[hans a]

I happen to have lots of old Omron relays. Im want to learn about RCL. Could i try to measure a coil and use it in an EQ?
Br Alex 

Edit; Found this so i guess i have a reason to get my Oscilloscope from the studio.
https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3344_AppNote&DocType=CS&DocLang=EN

[trobbins]

I have used a relay coil for a simple notch and bandpass single-pot filter for a small guitar amp - just glued the armature to the coil core to maximise inductance and got 0.2H, which resonated well with 68nF.

You should use a simple inductance measurement with the expected operating signal voltage (ie. no DC as per your link) if the inductor is to be used in an AC filter circuit.

[squarewave]

I would think they would distort pretty easily and there's sometimes a diode across the coil. But depending on the application that might not be undesirable. Higher voltage ones would have higher inductance and lower voltage ones would not distort as much.

What I would like to see then is an bank of them tied to a keyboard that adds DC causing the relay to close / open which causes the inductance to increase / decrease (by quite a bit I would imagine) and shift Fc. The slight non-frequency dependent and slightly random delay between pressing the key and the relay switching could be used to add an attack to the envelope (albeit very short 1-2ms without the aid of a cap). And then the two poles can switch in / out two completely independent signals. I'm not a synth guy but that sounds like a cheap way to do something interesting.

[hans a]

I have used a relay coil for a simple notch and bandpass single-pot filter for a small guitar amp - just glued the armature to the coil core to maximise inductance and got 0.2H, which resonated well with 68nF.

You should use a simple inductance measurement with the expected operating signal voltage (ie. no DC as per your link) if the inductor is to be used in an AC filter circuit.

Thanks, very good info. I have deconstructed the ones i have, but i will try with the whole armature next time. I will read up on AC-measurement.

[gyraf]

It will work, to a degree.

So will power transformers.

And audio transformers (remember you can connect pri and sec in series for increased inductance)

/Jakob E.

[hans a]

Thanks Jakob.  Ok, i will try to test.
Im going to set up a little test lab. An oscilloscope, a signal generator. Can i maybe use a SWR meter to measure resonance of the coil?
Br Alex

Edit; Seems like you can.

[trobbins]

Simplest calculation for L is just Vac across the coil and Iac through the coil.  A slightly more complicated calculation accounts for the coil DCR  to extract the inductance from the measured impedance.  Try to apply the expected AC voltage across the inductor.  Try not to use too high a test frequency, otherwise the shunt capacitance of the coil will modify the impedance - the first coil resonance frequency will indicate what is too high. 

I used either an Omron G2R or Finder 40 series or similar style relay, and just cut away the contacts and top casing, and left the base and coil terminals, as I need a small footprint.  I do have lots of pot cores and formers, but thought I'd try the relay coil.  The DCR is in the datasheet.

[gyraf]

..just remember that magnetic shielding is very, very poor...

/Jakob E.

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