Collapsing field on interposing relay.

ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Measuring the voltage on a small IEC contactor with 24VDC coil. I was using three recycled 9V batteries as a source. Contactor pulls in and holds at about 18 volts. Remove power and the voltage peaks at about 32 volts on the 100ms scale of a Fluke 87. Min/Max. Switch to 1ms and the meter indicates an overvoltage greater than the 4000v range.

Can I assume that the 100mv peak will go up proportionately with the applied voltage? 24 VDC applied would result in about 43?
Do I need to worry at all about the 1ms reading?
 
Notice ya got no replies yet.

Vpeak = Inductance * peak coil current divided by time it take to open the circuit. It is all the rate of change of current and inductance.

peak coil current is Vdc/coil resistance

time to open depends on how fast you pull the contacts apart. FET turn off can be as fast at 40 nanoseconds.

Let us say your coil current is only 100 mA, and typical coil inductance is 10mH
Vpeak = 1E-2*1E-1/40E-9 = 25kV.
So, ya obviously NOT opening the circuit with a fast FET, more like taking a 1/4 usec for the current to drop to zero as arc ... typical af pulling apart contacts fast by hand.

Put a backwards diode across the terminals, voltage will clamp at a few volts, the current decays very slowly, in this example case,
L/R = 10mH/270 ohms = about 1/3 millisecond. The coil current hold the relay closed about that long also.

Carry the L/R to extremes, a superconducting coil and low R diode, the relay may take many seconds to open after voltage removed.
Short the superconduting coil with a superconductor an relay will /latch/ closed.

etc etc.

So Can I assume that the 100mv peak will go up proportionately with the applied voltage?
Thus, yes, since the current goes up proportionally with coil voltage at dc. Assuming you remove the applied voltage at the same speed. Note that in the old points ignition in a car, a capacitors is used across the points to 'quench' the points arcing That arcing slows the opening time to milliseconds. Transistorized ignition systems have fast opening times, as fast as the 40 nanoseonds previously mentioned.
 
Last edited:
This reminds me of a 1970's - "Chattering Relay" tester that we used to use for early EMC (ElectroMagnetic Compatibility) testing.
There were several relays connected in a cascading fashion , where one relays contacts fed the next in line. The relays would then make a hell of a racket as they Chattered on and off. As each turned off it would generate high amplitude transients which were then coupled into the device under test. More modern testers are now referred to as EFT (Electric fast transient) generators.
 
ptonsparky said:
"I'll never need to use algebra. What the heck is x?" says I back in 9th grade.
Click to expand...
I just ran across a report that said only half of Americans can read at the sixth grade level. At this point I think I would be happy if when people got out of high school they could do sixth grade arithmetic, much less algebra.
 
You must log in or register to reply here.
Top