Messing around with coil frequency

[Eleven Cents]

Just out of curiosity, I decided to measure the resonant frequency of the stock coil on my Bounty Hunter Prospector. I used a voltmeter that can handle up to 50 kHz signals, and set the signal generator for a sine wave at 5 V peak to peak. Without the coil connected, the meter was showing about 3.5 V RMS, which seems about right.

As expected, with the coil connected in parallel, the voltage reading varied with frequency. At 1 kHz, it was only reading around 1 V. The maximum voltage was 3.379 V at 7.5 kHz. I've read that older BH detectors run at 6.6 kHz, so it seems my coil is out of tune. But at 6.6 kHz, the voltage was 3.355 V, a difference of less than 1%. So maybe coil resonance isn't very sensitive as long as it's close. I have a 4" coil on order, so I'll see what that one reads next time I go to the hackerspace.

I also tried measuring the voltage on the output pins of the control box, but I couldn't get a signal. The meter would show around 2 V for a split second, then go back to near zero. Maybe the detector sees that the coil isn't attached and shuts itself down. I might try building a breakout that can go between the coil and the control box.

 

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[Terry Soloman]

If you have this kind of time on your hands, I need my car washed.

 

[Loco-Digger]

I'd stop playing with the meters and get to swinging the coil. The 4 inch coil may help in trashy areas, but does not go very deep, maybe 4 to 5 inches on a quarter.

 

[Eleven Cents]

I'd stop playing with the meters and get to swinging the coil.

I wish I could! Lately my free time has only been at night.

 

[Carl-NC]

Just out of curiosity, I decided to measure the resonant frequency of the stock coil on my Bounty Hunter Prospector. I used a voltmeter that can handle up to 50 kHz signals, and set the signal generator for a sine wave at 5 V peak to peak. Without the coil connected, the meter was showing about 3.5 V RMS, which seems about right.

5vpp is 1.77vrms.

As expected, with the coil connected in parallel, the voltage reading varied with frequency. At 1 kHz, it was only reading around 1 V. The maximum voltage was 3.379 V at 7.5 kHz. I've read that older BH detectors run at 6.6 kHz, so it seems my coil is out of tune. But at 6.6 kHz, the voltage was 3.355 V, a difference of less than 1%. So maybe coil resonance isn't very sensitive as long as it's close. I have a 4" coil on order, so I'll see what that one reads next time I go to the hackerspace.

Was this on the TX coil or the RX coil? How did you connect the meter & generator?

I also tried measuring the voltage on the output pins of the control box, but I couldn't get a signal. The meter would show around 2 V for a split second, then go back to near zero. Maybe the detector sees that the coil isn't attached and shuts itself down. I might try building a breakout that can go between the coil and the control box.

I think on the Prospector the TX circuit won't run at all without the coil attached.

 

[Eleven Cents]

According to several sources I've found, vrms = vpp * .707

 

[RobRieman]

Vrms= .3535 * Vpp

 

[Terry Soloman]

Gosh, this is fun..

 

[Loco-Digger]

Too technical for me guys.

 

[RobRieman]

Damn Terry....you have them both in a picture even. Lol

 

[digger460]

Gosh, this is fun..

Terry, I think it's showing an enemy submarine at a thousand yards off your bow.

 

[nsdq]

I forgot all this stuff i don't use my brain for elect stuff anymore

 

[Eleven Cents]

I repeated the experiment with my 4" coil. Its resonant frequency is very close to 7.7 kHz. Maybe BH is making all their coils for their 7.7 kHz models and treating them as good enough for the older 6.6 kHz detectors. As with the stock coil, the difference in peak voltage when driven at 6.6 kHz was less than 1%.

 

[Eleven Cents]

Or maybe it's out of tune on purpose. A post on another board says, "Most [single frequency] detectors have the coil tuned off to one side of the running frequency to maintain phase linearity." I have no idea what that means.

 

[Carl-NC]

Or maybe it's out of tune on purpose. A post on another board says, "Most [single frequency] detectors have the coil tuned off to one side of the running frequency to maintain phase linearity." I have no idea what that means.

The RX coil, not the TX coil.

 

[Eleven Cents]

Thanks for your replies, Carl. As you can tell, I pretty much have no idea what I'm doing.

I built a breakout box with a DIN socket and banana sockets to make testing easier when I start trying to build my own coils. I had my 4" coil with me when I built it. With a nominal 5V from the signal generator, the transmit coil read 3.3333 V at around 7.5 kHz. The receive coil read 0.001 V in air, and ten times that when placed on a metal table.

I'm using an old HP 3456A voltmeter. I finally got around to reading the manual, which says it needs to be turned on for an hour before using it. I guess that's so the temperature can stabilize. It seemed to make a small difference in my readings.

 

[Professor of Engineering]

Hello Eleven Cents,

Fun experiment!

Vrms = [1 / sqrt(2)] * Vp or Vrms = 0.707 * Vp

This is for a sine wave signal a square wave has a different expression.

My students love the first semester: DC circuit theory, but when second semester: AC Theory comes around they quickly realized they should have paid more attention (Viva Laplace) in calculus class.

GL & Happy Experimenting

Doc

 

[Carl-NC]

11¢, it would help a lot if you could post even a crude schematic of what you're doing, then I could give better feedback. Generally, to measure the resonant frequency you want to have a series resistor between the sig gen and the coil. Geotech has lots of info on building coils.

 

[Eleven Cents]

I'm not well versed in software for drawing schematics, but I can describe it. I have a 5-pin, 240 degree DIN socket into which I'm plugging my coil. Pins 1-4 of the DIN socket are connected to four banana sockets with soldered 22 ga stranded wire. The pieces of wire are 1-2 inches long and run separately through a small plastic enclosure. I tried to keep the wires as far from each other as possible to minimize interference. I did not connect pin 5 to anything. I believe it's connected to the shielding of the coil wire.

When attempting to measure the resonant frequency of the TX coil, I ran a BNC-to-banana cable from the signal generator to the TX coil banana sockets in my breakout box, then stacked another set of banana test leads into the first and ran them to the voltmeter. When attempting to measure the induced voltage in the RX coil, I moved that second set of banana leads to the RX coil sockets.

What value resistor would you recommend in series with the TX coil?

 

[Carl-NC]

10k or so should work.