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
    0
    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
    2
    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
    I know about ohms law and reading circuits, I am no expert, mind you, but I know some. I guess since I haven't done any voltage tests, not many other than 120 on my house, it purplexes me somewhat. I know about amperage and ohms, or resistance checks, but the voltage is getting to me. Are you saying that when a switch is closed in series with the source and the load, when tested with a voltage meter, across the contacts of the switch, that the path of least resistance will stay across the switch instead of going through the voltmeter. Then a reading of o volts is read in the meter. This is the only thing I can figure on it. Now, if with the same series circuit and switch you test the contacts of the switch again with the switch open, you get a reading of 120 volts that the path of least resistance goes through the meter now, due to an open in the circuit, between the contacts of the switch. Does this also apply to a bimetal themostat. On to another point. In a series circuit, or paralell circuit, when a switch is in series on the nuetral or hot side of an AC circuit, you will have one side closer to nuetral than the other side of the switch. In another words, a switch that only opens and closes has one side more to the hot side and the other side more to the common side, but the common side of the (switch) may be in the middle of the conducting path or let's say a little closer to common on the line, but not right on the common intersection. I guess what your saying is the two node points, one on the switch and one closer to the common do not have to be eactly on the common side to get a voltage reading. In another words, in most circuits, the potential difference between hot and common aren't always close to the switch you are testing. And I am talking switches, not resistors, so when using a voltmeter, the lead on the common side of the meter, doesn't have to be on the common nuetral side leading back to the source, you can test for voltage between one contact of the switch to the other contact, and the switch itself doesn't have to be close to common from the positive side at all to get a reading on the meter. So, the meter will take over the path of least resistance in an open switch, when testing. Well, sounds confusing, me trying to explain the situation, but no I did not know that you can test between contacts on a switch in a series circuit and get a reading with the switch open, and no reading with the switch closed. So this tells me the meter takes the path of least resistance in a series open circuit and actually acts as an amp meter, but gives a voltage reading. Let me give an example, a regular light switch in the house is on the hot side of the wire. It opens and closes providing an open or closed contact to the light . If you place the leads of your voltmeter across the contacts of the switch when open, you will get a volt reading of 120 volts. Now if you close the switch you will get a reading of o volts. Can you explain this. I always thought you had to have one lead on the switch, positive side and one on the negative side, line neutral, to get a voltage reading, when taking voltage tests. If measuring voltage on the same line, between contacts of switch on the positive side there would be no reading of voltage, open or closed. Info appreciated. thanks.
     
    Last edited: Dec 11, 2008
  4. ssherwood

    New Member

    Aug 25, 2008
    9
    0
    (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.
     
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