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Transformer Turns Question

Hi guys,

I need to count the number of turns of wire on this relay core I have. Trouble is, there are way too many to do so manually. The DC resistance is over 500 ohms and the wire gauge is *very* tiny; there must be a couple of thousand turns at least. I've thought of an ingenious way of counting them and I'd like to know if it will work in practice.

What I plan to do is wind say 10 turns of wire around the outside of this core, which is effectively just a solenoid of course. Then I feed this outer winding with say - for example - 10mV of AC at say 10kHz. I determine the voltage output level from the 'secondary' with an oscilloscope. From the ratio of p-p input voltage to p-p output voltage I can calculate the unknown number of turns on the core solenoid from the knowns: the input & output voltages and the primary no. of turns.

What do you think? Will this work in practice?

Reply to
Chris
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I think you'll have so much leakage inductance that the number you get will be very rough.

Other methods:

  • estimate the volume of wire, measure it's diameter, get the length, then estimate the turns from the area.

  • Weigh the thing, estimate the weight due to copper, measure the diameter, get the length, ditto.

--
www.wescottdesign.com
Reply to
Tim Wescott

D'oh. Measure the DC resistance, measure the wire diameter, use the conductivity of copper or a copper wire table to estimate the length ...

--
www.wescottdesign.com
Reply to
Tim Wescott

Whoa! "leakage inductance"?? Where's that coming from already? Concentrically-wound transformers are a perfectly valid topology AFAIAA.

Reply to
Chris

Not so simple. The windings aren't single layer; they're stacked about

1/4" thick which really fouls up the C=pi*D part of the calculation.
Reply to
Chris

That can work. It might be better to drive the relay coil from a signal generator and measure the added winding voltage as the secondary.

For more accuracy, measure the DC resistance and the inductance of the relay coil, pick a good frequency, and do the math to account for the copper loss. Or Spice it.

Don't expect an exact turns count, but you should be able to get into the ballpark. If you "close" the relay (push the armature down) there will be less flux leakage error and you might get within 1%.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

Certainly *safer* I guess!

Not sure what you mean by "a good frequency" - how do I determine that? I also thought maybe pulling the iron core out (if possible) and replacing it with a ferrite rod in case saturation might be an issue?

Reply to
Chris

I would add more than ten turns. Why? I don't know, I just feel you would have more accuracy with 100 or 200 turns.

Mikek

Reply to
amdx

Well, you need to do a little math and maybe some measurement. The frequency should be high enough that the voltage drop in the inductance is above the voltage drop in the copper resistance. And stay below the self-resonant frequency.

A nonlaminated iron core in a DC relay will have a lot of loss at high frequencies, too.

If you are willing to disassemble the relay, you could use a better, closed core. Or unwind and count!

Just curious, why do you want to know the number of turns?

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

If you drive it from both sides, and compare the ratios, maybe you could compensate for that?

Reply to
Tom Del Rosso

snip

Good advice. And place the turns right on top of the existing coil, and use a much lower frequency and watch the excitation voltage because what you will have is a high turns count step up ratio and you could easily short turns in the coil, especially if it was 'scatter wound'.

Assuming a ten volt 60Hz excite on a 100 turn primary, the secondary would be at 500 volts. It is a 50 to 1 ratio.

So, one could easily determine what the actual turns count is by simply reading the 'secondary' voltage and the ten volts into 100 turns makes all the math easier too. Keeping the freq low is better as well, especially if it is as was said a non-laminated steel core.

Reply to
DecadentLinuxUserNumeroUno

Don't answer that, Chris. It's a trap. ;-)

Reply to
Cursitor Doom

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