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Old 01-22-2012, 05:45 PM   #1
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How do you measure DC voltage without a negative DC line (smoke detectors)


New heat detectors are making my smoke detector constantly alarm when hooked up. They're attic rated 200F heat detectors, and the attic's been around 50F here.

System is wired as shown in the diagram below. How do I measure the DC voltage being sent out on the relay's gray wire? Is putting the multimeter in DC voltage mode, and measuring with red prong to gray wire and black prong to the ground my best option? (I'd first disconnect the gray wire from the wire nut connecting to the other relay's gray wire and the smoke alarm's red wire.)

EDIT: FWIW, Relay instructions say the max output on the gray wire is 5mA.

If that's OK to do and my best option, would connecting black multimeter prong to ground vs neutral make any difference? Never seen anything about combined AC/DC systems...

There's only the one wire that uses DC voltage...

EDIT: These heat detectors act just as a light switch. They don't connect the two hot wires when situation is normal. They connect the two wires when there is a rapid increase in temperature (7F/min I think) or it hits 200F. They aren't an electronic device, and don't use a neutral. These units are rated for dusty attics, even though my attic is extremely clean now. (Insulation removed & vacuumed out a while ago.)



Last edited by darlingm; 01-22-2012 at 06:11 PM.
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Old 01-22-2012, 06:17 PM   #2
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How do you measure DC voltage without a negative DC line (smoke detectors)


If it's alarming I don't think you need to check the DC voltage as that part seems to be working.
Are the heat detectors compatable with this system?
Could the heat detectors be normally closed contacts when they really need to be the opposite?


Last edited by zappa; 01-22-2012 at 06:21 PM.
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Old 01-22-2012, 06:24 PM   #3
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How do you measure DC voltage without a negative DC line (smoke detectors)


I'm guessing the relays have a 120 VAC coil and the heat detectors complete the circuit when activated? Does that sound right?
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Old 01-22-2012, 06:39 PM   #4
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How do you measure DC voltage without a negative DC line (smoke detectors)


Brand new heat detectors, relays, smoke detector, and wiring. Won't stop alarming until I unplug the smoke detector.

Kidde HD200F heat detector, Kidde SM120X relay, Kidde PE120 smoke detector. The SM120X relay literature says it's for the HD200F, and lists the P120 as a compatible smoke alarm model.

Did some more looking online. Found a few websites that said smoke detectors connect the ~ 9v DC line to the AC neutral.

So, did some multimeter checking, and now I'm very confused.

* With everything else disconnected, voltage across the unswitched power cable hot and neutral is 122v AC. Everything's good here.

* With only one heat detector and its relay connected, voltage across the relay's gray wire and the AC neutral fluctuates between 8-10v DC. This means the smoke detector is operating properly, because the relay is giving it the alarm signal. Also, disconnecting the gray wires makes the smoke alarm stop alarming.

* With circuit off, across the heat detector's white relabeled black source power and the black return power, infinite resistance (no connection) with everything disconnected from this heat detector. Same measurement at both ends of cable. That's good, without a heat problem, there should be no connection.

* Across the relay's black return power wire and the neutral, 17-18v AC!

How the heck can a heat detector acting as a switch, without any electronics (at least any that need a neutral wire), receive 122v AC and return 17-18v AC? It's a screw connection to the heat detector, which is very solid. Shouldn't be returning anything. Heat detector isn't hot or anything, and there's no arcing.

EDIT: I've checked each relay and each heat detector separately, and they're acting the same way.
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Old 01-22-2012, 06:44 PM   #5
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How do you measure DC voltage without a negative DC line (smoke detectors)


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Originally Posted by zappa View Post
I'm guessing the relays have a 120 VAC coil and the heat detectors complete the circuit when activated? Does that sound right?
More specific to this, my understanding is that the heat detector acts as a switch, getting 120v, and outputting 120v on an alarm. My understanding is that when the relay receives 120v on its black wire, that it sends out ~ 9v DC on its gray wire.
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Old 01-22-2012, 06:53 PM   #6
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How do you measure DC voltage without a negative DC line (smoke detectors)


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More specific to this, my understanding is that the heat detector acts as a switch, getting 120v, and outputting 120v on an alarm. My understanding is that when the relay receives 120v on its black wire, that it sends out ~ 9v DC on its gray wire.
I think we are on the same page. Heat detector contact closes and activates the relay, just like flipping a light switch for a bulb.

