Discussion in 'Homework Help' started by bonkers, Dec 18, 2008.

1. ### bonkers Thread Starter Member

Dec 11, 2008
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Let me begin, by saying I didn't know there was a homework help forum here. I am taking an electrical course in appliances and refrigeration and there are a lot of questions on electrical troubleshooting. I have asked in other forums on this site and got some pretty good answers. I have another question that pertains to my homework, When testing for voltage, 120, across a switch or a timer, (and I am assuming a timer is the same as a switch,) to a load, would the reading read the same as just testing across the switch without the load. If open would it be 120 on the meter and closed 0 on the meter. I don't have a way to verify this, I have no equipment to work on, it is all guess work. Help appreciated.

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
1) If you have a switch which drives a load:

The voltage across the switch will be:

a) 120V (or whatever) when the switch is open (no current)

b) 0V when the switch is closed (current flowing)

2) if you have a switch with no load:

The voltage across the switch will be 0V in both cases (open or closed).

3. ### bonkers Thread Starter Member

Dec 11, 2008
14
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Must have taken you some time building that project of yours, cool, I like it. Now I am getting somewhere, O.K. now since the term on inductance and reactance was mentioned in an earlier discussion and how the readings on a voltmeter would be different betwen a resister and a switch, one having inductance and reactance and the other not, if we have a capacitor circuit on an ac line, capacitor in series with a load and a switch would the readings across the switch be any different than if there wasn't a capacitor in the circuit. Thanks, help appreciated.

Last edited: Dec 18, 2008
4. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,402
1,221
Bonkers,

Looking at the schematic in your second link, can you tell me if the Air Conditioner is cooling?

Do you know why the left hand CC relay contacts read 0 volts and the right hand contacts read 115 volts?

Why would the right hand and left hand CC relay contacts read 0 volts when the contactor changes state from closed to open?

5. ### bonkers Thread Starter Member

Dec 11, 2008
14
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In answer to your question, It looks to me like it is cooling, looks like all switches are closed and there is voltage across the compressor. I think the course I am studying on is not explaning voltage measurments very well. I'll need to upload an example circuit so I can clarify some things, but it will have to wait until after christmas. Thanks for help.

6. ### mik3 Senior Member

Feb 4, 2008
4,846
63
The link description is correct because the open switch is in parallel with the closed switch, thus essentially you measure the voltage across the closed switch.

7. ### mik3 Senior Member

Feb 4, 2008
4,846
63
What is inside TSTAT?

8. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,402
1,221
Bonkers,

Here is my commentary ....

I assume the diagram is valid for your system and I assume those contacts are not representing a solid state device controlled by a microprocessor to supply the 115V to the various parts of the unit.

T'Stat I take is thermostat ... I seriously doubt there is more than 24 volts AC coming from the Thermostat.

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9. ### bonkers Thread Starter Member

Dec 11, 2008
14
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See, not that easy is it? Not sure about what is in the TSTAT, but I believe it measures ambient tempeture inside room. I mean if you set it for heating mode, the the ambient temp inside room would control the tempeture sensing of the TSTAT, and would then turn on the RV VAlve. The RV valve redirects the freon flow in the opposite direction so that the outside coil is now the cooling coil and the inside coil is now the heating coil, in normal freon flow the inside coil is for cooling, and the TSTAT would be set for cooling and the RV valve would be denergized. The defrost control is for the condenser coil, outside coil. It energizes the RV, reversing valve so it will defrost the outside coil, this usually only happens when the outside coil frosts up, due to excessive cold tempetures that would cause poor air flow over the coil which would cause poor performance of unit. Inside the TSTAT, i don't know, other than it's function is to turn unit on in heating mode and cooling mode depending on what mode you want it in. I would guess the TSTAT is used to monitor inside room tempeture say if it is 55 degrees inside and you want to bring tempeture up to 80 degrees, setting the TSTAT in heating mode would probably bring the unit up to operating parameters of unit for the room to reach that tempeture, then when it reaches the proper heating tempeture I guess the TSTAT shuts off the unit or compressor then restarts the unit when the tempeture goes down again. In the illustration I just look at the TSTAT like a switch. there are many types of TSTATS, but just look at this one like a switch.

Last edited: Dec 24, 2008
10. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,402
1,221
Bonkers,

If that is a functional diagram, 115V will be at the points I mentioned. How the actual switching is done inside the device (t'stat and switch) is immaterial. And since it appears to be all solid state devices, the rules for voltage measurements acrossed open and closed switches do not apply when using that diagram.

11. ### bonkers Thread Starter Member

Dec 11, 2008
14
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I am trying to make a long story short. I have not checked on whether the TSTAT is a solid state device or not. The instructions never indicated that. By looking at the diagram, I assume it is a switch, and would work like a switch. But, when you say solid state device controlled by a microprocessor, no there is no microprocessor. The comments you provided I have allready assumed as true, but thanks for verifying. The test question asks two things. There is 0 voltage on the RV, doesn't ask which side. And, there is 120 volts across the contacts of the defrost control to RV. So, if I were to put the voltmeter across the contacts of the defrost control to the RV there would be a reading 0f 120 volts, so I would assume that there would be open contacts at the defrost control according to the laws of voltage measurements across a switch in a series circuit. Meaning-open switch=voltage across meter. Closed switch=o volts across switch. I know about the common measurements to the hot sides of contacts or from common to line 1 or neutral to line 2. As you state. As far as solid state devices and non solid state devices, that is not included in the study material. I will go over the material, and see if I can make a conclusion. Thanks! Appreciated!

12. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,402
1,221
It does.

For the purposes of your test, yes, I would suspect the defrost control. I do not like the fact that the Compressor is running, unless I'm missing something in the heat transfer cycle.

13. ### bonkers Thread Starter Member

Dec 11, 2008
14
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I just took some wild guesses, finished the dam thing and sent it in. I saw several answers to the tests questions. I'll wait and see what my score was, when sent back. Help appreciated. I'll let you know whether I passed or not. Thanks.