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I have one side of a 125VAC panel-mount indicator connected to the AC load points of a solid state relay. The control points are hooked to a microcontroller with a basic program that switches the indicator on and off depending on characters that arrive on the UART.

The problem is that even when no voltage is applied to the control points, the indicator light still glows dimly. When I engage the control points, the indicator lights up brightly.

At first I thought the SSR had failed, so I ordered another one from a completely different company, but it's doing the exact same thing. Is it just a coincidence that I got two broken SSRs in a row from different manufacturers or (more likely) am I doing something wrong?

EDIT: Here are the SSRs I've tried.

  1. ECE ESR5102401000 24-240VAC/10A 3-32VDC
  2. SHZHE SSR-25DA 24-380VAC/25A 3-32VDC
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    \$\begingroup\$ Some SSRs contain snubber circuits which can leak small amounts of AC even when they're off. You need to specify which SSRs you've tried in order to get a good answer to your question. \$\endgroup\$ – Adam Lawrence Jan 15 '13 at 18:08
  • \$\begingroup\$ Sorry. I've added more info to my question. \$\endgroup\$ – David Brown Jan 15 '13 at 18:14
  • \$\begingroup\$ No need to apologize. \$\endgroup\$ – Adam Lawrence Jan 15 '13 at 18:15
  • \$\begingroup\$ @Madmanguruman - +1 for the point regarding the snubber circuit. This will be found in almost all SSRs designed to switch high amperage AC loads. A good question to DavidBrown is to ask now small of current it takes to dimly light his front panel lamp. He may need to select another type of indicator. \$\endgroup\$ – Michael Karas Jan 15 '13 at 18:27
  • \$\begingroup\$ The indicator is only being used for testing. I was hoping to connect a heating element to the SSR once I got it working. Ideally, I'd like the SSR to stay open completely when the control side isn't energized. \$\endgroup\$ – David Brown Jan 15 '13 at 18:31
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You don't have enough of a load on the output of the SSR.

Most SSRs contain a snubber network that consists of a resistor and capacitor in series and then connected in parallel with the output terminals of the SSR. The capacitor allows some leakage current to flow.

This leakage current is enough to illuminate neon lamps and will even cause some 120 Vac strobe lights to flash periodically. Connecting a small incandescent lamp (5 Watt Christmas bulb) across your load terminals is usually enough to swamp out that leakage current.

Note: the snubber network is required for the triac inside the SSR to not false-trigger when certain conditions are just right. But the snubber does introduce its own set of problems.

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You need to check the input current on the DC side, the led on those relay will blink even if the circuit is not closed. I have 25A relay and that one requires 50mA on the DC side to activate the relay. i think the 10A relay needs like 25mA.

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Are you driving the relay high (with 5 Volts), or are you sinking (taking the pin low)? Some MCUs sink more current than they can source. I have similar setup with 80 relays, 3 different kinds and current abilities, all running from an Arduino Leonardo/UNO with no issues. Try this, if you don't already have it this way: 1) put 5 volts on the + side of relay input.
2) Take the common (-) side to a pin on your MCU, Arduino or PIC 3) To drive it, enter a 0 (or LOW) instead of a 1 (or HIGH) in your code, eg digitalWrite (relay, LOW). This drives the pin low and will trigger the relay.

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