Impedance matching with a 20KHz signal

Thread Starter

8dm7bz

Joined Jul 21, 2020
161
Hello,
I have problems with driving a coil with a 20KHz signal. I made some tests and will attach them to this thread. My setup is as follows: An AD9833 is used to produce a 20KHz 5Vpp square wave. Now someone on this forum told me to either use a buffer amplifier or a mosfet to drive a coil. The coil in question is one of those. It is 3 coils wrapped orthogonal to eachother. At the moment I only try to drive one.

I tried the mosfet variant and ended up with the results in the attachment. The mosfet basically peaks to a very high voltage and isn't stable at all.

Then I tried using an op amp. First thing I noticed is that it has some problems with slew rates, I think that is what it is. And the voltage across the coils are unsatble as well.

If you have any opinions on what to change, I highly appreciate them.

My goal is to drive one coil only. I will time multiplex the signal in the end.

thanks,
8dm7bz
 

Attachments

Audioguru again

Joined Oct 21, 2019
2,090
Why didn't you look at the datasheet for the IRF540? It shows that it needs a gate-source voltage of 10V but you have only 4.7V.
Some of them barely turn on when the Vgs is 4V.
Use an IRL540 instead because the "L" means that its gate-source voltage is 4.5V logic-level.
 

Thread Starter

8dm7bz

Joined Jul 21, 2020
161
Excuse me @Audioguru again that was my bad. The schematics actually don't reflect what parts I used. I just made the schematics afterwards and forgot to mention the parts in use.

The mosfet that was used is the IRFZ44N
The op amp is a LM358P

my bad
 

Audioguru again

Joined Oct 21, 2019
2,090
Excuse me @Audioguru again that was my bad. The schematics actually don't reflect what parts I used. I just made the schematics afterwards and forgot to mention the parts in use.

The mosfet that was used is the IRFZ44N
The op amp is a LM358P
Again, the IRFZ44 Mosfet might work if you are lucky to get a sensitive one. Use a IRLZ44 instead. See the "L"?
The LM358 is too slow (it is low power) to make a good 20kHz squarewave.
 

Attachments

BobTPH

Joined Jun 5, 2013
2,602
Why are you trying to drive a coil with a square wave? A coil has lot’s of inductance and the current in it cannot change instantly like a square wave does. What are you trying to do? If you want the max current through the coil, you would put it in series with a capacitor to make it resonate at 20KHz, but then it would turn into a sine wave. Is that what you want?


Bob
 

Thread Starter

8dm7bz

Joined Jul 21, 2020
161
@Audioguru again thanks. I think I got lucky then, because the mosfet is switching correctly. But after more experimentation, I think I will abondon the mosfet and take an op amp

@BobTPH yes that is exactly what I want, the end result should be a sin wave. Do you have any suggestions how I would do that at 20KHz. Like what are the calculations I have to do ?
 

BobTPH

Joined Jun 5, 2013
2,602
You put a capacitor in series that has the same impedance (actually the negative of the impedance) as the coil. So you have to know the inductance of the coil. Then use the formula for a resonant LC circuit:

f = 1 / (2 pi sqrt(LC))

solve for C and plug in the f (20000) and the L in Henries, and you will get C in Farads.

Note that this will create a zero impedance, or rather, a very small one, so you may want to detune it a little to control the impedance.

Bob
 

Thread Starter

8dm7bz

Joined Jul 21, 2020
161
@BobTPH thanks for the help. I just realised that that is not the thing I want to do. I'm still very unexperienced in electronics. I will move to the solution @Ian0 suggested over in this thread.

thanks a lot though :)

8
 
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