# Simple electrical question

• posted on July 17, 2007, 4:33 pm
I have a clothes dryer with a 120VAC push to start button. In measuring the terminals, both sides are hot (120V) with respect to ground and zero volts across the two terminals. Why is that?
I thought there was a short on one side of the push button but the dryer is working properly. Normally it should be hot only on one side.

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• posted on July 17, 2007, 5:32 pm
** Frank ** wrote:

What are you measuring those voltages with?
If it's a meter, what kind, digital or mechanical movement?
Can you locate a schematic diagram of the machine wiring? That should tell the tale.
Jeff
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Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)

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• posted on July 17, 2007, 5:42 pm
Jeff Wisnia wrote:

Could be there's an electrically latched relay is the setup which closes a set of contacts that "short" your push to start switch. When the dryer is stopped by the timer running out or someone opening the door, power to the starting circuit is cut off and the latching relay drops out.
Capice?
Jeff
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Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)

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• posted on July 17, 2007, 5:33 pm
** Frank ** wrote:

First guess is there are more terminals and you're measuring two inputs or an auxiliary feed/interlock.
Second is perhaps you're using a high-impedance meter and one of these is a so-called "phantom" voltage...as Jeff is getting ready to try to determine. :)
Try a neon tester or a light bulb on a set of leads and see if the voltage is still there on both terminals if there really aren't any others.
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• posted on July 17, 2007, 10:34 pm
On Tue, 17 Jul 2007 09:33:24 -0700, "** Frank **"

Any chance it's a normally closed pushbutton? If you press it when measuring across it, does the voltage go from zero to 120?
Seems odd that they would use a NC switch, but that's the only thing that explains what you measure...
Paul F.

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• posted on July 17, 2007, 10:40 pm
On Tue, 17 Jul 2007 18:34:05 -0400, Paul Franklin

Paul, you correct. There is a normal situation, no volts across switch is a closed switch. With switch open there should be 120v measured across contacts. jesse

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• posted on July 17, 2007, 11:37 pm
Jesse wrote:

If it were just a mechanical contacts switch...I think those buttons control a relay, not a mechanical switch in on my GE I know there are some other interlocks besides just two terminals. So, as noted before, it's possible on at least some that he isn't measuring what he thinks he is...
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• posted on July 18, 2007, 5:36 am
Paul Franklin wrote:

Nope; could be N/O with a holding-on contact in parallel; this is a common arrangement in electromechanical control systems.
--
"Take sides! Always take sides! You may sometimes be wrong - but the man
who refuses to take sides must *always* be wrong! Heaven save us from

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• posted on July 17, 2007, 11:53 pm
On Tue, 17 Jul 2007 09:33:24 -0700, "** Frank **"

If the start button works like it is supposed to then you are mistaken about the measurements.
For you to get 0 across the switch and 120V on both sides of the switch the switch has to be closed.
If your measurements are correct then the switch is shorted closed which would cause the dryer to constantly be in the start position.

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• posted on July 17, 2007, 11:56 pm
** Frank ** wrote:

We could make more intelligent guesses if the OP told us whether those measurements existed with the dryer running, off or either.
Jeff
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Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)

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• posted on July 18, 2007, 12:51 am

The push button is normally open. Measurements existed with dryer off. Push to start means the contacts short across the terminals. I use a very old but reliable analog Simpson 260 meter for the measurements.

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• posted on July 18, 2007, 2:07 am

I think, as some other posters have mentioned, that once the button is pushed the timer, a relay, or a centrifugal switch on the motor closes contacts in parallel with the switch. This keeps the dryer running after you release the start button. It's a common system used in motor controls with seperate start and stop momentary pushbuttons. However, this would require your measurements to have been made with the dryer running. Basically, if there is zero volts across a normally open switch, closing it cannot do anything, as would be required for the dryer to start when the button is pressed. You might want to check your measurements again. There could be a schematic diagram somewhere inside the dryer.
Don Young

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• posted on July 18, 2007, 3:58 am

The schematic shows the PB is in parallel with the motor relay NO contact and a solid state circuit board called "even heat control". After start up, the PB is sealed by this motor relay contact providing 240V across the motor windings. No ideal why both sides of the contact are at the same potential before start up - goes against theory. Too many other things to do right now but when I get around to it I'll open the control panel up and measure it again.

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• posted on July 18, 2007, 4:13 am
** Frank ** wrote:

Looks like you and I both have curious minds....
And als Simpson 260s...Mine has been with me almost 50 years now. <G>
Do let us know what the answer to the mystery turns out to be.
But my curious mind also wants to know what prompted you to make those measurements in the first place... <G>
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)

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• posted on July 18, 2007, 1:53 pm

I inherited the Simpson 260 along with the Health Kit scope from my dad - he had it as long as I could remember, perhaps after WWII.
As for what prompted me to measure to make the measurements, the wife told me to fix the dryer as it didn't run. It turn out the thermal fuse was blown.

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• posted on July 18, 2007, 4:18 pm
** Frank ** wrote:

I inherited my dad's jeweler's hand tools....
And I still get a little "farklempt" whenever I have an occasion to use one of them and see his initials scratched onto it somewhere.

Obstructed vent I'm thinking....<G>
Jeff
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Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)

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• posted on July 18, 2007, 4:27 am
** Frank ** wrote: ...

I'm thinking you're then measuring a voltage from that board...
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• posted on July 18, 2007, 1:53 pm

Could be, but if both sides of the PB contacts are at the same potential (also same phase) when its open then there won't be any current flow across these contacts into the motor windings when its closed. I must have missed too many classes during Circuit 101.
The neon tester was a good idea.

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• posted on July 18, 2007, 2:07 pm
** Frank ** wrote:

Yeah, but I'm guessing one side or the other will disappear when whatever logic on that board is activated or that it is, in fact, just a phantom held by a high impedance looking back into the output of the board...
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