simple coil measurements

Thread Starter

richbrune

Joined Oct 28, 2005
106
I recently had a helper wind several coils for traffic detectors, essentially the same as the ones that trigger traffic lights. The coil is supposed to be 3 turns, 48" diameter, and placed into the roadway so that the three turns, (length) of the coil is a total of 3/8" high. The problem is that one of the detectors doesn't work, and I suspect that either my helper lost count or got lazy, and the the coil doesn't have the correct Number of turns on it.
I don't have an accurate enough resistance meter to determine if the total length of the wire is correct, as the resistance of the wire is very low.

1)Is there a SIMPLE, portable, circuit that I can build to check that the correct number of turns was wound, after the roadway is sealed? (I'd rather not pack an ocilliscope and signal generator out there and set up)
Thanks, Rich
 

Thread Starter

richbrune

Joined Oct 28, 2005
106
Yes, and the problem is the "lead in" to the coil is manually twisted for cancellation of "noise" that enteres the wires leading to the coil. That means that without knowing exactly how the lead in was twisted, I can't know what the total lenght of the wire would be in any case. Is there a circuit that can sense the difference in reactance between a coil with two turns and three?
Thanks again,
Rich
 

Gadget

Joined Jan 10, 2006
614
An inductance meter.... a Grid dip oscillator.... but I would suggest injecting high frequency AC signal into the coil, making a pickup using another coil, placing it near the centre of the buried coil, hooking the pickup to a meter that will read the AC voltage... and comparing the readings of the good coils vs the bad. (Sort of a loosely coupled transformer). If the voltage induced into the pickup is 2/3 the voltage induced from the good coils, then there is only 2 turns on that bad coil.
 

Thread Starter

richbrune

Joined Oct 28, 2005
106
Thanks, Gadget!

That sounds like a great idea. Should I shoot for the corner freqency of one of the coils as the "high frequency" AC signal? Also, if there is a vehicle over the loop, would that increase the coupling between the coils?

Thanks again, Rich
 
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