Controlling a contactor with an Arduino

Hi,
I have a project to control my water heater remotely. I bought a simple relay from DX () and for testing purposes connected the system to a 60w 220V light bulb and all worked well.
Since my water heater is 2.1KW I've decided to use 2 level switch, the arduino will turn on the relay and the relay will switch the contactor ( B6-40-00 datasheet), The contactor's coil is 240V.
When I switch the contactor (using the relay, through the Arduino) even without load, switching the relay off through the Arduino hangs the Arduino completely.
A friend suggested that this is an EMI problem, and I should increase the distance between the contactor and Arduino or use a zero cross switch, but if so, why when I used only the relay to switch I did not have this issue. I suspect that the real problem is that when the relay switches the contactor off it's coil discharging is the cause of the problem (even though I can't explain what exactly causes the hang).
Any help how to solve this will be appreciated, also do you think I can use the relay to control my water heater without the contactor?
Thanks

Use a IN4007 in the line which powers the MCU so that noise due to back emf of Contactor coil doesn't affect MCU power. Also shield the MCU.


For a much less price you get GoodSky GS-SH-112D or Tyco Schrack T9AS1D12-12 T-Relays which are of better quality. I am not promoting GoodSky or Tyco products. I am just mentioning this because they are better and low priced than the relay you are using. They are also 12V 30A 230V contact type relays. They also have other versions which work at 5V and other voltages for DC Coil.

https://www.goodsky.co.uk/products.php?model=GU&search=Go

https://www.te.com/catalog/pn/en/1-1393210-3

Hi,
Thank you for your reply, but couldn't the problem be due to EMI and not EMF?

JitterJaw said:

I suspect that the real problem is that when the relay switches the contactor off it's coil discharging is the cause of the problem.

Click to expand...

This seems to be exactly the root of problem, due to the fact that once it is required a substantial force for switch between contacts of Contactor, inductance become very large.

Note that although a relay also employ the same physical principle, the magnitude of current and inductance involved are much smaller if compared to a Contactor.

Anyway, post the schematic of your circuit, due this way we will be able to give you accurate suggestions for turn around the problem you´re facing.





+++

It sounds like an EMI problem to me.

1. You should have a reverse biased diode across the smaller control relay coil. I assume that is a DC relay switched from a transistor connected to an output port. The diode will dissipate the back emf from this first relay.

2. The small relay is switching a 240v contactor coil. In this case you need an AC snubber connected across your control relay contacts to absorb the back emf when the contactor coil is switched off. The snubber is just a resistor and capacitor in series across the relay contacts. Apart from reducing EMI it will save your relay contacts from arcing and burning out.

Google AC Snubbers and Switching inductive loads.

Update: Sorry just reviewed your post and the relay data, so 1) above seems OK, but still think 2) is what you need to do.

Hi,
Here is my schematic.

I don't know the exact implementation of the opto-isolated relay, so I left it dangling. I assume there is a resistor between the opto-diode and the Arduino's output (If anyone knows otherwise please say so). Notice that the opto-isolated relay uses the same 5V and GND as the Arduino. I modeled my water heater as an RCL, even though I think it is mostly R and L. Also not sure how contactor is implemented, this is only a guess.
Thanks

beetlejuice said:

It sounds like an EMI problem to me.


2. The small relay is switching a 240v contactor coil. In this case you need an AC snubber connected across your control relay contacts to absorb the back emf when the contactor coil is switched off. The snubber is just a resistor and capacitor in series across the relay contacts. Apart from reducing EMI it will save your relay contacts from arcing and burning out.

Click to expand...

Hi,
I hope this question isn't too dumb, but can an RC snubber be replaced with an MOV? if yes, what should be the specifications of the MOV.
Thanks