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I'm currently using the following opto-isolated relay board: URL

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Everything is working correctly, I can switch on and off a small light without any problems. Now as soon as I plug in a larger load like a water pump (120V 1.2A) it flicks on and quickly off. Now I would expect this if the relays were under powered but they are 120V / 10A relays. Am I missing something, is there something else in these board designs that are limiting me?

Any help is much appreciated.

Updated:

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Solution: I have stumbled on to the problem trying your guy’s battery suggestion. I have a shift register that is consolidating the input pins to the relay board. When I was hooking up the battery I decided to bypass the shift register too. Low and behold it works both battery and direct power. It seems the shift register is causing all kinds of problems with the inputs, the worst being it is terrible inconsistent (Fails to shutoff sometimes). I will create another question targeted around the shift registers once I know more. Thanks for you help guys.

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2 Answers

up vote 4 down vote accepted

The switching of the pump causes much EMI (ElectroMagnetic Interference). This can be conducted, like Wouter suggests, and then an easy way to test is indeed temporarily use a battery, so that the interference can't come in through the power supply. If it works OK on a battery you'll have to decouple the power supply better. Filters! You can use a mains EMI filter, like this one, built into the socket:

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Also decouple input and output of the power supply with capacitors, and the power connections of the microcontroller and other ICs. Use 10 µF parallel with 100 nF, as close as possible to the power supply pins.

If the battery doesn't help then the EMI is radiated. Also here decoupling with capacitors should be the solution.

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This may be it but I’m a little confused as I thought it was isolated, just to clarify the setup a bit better: The Relays are powered by a separate 5V wallwart (Jumper 2 at the top, not connected to the microcontroller). The microcontroller is powered separately and its output pins are opto-isolated away from that 5V wallwart. I will try powering both the microcontroller and the replay power source from batteries and see what happens. – Exist Aug 12 '12 at 17:55
@Exist - It's not the relay's power supply which is the suspect, but the pump. This may cause EMI on the mains, through which it enters the microcontroller's power supply. It's a possibility, not sure. Can you try Wouter's suggestion of the battery? – stevenvh Aug 12 '12 at 18:05
I came across the problem trying the battery. Issue added up top. – Exist Aug 12 '12 at 21:28
up vote 4 down vote

This sounds to me like your water pump is interfering with your microcontroller via the mains power. To test: try this with the microcontroller powered by a battery.

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Is that EMI induced , which we need to find and break the Antenna or is that line noise so we would need a line filter and a X-capacitor ? – sandun dhammika Aug 12 '12 at 9:30
@sandun - Wouter speaks of conducted EMI, whereas I also talk about radiated in my answer. – stevenvh Aug 12 '12 at 10:10
Some more background, if it helps. The microcontroller does continue to function. I can switch on and off other relays with lighter loads. The relays that have pumps attached just immediately switch off as soon as they are issued the on command. – Exist Aug 12 '12 at 18:00

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