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.
Not really a simple question...Your description leaves me scratching my
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.
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.
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
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.
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...
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
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.
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.
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.
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
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>
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
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.
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
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