# voltage tests and thermostats ac circuits

Discussion in 'General Electronics Chat' started by bonkers, Dec 11, 2008.

1. ### bonkers Thread Starter Member

Dec 11, 2008
14
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I have been checking out info on voltage tests for air conditioner circuits. And I am a bit perplexed. I will admit, I am not an expert at voltage tests. Anyhow I always thought you measure voltage from line in to line out. Positive to negative. The info I have been reading is telling me to measure a switch or thermostat across a line in series. One end of thermostat to the other end with the thermostat in series with the line. A reading of 120 volts means circuit is open, reading of o volts circuit is closed. How can you get a reading with voltage in series with the switch. I always thought voltage was tested in parallel across from line in to line out. Not across a switch in series say on the same line. Thanks! Info appreciated.

2. ### leftyretro Active Member

Nov 25, 2008
394
5
Well the instructions are correct. And you are also right in that MOST voltage measurements are performed with one meter lead on circuit common (or ground or neutral if you like) and the other meter lead used to measure for voltage at various parts of the circuit. Not sure what you mean by line in and line out, not the best of terms I think for this discussion.

One thing you might try is to draw a simple circuit of a battery, a switch and a lamp all in series. If the switch is closed of course measuring across it's switch contacts will read zero volts. Now draw the switch with open contacts and then think what your meter will read? A voltage. If you look at your drawing and trace the wiring back to the source voltage you will see that you are indeed measuring parallel with the voltage source. I could go on and explain that your voltmeter most likely looks like a 10meg ohm resistor to the circuit and that is so large compared with the resistance of the other components that for all practical purposes that the total circuit voltage will be measured by the meter.

Learning ohms law is one of the best first topics to help one understand basic electronics circuit behaviour. Almost all the rest of electronics theory builds up from there.

3. ### bonkers Thread Starter Member

Dec 11, 2008
14
0

Last edited: Dec 11, 2008
4. ### ssherwood New Member

Aug 25, 2008
9
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(Assuming 120vac wiring) Bonkers, maybe I can state it a different way. When a switch is closed, both sides are hot, so there is no voltage potential from leg to leg, so you read 0 vac.
When it's open, one side's hot and the other not, so often you will see a potential and will read your 120vac...but that assumes the rest of the downstream wiring is completed to ground or neutral -not always the case for HVAC equip.
I do not often use this technique, I just take one end of my meter to ground and follow the hot, because there are situations where there are more switches, relays, etc. not closed, which can throw you off with the first technique.

Having said this though, sometimes you read voltage through relay coils, motors, and other devices -to ground, which can trick you into thinking a circuit is completed when in fact it is not. So method 2, testing to ground, is not always functional either. One needs to be aware of which types of devices you are testing and how they work. Schematics are very helpful for T-stat, HVAC troubleshooting.

Hope this helps.

Last edited: Dec 13, 2008
5. ### bonkers Thread Starter Member

Dec 11, 2008
14
0
Thanks for info, I pretty much got it figured out. Though i have a slite different view on it. I look at it, now, like, the meter is in paralel with the switch, and if the switch is open, the meter takes the path of least resistance, or maybe not least resistance, but since the meter has a high resistance, not sure on the exact resistance of the meter, but since the path of current has no where to go, obviously it must go through the meter on an open switch. But, I had always thought that amps could only be measured in a series circuit, not voltage, that is in the same situation. I always thought you had to measure across the line voltages, positive and negative, not between a switch or resistor, for voltage that is. One thing that I think is of importance in the tutorial is to show that the meter is in paralel with the switch, there is no mention of this, other than the meter must be in paralel across the switch, but the diagram shows the meter across the switch in a paralel circuit, not a series circuit, and the diagram that does show the series circuit with the meter and switch does not explain that the meter is in paralel with the open or closed switch, and some newer techs like myself, might not understand it, or notice it, but I am glad I did figure it out. thanks for help.