I am about to design a custom PCB that is supposed to switch a 230 V inductive motor with a 750 W rating over relays.

I have the "option" (since I happen to have them around) to do this using prebuilt relay circuit boards built for Arduino use, but honestly I find these to be quite concerning and flimsy and wouldn't trust these more than to switch low voltage low current loads for quick and dirty switching setups (like I do for a garage door opener I have modified to also work with 433 MHz remote inputs.) They are rated to work at 230 VAC at 10 A (at least the relay has it printed onto it, I find this to be an important difference) but I find the distance between the solder borders of the pins to be quite small with the bare minimum of 1.6mm which does not even get close to the minimum spacing of 2.5 mm that I calculate with the 325 V peak voltage +10% allowed mains variance.

Therefore I would like to do this on my own and integrate the relay circuit onto my PCB that is currently an ATtiny MCU operated over a 5 V line with a couple of port expanders and H-bridge drivers powering DC motors over a 12 V line.

I am aware about the design and safety guidelines outlined in IPC2221B and IPC9592B, trace widths and thicknesses, clearance and creepage distances and plan to outscale these by a couple of factors. The PCB under the relay will also have a slot milled to create an air gap between its 5 VDC and 230 VAC terminals. Furthermore, I would like to switch live and neutral using their own individual relays.

I have a couple of questions and assumptions I would have liked to have looked over, and in general, further input that I have missed so far.

  1. I assume given the inductive load of the motor that regardless of the amperage rating of the relay I should use one certified for high inrush currents. Is that correct?

  2. I plan on using logic level MOSFETs that I already have and am familiar with to drive the relays. I see on some occasions that people suggest optocouplers to switch relays on and off and advertise this choice with safety reasons. I don't see a benefit for that in my scenario here, as I am using the MOSFET to switch 5 VDC. Furthermore, are relays designed or should I expect them to fail in such a way that they will short their outputs with their logic inputs? In that case what good is an optocoupler going to do that is switching the relay on and off since the main 5 VDC will be shorted in either case? Is this something that is used for other switching scenarios?

  3. What terminals should I use for the 230 VAC wires? The prebuilt Arduino relay boards use very garden-variety 2.54 mm pitch screw on terminals. I'm not at all happy with the distances there (1.6 mm between solder joints,) especially since I expect dust to get onto the board eventually, and they just look very brittle to me. Are there bigger higher pitch screw on terminals specifically for this design, or safe clamp-on solutions? (Even though I'm going to isolate everything exposed with hot glue.)

  4. I noticed on all example schematics of the relays' datasheets that none of these have a current limiting resistor put in front of the coil of the relay. This seems to me to be a clear mistake and I am wondering why it is being left out in the schematics. Yes, the schematics referred to MOSFETs and not BJTs.

  5. Other things I am missing?

I plan on ordering the parts along with other ones on my next Mouser order, so feel free to directly link to components on their site if you have any recommendations.


2 Answers 2

  1. Use a relay designed and specified for a motor load at least as high as what your motor indicates on the nameplate (watts/HP and LRA) if you want predictable life and reliability.

  2. Optoisolation can help deal with EMC issues since relays do not provide perfect isolation, particularly to AC. If your circuit resets or goes haywire when the relay drops out then you probably should have used optoisolation (or had better layout).

  3. There are lots of PCB-mount terminal blocks (as well as connectors such as Molex and individual spade connectors). For example you might use ones with 7.5mm pitch. Consult a distributor and from there catalogs. Some relays are available with spade connectors on the top so the mains never makes it to the PCB.

  4. Relays, when used within the rated voltage range, do not require a series resistor for the coil, no matter whether they are driven by a MOSFET, BJT or another relay contact. When driven by a semiconductor they generally require some coil suppression though, so the energy stored in the coil inductance does not kill the driver. A resistor and diode across the coil is one way of doing that. A simple diode across the coil is another way of doing it (but reduces relay life).

  • \$\begingroup\$ Thank you for your reply! Am I right to assume that for 2) to make sense I would need a separate 5VDC line isolated from my MCU to drive the relays? And in regards to 4), I have tried to look up but haven't found any arguments for why the diode should be paired with a resistor. I see two advances in regards to that topic, one being that one should just use a higher rated diode, the second being that the resistor and diode are part of a snubber circuit (however the capacitor is missing here). Could you please shed some light on this, ideally with links to some reading material? Thank you! \$\endgroup\$ Oct 25, 2022 at 14:12
  • \$\begingroup\$ On second thought, given your and vu2nan's comment it seems to me that all things considered (motor load rating, emc, mounting, isolation), I would be better off and safer using a sufficiently sized SSR with the respective terminals, i.e. RA4850-D12. What do you think? \$\endgroup\$ Oct 25, 2022 at 15:07
  • \$\begingroup\$ Something like an Omron G7L is likely rated adequately. You can, and usually should, use a higher voltage for the coil (eg. 24VDC or 12VDC) if you have an isolated supply for the coil. I don't see HP ratings for that SSR. \$\endgroup\$ Oct 25, 2022 at 17:08

It's standard practice to control a single phase 230 V AC - 0.75 kW induction motor, drawing a full load current of 5A and a starting current of 30 A, using a 3-phase size 00 contactor with thermal overload protection.

Here's the schematic.

enter image description here

A relay, capable of switching the contactor coil current, could be located on the PCB.


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