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Topic: [SOLVED] RS232 output Down after relay/solenoid use (Read 455 times) previous topic - next topic


Dec 15, 2017, 01:51 pm Last Edit: Dec 19, 2017, 08:50 pm by AngeloGomes
Hi guys, I need some advice, as my electronics skills are too basic.

I design(if we can say this) a circuit that uses an arduino and the 8 pins of a PCF8574 to control the 8ch relay board. Actually arduino receives a command from the computer, using the rs232 communication port. After is parses the command it activates the relay channel and sends back to the computer a message saying success or error.

Each relay channel is connected to a 110VAC solenoid valve with the following specs (11W , starting 40VA, sustain 22 VA).

Everything runs fine for, maybe  4x-5x I switch a relay, but after sometime, the RS232 output from arduino stops to send data to the computer. I´ve found that if I disconnect the power(110VAC), i.e., without load, everything runs smoothly, so the problem is related to the solenoid.

After I read several topics here, I´ve found that many people already experienced this too. Some people say to use snubber diode, a RC snubber, use ferrite cores. For me, these are OK, the problem is to decide what to use in my case ? Which diode ? values of R and C ? Where exactly should I  put them ?




Just to add.

As a VAC solenoids, it seems that the options are MOV, TVS diode or RC snubber


If the comms is lost when relays close, it's more likely to be wiring that is unshielded, not twisted pairs or run too close and parallel to the 110vac wiring.

If it happens on relay opening, then r/c snubbers or mov's may help.


I think you should first evaluate your wiring.

Ideally ALL the Arduino and Computer stuff should be physically on one side (lets say the left side).
ALL the 110V wiring etc should then be on the right side.
The further apart the left and right wiring can be the better.

Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.


Hi guys, thanks.

I already did these steps, the 110VAC, the 8ch relay board are well apart from arduino, actually, I made a box(metallic) , where the the relay board are inside and wires for the solenoid valves go out. The 8ch relay board  inside the box, is connected to arduino by a 50cm flat cable.

I noticed the comm output from arduino stops when some relay opens, so it is  a case for a snubber.

I found some vendors selling RC snubber  in my country, but I am a little bit confused.

One vendor sells snubbers for solenoids  with  power <8VA and <12VA, but my solenoids are rated at 22VA. So it seems that none of them can be applied to my case.

What I need now is to know what are the values for R and C, so I can assembly/mount the snubber by myself.

Can someone help me with this ?




For a test I would  try a 0.01 µf and 100 to 150 ohm resistor.   Capacitor must be rated for 250VAC.   If you can't find a 0.01 µf you can try a 0.1µf  (but I would recommend 0.1µf only for testing).

Even if the above works,  you still have an issue with the path the original spike was taking.  If not addressed you will likely have periodic issues due to ESD or other spikes on the AC line.

One thing I would look at is, are the valve bodies (metal?) grounded?

Does your relay board have relay contact clad nearby the input (from the driver) clad?

Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.


Hi John. Thank you for your help

I will try to mount the rc snubber next monday and I will back here to report the results.

The relay board is this one:


Did you use a separate 5volt/1Amp relay supply, connected to JD-VCC and relay ground (jumper removed).
And did you  _NOT_  connect relay ground to the PCF8574/Arduino, just relay VCC and the inputs.
So you  _DO_ have opto isolation.


Yeah, I did it. The relay board now has a external 5VDC power supply and the GND pin on that set of channels pins is free.


Hi guys, just to let you know that I solved it with the RC snubber.

I used a 0.22 microF  650V capacitor,  and a 120 Ohm 2W resistor.

I put a RC snubber in parallel to the solenoid valve.

Everything now works as expected.

Thank you all.


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