Diode across inputs to solenoid?

[Butter]

I was looking at the solenoid on a contactor today. It had a diode
across the two leads and I've been wondering why. The contactor is
used to turn on a Miller mig welder. Its rated for 120Volts. I
checked it when it was disconected and i got open one direction and
420 ohms the other. I don't remember what a typical value would be in
the closed direction. I put it all back together and found 19V AC
when the thing was running. Since its rated at 120 V I am a bit
confused about this situation.
What is diode there for? Should i be reading DC Voltage across the
solenoid?
Rosco

 

[DJ Delorie]

When you put current through an inductor (like the coil in your
solenoid), it builds up a magnetic "charge"[*], much like a capacitor
stores electric charge. Thus, it resists change to *current* much
like a capacitor resists change to *voltage*.

So, you energize the coil by putting a current through it. It builds
up its magnetic charge. You disconnect the power, but the coil wants
to make the current keep going anyway. You now have a fixed current
across a *very* high resistance. V = I x R, so you get a *very* high
voltage.

The purpose of that diode is to provide a *low* resistance in that
particular direction, so instead of getting a couple hundred volts
across your circuitry you get a couple milliamps through the diode.
Depending on the circuit powering the coil, different types of diodes
(or other devices) might be used for this purpose.

Basically, it "discharges" the coil safely when you disconnect power.

Also, note that contactors have ratings for the coil and separate
ratings for the contacts. Make sure you're reading the right ones.


[*] Yes, I know these aren't the right scientific terms, but they
offer a good analogy.