How do you measure DC voltage without a negative DC line (smoke detectors)

zappa

I think he had everthing hooked up.

zappa

Now, why this is happening I have no clue. they should be like totally open mechanical switches I would think.

zappa

Will your meter measure high resistance like 100K or even higher? The only thing I can suggest is to remove your detectors from the wiring and measure across the contacts. I think they should be very high resistance but they seem to have a low enough resistance to operate the relays when they shouldn't be.

gregzoll

http://www.1728.org/project3.htm Scroll down to the section regarding alarm circuits. Also http://www.electronicspoint.com/latc...ent-t3986.html even though it is about SCR's, it still applies.

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darlingm

Ahh, thanks, missed the edit. Can't tell if they are actuating for sure. They're wrapped in some type of black plastic-type material. You can see a picture of it at SM120X Relay Module.pdf. It's the one on the right.

Sort of... I'm using the ideal in-sure wire connectors, so I have to kill the circuit while making changes. Speakerphone + wife makes it easier, though.

Ahh, you might be dead on. Looked into induction/phantom voltage, and found discussions like Phantom voltage reading on switch loops.

Never considered that you'd have induction from the source hot to the return hot on a switch loop, makes perfect sense though. Looks like the amount detected varies on each multimeter model based on its resistance, but that thread discusses 15v, which is dead on to the 17-18v I'm detecting.

I'll measure the current later tonight or tomorrow to confirm that it's phantom voltage with extremely minimal current.

Perhaps the 17-18v was a red herring...

darlingm

... Missed there was a second page. (Thanks for all the responses guys!)

That's my understanding of them, as long as they don't get hot.

Seems to. Finger on one hand to the other initially reads 14 million ohms but fairly quickly starts reducing toward 6 million ohms.

I have measured the resistance disconnected from power. Both at the unit, and across what would be the source hot and the return hot, it registers 0.L (that's the letter L) ohms which is how this unit indicates infinite/higher than measurable resistance. I'm thinking the phantom voltage was putting me in the wrong direction, and perhaps the heat detectors aren't connecting anything.

Awesome links, I'll read up on those.

darlingm

Just realized, this may be very simple. Perhaps the relays are sensitive enough to send out the 9v DC (max 50mA) if they see 17-18v AC even with no current to speak of. Maybe I can't use a switch loop, and will have to run two romex's for each heat detector. Boy that would stink.

zappa

I can't buy into that theory plus Kidde is a reputable company and they wouldn't design something that would false that easily. When you disconnect the sensors the relays drop out and function normally.

Give them a call and see if they have any ideas. Please check back in when you get the system working and let us know.