Coil Question
Cenlasoft
Posts: 265
Hello,
I want to thank everyone for the past help on the "Coil as sensor" post. I really have LC circuits and Resonance down pat. I can make LC circuits and I understand how they work. My question is:
I have a series LC circuit with a 560 pF capacitor and 1.236 uH coil. I used my signal generator and found resonance at 221 KHz. The voltage across the coil is 23.5 v at resonance. My next step was to convert the voltage to dc with a diode. I used Beau's inductance proximity circuit as a guide. I tried to use the pseudo peak detector (diode and .01 uf). The voltage (23.5 v) was supposed to be the same, but I get only 2.5 v DC across the capacitor (.01 uf). I measured this with both multimeter and scope. I must be missing something. When I get this solved, I will use my new Prop demo board to excite the circuit. I plan on using a 0831 ADC to measure the DC. The coil will act as a sensor to test metals.
Thank You,
Cenlasoft
I want to thank everyone for the past help on the "Coil as sensor" post. I really have LC circuits and Resonance down pat. I can make LC circuits and I understand how they work. My question is:
I have a series LC circuit with a 560 pF capacitor and 1.236 uH coil. I used my signal generator and found resonance at 221 KHz. The voltage across the coil is 23.5 v at resonance. My next step was to convert the voltage to dc with a diode. I used Beau's inductance proximity circuit as a guide. I tried to use the pseudo peak detector (diode and .01 uf). The voltage (23.5 v) was supposed to be the same, but I get only 2.5 v DC across the capacitor (.01 uf). I measured this with both multimeter and scope. I must be missing something. When I get this solved, I will use my new Prop demo board to excite the circuit. I plan on using a 0831 ADC to measure the DC. The coil will act as a sensor to test metals.
Thank You,
Cenlasoft
Comments
The diode and .01uF are loading the coil and inductor to the point that you are "de-tuning" your LC circuit. You MUST make this connection very high impedance to minimize the loading on your LC... a 10Meg 1.5Meg voltage divider from your signal will reduce the 23.5V down to about 3V. And the .01uF is too large try something near 100pF.
Give me a few minutes and I will draw up a schematic
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Philldapill·is correct ... Are you sure that's not 560 nF or 1.236mH·? either one would give you about 190kHz.
Anyway·attached is a schematic that shows how to read an inductor with·very·minimal loading on the LC tank.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Thanks,
The capacitor I have is from a Radioshack pack and its marked '561 1kv'. I measured it with an LCR meter and it was .560 nf. I converted the value to Farads which was .00000000056 F. Am I right? Its one of the few caps that in combo with my coil gives me good resonance. I remeasured the coil and its 1.236 mH. I will try the circuit you gave me. I am an archaeologist and went to St Louis, Mo. last week to give a paper on nondestructive testing of artifacts. It was a comparison of techniques. I mentioned Parallax and the propeller. Lots of interest. They have a lot of grant money. I will apply for some to see if Parallax products can be used on cultural materials.
I'll let you know about my results.
Thanks,
Cenlasoft
I just looked at my original post. The coil is 1.236 mh. sorry.
Thanks,
Cenlasoft
A measured 221kHz agrees much better with the calculated 191kHz ... Keep us posted on your results, and if there are any questions you have please feel free to ask.
"St Louis, Mo." - Hmm I spent many weekends in MIZZOU during the time I went to Westminster College in Fulton, Mo. MIZZOU has a very nice campus from what I remember.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.