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I'm trying to use a power relay (PR26MF) to turn on and off a device. The device takes a 9v supply and typically draws ~250mA. I'm finding that the relay turns on just fine, but when I try to turn it off, it won't: it remains latched in the on state.

The datasheet mentions that a snubber circuit across the load may be necessary. I tried adding one (with a ~2ms time constant). It had no effect.

I measured the resistance across the output port of the relay without the load attached; it works as you would expect: ~inf resistance when it's off and low resistance when on. It seems to turn on and off just fine in that setup.

I tried a different load - using the SSR to drive a 5V mechanical relay. The SSR still exhibited the latching behavior.

stevenvh
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Shawn J. Goff
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3 Answers3

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One big question is whether this is an AC load or not. The solid state relay you are using is meant to switch AC loads only and WILL NOT shut off until the zero crossing occurs.

If you are using it for a DC load, then it'll stay on once switched on, and will not turn off until the current goes to 0 (usually at zero crossing with an AC power supply).

If it is an AC load, then you may need to design your snubber more carefully to match your load and power supply. You haven't given us enough information to help you with that part, though.

If you are trying to switch a DC load, then you need a different type of solid state relay.

Adam Davis
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    That's the ticket! I actually had another ss device here and had already determined it wouldn't work for that very reason. Doh! – Shawn J. Goff Mar 29 '10 at 18:02
  • What you mean by different type of relay? can you please leaborate and suggest whcih one to use? Coz i am facing the same problem.I used SSR to switch on 12vDC pump via 89s52 controller through moisture sensor.when the senor is dry (logic 1) the motor is switched ON. and when senor is wet (logic 0 ) my program will switch off the relay, BUT relay is still ON. i checked the port pin statuc which controls the relay input its working fine without connecting to relay. –  May 21 '15 at 07:04
  • @user77011 It's likely that your SSR is rated for AC only. You need to find one that is rated for DC, and the current your pump needs. If your pump isn't very large, then this would probably work fine: http://www.digikey.com/product-detail/en/CPC1909J/CLA281-ND/1277136 – Adam Davis May 21 '15 at 12:32
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The relay in question is for AC not DC and therefore will not turn off the 9 volt supply to your device as it never reaches zero crossing. you need to get a relay for DC loads then it will turn off. the PR26MF series has both type zero and non-zero crossing but are used for AC loads not DC.

Marathonman
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Suggest sending a zero pulse in the load circuit every so often .... keep the pulse time low such that any load like a motor doesn't stop as this might shorten its life. If the load is not at risk send a reset zero pulse every minute or so.

Nyge
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