Trouble shooting hvac controls

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JdoubleU

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In our green house we have windows that are controlled by the HVAC controller. The controller has a 24 volt ac output that energizes a relay that opens the windows. The window were not working so I went to check the output voltage of the controller. It read 24 volts even when it was not energizing the relay. The HVAC guy said it was because I was reading the bleed through of a triac. What does this mean?
 
It means that you are using a digital multimeter that has a high impedance input as nearly all do. Measure the voltage (across the coil) with the relay coil connected. Don't try to measure an open circuit voltage.

-Hal
 
Triacs, like most transistors, leaks a very small amount of current in the off state. I bought a Fluke SV225 which work very well when working on Triac output modules
 
GMc said:
Triacs, like most transistors, leaks a very small amount of current in the off state. I bought a Fluke SV225 which work very well when working on Triac output modules
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I have a fluke 117. Should I see some kind of change in voltage from my meter if I wanted to make sure that I was accually get my output voltage.
 
Try jumping a wire across the NO contacts to see if the shade opens,

nice way to half-split if you're nuking out on the controller.

Those outputs off from the triacs do need a resistance to bleed thru, which is a consideration with solid-state relays, but not mechanical kinds.

Check out the RIB MU1C.
 
Jakewhis said:
I have a fluke 117. Should I see some kind of change in voltage from my meter if I wanted to make sure that I was accually get my output voltage.
Click to expand...

If the 24v your reading now is from leakage current, placing the SV225 on your meter will drop the reading to 0v or near 0v.

Here is a link to the instruction sheet. It states it's only good for above 60v dc or 30v ac. I would guess that it would still work on your application.
 
That's why I still use my old analog Sperry Snap 6 if there is a funky control problem when diagnosing HVAC controls....had to learn that the hard way....Has anyone here made a thermostat go up in smoke?...I have.
 
Dude, did that little mercury bobble blow up??

I heard about a guy who did that to four thermostats in succession until he asked Home Depot why his tstats were blowing up!
 
stevenj76 said:
Dude, did that little mercury bobble blow up??

I heard about a guy who did that to four thermostats in succession until he asked Home Depot why his tstats were blowing up!
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It's actually very easy to do, most of the time it originates from having two 24v. sources bucking each other, heating controls/transformer vs. cooling. That's why most L.V. t-stats have Rc (cool) and Rh (heat) terminals, to isolate the power.
 
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