Voltage question

Cenlasoft

Hello,
I used a series LC circuit (560 pF cap and 2.14 mH coil) and excited it using a prop demo board. My scope reads 55 volts peak to peak and 20 volts RMS. Everything is fine until I use a diode to convert from AC to DC in order for me to use an ADC8031 to read the results of the voltage change when I bring a nonferrous metal near the coil. The coil is actually a probe. When I measure the voltage with the scope at the diode, I only get about 3 volts DC. I have checked everything I think and I even used a signal generator to excite the LC circuit. Thanks for any help.
Cenlasoft (Curtis)

mctrivia

what is the peak to ground voltage without the diode?

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
My new unsecure propmod both 1x1 and full size arriving soon.

Rayman

Sounds like maybe your AC to DC conversion circuit is loading down your resonator (essentially lowering the Q)...

If so, you can use an opamp or maybe just a series impedance to increase the load impedance...

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
My Prop Info&Apps: ·http://www.rayslogic.com/propeller/propeller.htm

Cenlasoft

Thanks,
I'll try the load resistor.
Curtis

Beau Schwabe

Cenlasoft,

Rayman is correct... you need to create a very high impedance voltage divider here before you take any measurements... typically I use a 10 Meg resistor in series with a 1 Meg resistor here... The 1 Meg resistor goes to GND, while the 10 Meg resistor would go to your coil/cap. Place your scope between the 10 Meg resistor and the 1 Meg resistor. The ADC should have no problem with the high impedance, keep your wires as short as possible though from the (coil/cap/10Meg/1Meg/ADC)

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Cenlasoft

hello,
I tried the high impedance voltage divider and the voltage now reads in the low millivolts range.
My circuit is as follows: The prop pin excites he LC circuit ---> Cap/Coil/10 meg/1 meg/diode/vss
I tried different test points; across the coil, at the voltage divider junction, across the cap. across the coil I get 51 vPP, 21 vRms, when I put diode and no voltage divider, I get 3.5 v DC.
Thanks,
Cenlasoft

kwinn

Can you post a schematic with the point where you measured with the scope indicated?

Philldapill

Sounds like whatever you are measuring the voltage divider with, has a low impedance by itself. Basically, you are creating ANOTHER voltage divider with your measurement device. The best thing to do, is just buffer the signal with an opamp to begin with.

Cenlasoft

Hello,
Thanks for your reply. These are my quick circuits and test points.
Cenlasoft

kwinn

Most scopes have a 10 meg input impedance, so if you connected the 10 meg resistor to the diode and the one meg from ground to the 10 meg you should read about 1/20th (half because it is rectified, one tenth because of the voltage divider) or about 2.7V peak on the scope. Being half wave that would be about 0.95V RMS.

Is the diode a signal diode? A rectifier diode for the AC line is too slow for this.

Can you scope the voltages at the 3 points in the attached drawing?

Cenlasoft

Hello,
I just received a new two channel digital oscilloscope today. Its a BK Precision 2530. I tried the LC circuit and it was amazing. Everything was what it was supposed to be. At resonance, the circuit (prop/cap/coil/diode) gave be 54.4 V DC. My other portable scope was the problem. I still want to maybe use a 741 op amp to buffer the old scope from the circuit as was suggested. I want to thank everyone for their expertise and generosity. I will now divide the 54.4 v dc and send the output to a ADC0831. Any suggestions would be appreciated. I will post and let everyone know of my results.
Cenlasoft