04:50:51 pm
content: Yes, the relay status will show that the heater is powered.Let's imagine a situation - the heater is broken, then there is no current, but it is poweredRegarding safety, I would choose an opto relay.

author: Gyro
author_rank: Super Contributor
timestamp: October 22, 2024, 05:33:43 pm
content: Both of the above are valid. If youreallywant to know what's going on, use both a CT and Opto. I'm thinking here of the possible situation of a (water) heater that includes its own safety thermostat.

author: floobydust
author_rank: Super Contributor
timestamp: October 22, 2024, 06:38:36 pm
content: OP I think it won't work reliably because power relays contact material (like AgCdO) are no good at (signal) low voltages and currents. Wetting voltage and current can be 10V at 10mA or worse depending on wiping action and contact style.Even then, you still need an AC-rated coil and possibly special high duty-cycle coil if the heater is on most of the time. Those relays don't cook their coil when always on.Your circuit could default to normally heater-off always having contact current, fail safe scheme or use both the NC and NO contacts but the problem of oxide/passivation layer building up is still there.I second using a low current like 1mA opto-coupler to do voltage sense. I likeMOC217Mand a tiny bridge rectifier and three resistors on the LED side (so it won't activate at say 100V).You can also use a CT which can power an opto-coupler or go into the MCU, for current detection. You just need presence/absence I think.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: October 23, 2024, 03:04:30 am
content: use a non-contact voltage detector. 3 transistors, easy peasy.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: October 23, 2024, 03:53:36 am
content: this was developed as a product for monitoring fixed-installation water heaters, but never went anywhere:cheers,rob :-)

author: Circlotron
author_rank: Super Contributor
timestamp: October 23, 2024, 10:15:34 am
content: The purpose of the AC mains sensing is to determine when a water heating element has been powered off because it has reached the right temperature so a process can proceed. Sensing voltage is enough. No need to sense current because if the heating element was open cct the water would never get hot so things would stop right there.

author: Circlotron
author_rank: Super Contributor
timestamp: October 23, 2024, 11:04:12 pm
content: Had a look at using a small signal relay and suitable voltage dropping circuit to power the coil but these tiny relays don't seem to have the voltage isolation capabilities of physically larger high current relays. I have since discovered a HCPL3700 AC/DC to Logic Interface Optocoupler that looks to be just the ticket.https://www.farnell.com/datasheets/2729322.pdf

author: ajb
author_rank: Super Contributor
timestamp: October 24, 2024, 03:56:01 am
content: FYI in the industrial world there are DIN mount opto relays for this sort of thing:https://www.automationdirect.com/adc/shopping/catalog/relays_-z-_timers/optocoupler_relays/52507

author: Circlotron
author_rank: Super Contributor
timestamp: October 24, 2024, 11:15:45 am
content: That's interesting. I'll consider that.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: October 24, 2024, 11:52:59 am
content: Quote from: Circlotron on October 23, 2024, 10:15:34 amwould you not then be far better off using a temperature sensor placed in the liquid?cheers,rob :-)

author: Circlotron
author_rank: Super Contributor
timestamp: October 24, 2024, 11:59:24 am
content: Quote from: robert.rozee on October 24, 2024, 11:52:59 amProbably, but the board has to be a drop in replacement for an existing board as well as a service replacement. So gotta fly with whatever is already there.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 24, 2024, 12:55:15 pm
content: You can also measure the voltage across the relay. If there is voltage, the heater is off.You will still be able to detect a change in the relay, the minimum switch requirements of the relay are met by the load, and you don't need a flying neutral lead.Oh yes, drawback, few mA of optocoupler leakage.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 26, 2024, 12:16:10 pm
content: Not dirt cheap but cheap enough. NZ$10 (ex delivery) should cover it even with the H11L1M. Mouser prices.1. See attched. OK, I've used 4k7 not your 5k1 but what the hell.2. If your micro doesn't have Schmitt trigger inputs add a gate or use logic optocoupler egH11L1MThe Zener needs to be any 1.3W (or higher) type but you can use 9-16V ones. The MOV is optional but I'd keep the polyfuse eg 0ZRE0005FF1C.C1 should class X. eg R49AN368000B1M I and your insurer advise you not to scrimp on this.C2 is a 25V cap, it can be of a higher value if you like, say 220 to 470u. That lowers the time taken to detect the heater switching a little but that probalby doesn't matter.Put a LED is series with the opto led for a visual indication. If you do, lower R3 to say 470R. Suck it and see.

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 26, 2024, 12:44:41 pm
content: In the strictest sense of the word, “powered or not”, interpreting it as if the device is actually drawing power from the line, a current sensing solution as shown previously is the way to go.You may use either a CT or hall sensor. Your choice.