I'm building a magnetic levitation analog circuit. I realize there's a handful of ways to do this and I came up with my own idea ( which is partly why is doesn't work ... ) and would like to continue exploring this idea even if later it isn't as stable as other designs.

The setup is a IR LED and photodiode ( setup reverse bias ), that goes to an inverting amp ( IC OP07 what i had..), which then heads off to a push pull mosfet driver ( IRF 510 and IRF 9510 ).

My electro magnet is about 50 ohms wrapped around a pipe with ferrites in it. I added a lot of ferrite which I regret since the field doesn't change directions very fast.

I skipped the differentiator ( would like to try later ).

Power supplies are +15 and -15 VDC, with current limiting on the -15 since it's a wall wart. The positive supply, is good up to 10 A, it's a beast.

The gates are getting appropriate pos and neg voltages. And, with a pure resistive load ( 75 Ohm ) , the current drawn is what I'd expect. However, when the coil is connected, only the P channel mosfet draws current.

I'm wondering if the inductance of the coil is in causing this ?

UPDATE: I didn't include the IR LED in the diagram. It runs off the 15 V rail with a 1K. I have -0.03 V on my meter, when I check the ground. I have assumed this was from the photodiode. That's why I put the small ref volt on the OP1'a pos input.

On my meter, I do see a +15 to -15 V swing on the output of OP1, as I block and unblock the photodiode.

enter image description here Updated schematic fixed per gsills comments.

  • 1
    \$\begingroup\$ A schematic would help far more than a word-picture does. \$\endgroup\$ Commented Jan 12, 2014 at 8:36
  • \$\begingroup\$ What Anindo said BUT it sounds like your circuit is not correct - odds are drive to FET gates is not going +ve for whatever reason. SO - Circuit diagram please. \$\endgroup\$
    – Russell McMahon
    Commented Jan 12, 2014 at 10:44
  • \$\begingroup\$ Schematic coming up... \$\endgroup\$
    – wbg
    Commented Jan 12, 2014 at 19:30

1 Answer 1


If you are trying to use the FETs as a source follower current buffer for the Opamp, the sources of the FETs need to be connected to each other and the coil.

The IRF9510 is upside down as drawn in your schematic. That would allow current to flow through the coil (or N FET if it is turned on) and through the body diode of the P FET. This will happen no matter what the state of the gate of the P FET is.

After you correct the orientation of the IRF9510 the circuit should be able to apply about +/-11V across the coil. Gate to source threshold voltage of these FETs is about 4V, so that will limit the voltage range of the source to 4V less than the bias voltage. Also, since the two gates are tied together there will be a large area for crossover distortion between conduction of the IRF510 and the IRF9510, which may or may not be a problem for you.

Edit: Also just noticed that the ground reference at the batteries in the schematic must be wrong. Ground should be at the battery +/- common point to get +/-15V.

  • \$\begingroup\$ Thank you for catching the newbie errors. I'm not concerned with the cross over distortion since the voltage swings very rapidly from abt -11 to +11 very quickly. This is crude circuit for sure. Once it "works" and I can try to balance the target magnet, then I'll revise for functionality. My background is in physics and my electronics courses were minimal so I'm happy to relearn basics and build my fundamentals right now. \$\endgroup\$
    – wbg
    Commented Jan 13, 2014 at 2:13

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