Can you safely measure across the 2 contacts on the heat detectors with power applied? Should be 120 VAC but I don't know where the 17-18 volts are coming from. Stumped
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Old 01-22-2012, 06:56 PM   #7
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How do you measure DC voltage without a negative DC line (smoke detectors)


What would happen if you disconnected both heat detectors? That would be a good test also.
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Old 01-22-2012, 07:02 PM   #8
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How do you measure DC voltage without a negative DC line (smoke detectors)


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Originally Posted by zappa View Post
I think we are on the same page. Heat detector contact closes and activates the relay, just like flipping a light switch for a bulb.

Can you safely measure across the 2 contacts on the heat detectors with power applied? Should be 120 VAC but I don't know where the 17-18 volts are coming from. Stumped
Yeah, I avoid stuff like that when possible, but I'm comfortable doing it... Just go slow.

LOL, 90v AC across the 2 contacts? I'd understand 122v - 18v, or 104v, but not sure why 90v.

The 17-18v AC is if I measure with power applied across the heat detector's black return power (that would regularly go to the relay, but is disconnected for measuring purposes) against the neutral coming in on the unswitched power cable.


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What would happen if you disconnected both heat detectors? That would be a good test also.
With both heat detectors disconnected, the smoke alarm doesn't alarm.
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Old 01-22-2012, 07:09 PM   #9
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How do you measure DC voltage without a negative DC line (smoke detectors)


There must be some voltage drop across the relay coils. Not knowing the circuit their may be something else hooked up sucking the voltage down. If it was just the relay coil you should be seeing close to your input voltage across the detectors as there is no load in the circuit.

Other than disconnecting the detectors to see what happens I can't think of much else to check.

Are the relay covers see through? Is there a way to see if they are actuating.

Last edited by zappa; 01-22-2012 at 07:12 PM.
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Old 01-22-2012, 07:11 PM   #10
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How do you measure DC voltage without a negative DC line (smoke detectors)


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There must be some voltage drop across the relay coils. Not knowing the circuit their may be something else hooked up sucking the voltage down. If it was just the relay coil you should be seeing close to your input voltage across the detectors as there is no load in the circuit.

Other than disconnecting the detectors to see what happens I can't think of much else to check.
Thanks for your time. I'm very confused and stumped too, this isn't making any sense. I'll see how good their customer service phone number is tomorrow...
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Old 01-22-2012, 07:13 PM   #11
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How do you measure DC voltage without a negative DC line (smoke detectors)


Can you tell if the relays are actuating? I added a line to my last post.
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Old 01-22-2012, 07:15 PM   #12
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How do you measure DC voltage without a negative DC line (smoke detectors)


Can the relays be unplugged? (socket type). Another good test.
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Old 01-22-2012, 07:21 PM   #13
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How do you measure DC voltage without a negative DC line (smoke detectors)


Sounds like you are picking up some induction (Phantom) voltage there.

It may not be an issue with everything connected ....
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Old 01-22-2012, 07:23 PM   #14
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How do you measure DC voltage without a negative DC line (smoke detectors)


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With both heat detectors disconnected, the smoke alarm doesn't alarm.
Sorry,,,,I missed this answer. That is the problem right there.
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Old 01-22-2012, 07:24 PM   #15
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How do you measure DC voltage without a negative DC line (smoke detectors)


The DC is used as a "Latching" current. Alarm sounds, the relay closes and sends the dc out to the others and the panel to complete the circuit. Alarm does not sound, no flow of dc current. Very simple. Works just like a water sprinkler system. no current, water does not flow to the heads. Energize the relay's to open the valves, water starts flowing through the system out to the sprinkler heads.

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