Voltage to current converter circuit

[Bharanidharan]

hi,

I am interested to know if 5V AC signal can be converted to 200mA current and fed to a inductor. Can someone help me with designing this circuit please? or give me idea on how to do this?

Thank you very much.

Bharani

 

[(*steve*)]

Well, a 25 ohm resistor will convert 5V RMS AC into 200mA RMS AC.

However I suspect that's not what you want to do.

Perhaps you can explain what you want to achieve, not how you think it can be achieved.

 

[Bharanidharan]

I require to excite an inductor of value 135uH using current values between 200mA - 1 A.
I am using a waveform generator that is connected to a opamp which produces voltage of 2.5V. this is then sent to the multiplexer and then to the inductor array( 16 coils totally, 8 coils transmitter, each switched using the MUX individually ).

Multiplexer current limit is 20mA, so I require an constant current source after the multiplexer that produces the current of 200 mA to 1A. and after this the induced voltage on the receivers need to be sent back to the microcontroller using the multiplexers again, and i believe this increase in current might blow up the multiplexer....

So how can i design this ? the ac source has a frequency of 20kHZ supplied to it. Also, please recommend a free simulation software.

Thank you.

Bharani

 

[(*steve*)]

20mA and 2.5V is plenty to turn on a darlington pair. Presumeably the waveform is a square wave? And it varies from 0V to 2.5V?

Check out the resource describing how to use a transistor to switch a load. In your case the load is the inductor.

At this point I start losing track of what you want to do. I've ignored the multiplexer stuff (other than the current limit). I'm not sure where the induced voltage comes from...

Perhaps (as I suggested before) you should tell us what you're trying to achieve, not how you think it can be achieved.

 

[Bharanidharan]

I am trying to analyze a metal piece in between an circular inductor array. I use about 8 inductors and use 4 of them as transmitters individually by switching and 4 of them as receivers (same as in transmitters) . All I require is a constant current source that can produce 200mA - 1A from a 2.5V at 20KHz sine wave frequency. The analysis is using a self developed software.

If you have any further questions let me know.

Thanks

 

[(*steve*)]

Are you saying you want to put a constant current through an inductor (or series thereof) switched at 20kHz?

If so, you can't do that. You can't instantly change current in an inductor (well, not without an infinite voltage).

 

[KrisBlueNZ]

Perhaps he means he wants to feed a 20 kHz sinewave into each inductor, with an amplitude of between 200 mA peak and 1A peak?

What is the inductance? The DC resistance?

How closely coupled are the other inductors? How much energy will an inductor receive from the inductor opposite it?

Will the four "transmitting" inductors all be driven with the same signal?

 

[Bharanidharan]

Sorry, if i was confusing Steve, Kris is right. I am trying to feed the inductor at 20Khz frequency of current peak between 200mA - 1A. The inductance is of 135uH and DC resistance is of 3 ohms.

The inductors are placed at equidistant of 1cm from each other. If the peak current being supplied is 200 mA, thenI suppose the energy will be 5.4UJ.

And yes, the four tranmitting inductors will be driven with the same signals.

I think, it is kind of converting the AC voltage to AC current to drive the load.

Hope this helps.

Thanks

 

[KrisBlueNZ]

X_L = 2 pi f L
= 2 × pi × 20,000 × 135 × 10^-6
= 17Ω

Getting 1A RMS into a 17Ω inductive reactance would require an amplifier with a fairly agile output, I think. It's certainly outside my experience. And I don't think a hard drive servo chip would do it, although it might be a good idea for you to read up on them.

Have you considered making the inductor/electromagnet part of a tuned circuit? The capacitor value comes out at almost exactly 0.47 µF. Is it a coincidence that it's so close to a common preferred value?

Do you need to rapidly change the amplitude of the current waveform in the inductors? Or will it stay steady during each test run?

 

[Arouse1973]

I don't know of a way to change the current in an inductor if the frequency is fixed and the voltage and the inductor. You will have to change one of them. I guessing the voltage or frequency would be the best option? I might be missing something here sorry.
Adam

 

[Arouse1973]

What about an audio power amp like the TDA2004. It will be running at max though, 20W = 17R+3R =20R at 1 A.
Thanks
Adam

 

[KrisBlueNZ]

What about an audio power amp like the TDA2004. It will be running at max though, 20W = 17R+3R =20R at 1 A.

I don't think so, but I'm getting out of my depth here. I'm waiting for the answers to my questions before I know how deep the water is!

 

[Arouse1973]

I don't think so, but I'm getting out of my depth here. I'm waiting for the answers to my questions before I know how deep the water is!

I must be missing something here then? Maybe he can draw a schematic to show what he means.
Adam

 

[BobK]

Unless the inductor is reacting with something other field, the inductive reactance will not result in any power usage. You only need to worry about the 3 Ohm resistance for power. I would think an audio amp capable of putting 1A into a 3 Ohm load would work okay.


Bob

 

[Arouse1973]

Good point Bob. But what will the amp see in this case, 3R or 20R? At 20KHz. The load might not develop 20W but the amp can only supply 20W.
Adam

 

[BobK]

That is really meaningless, it will see different things at different points in the waveform. The voltage and current will be out of phase. I did a simulation and the peak instantaneous output power was 10W. The average power was 3W. This is with a 20KHz 1A peak sine wave output.

Anyway, one rated at 20W should have no problem at all, even if I am wrong.

Bob

 

[Arouse1973]

I doubt very much your wrong Bob.
Adam

 

[(*steve*)]

OK, assuming you're driving this impedance using an Audio amplifier, the current through the inductor will be determined by the amplitude of the signal and the inductance of the inductor.

The relationship should be calculable (especially since you have a wise tolerance).

With an audio amplifier having constant gain, you could increase the output level of the signal generator with frequency to maintain a close to constant RMS current through the inductor. Conversely you could change the gain of the amplifier with frequency.

Placing a small resistor in series with the inductor and measuring the voltage across it would allow you to measure the average current through the inductor. For initial trial purposes, you could use a multimeter (on an AC voltage range).

 

[Arouse1973]

That is really meaningless, it will see different things at different points in the waveform. The voltage and current will be out of phase. I did a simulation and the peak instantaneous output power was 10W. The average power was 3W. This is with a 20KHz 1A peak sine wave output.

Anyway, one rated at 20W should have no problem at all, even if I am wrong.

Bob

Hi Bob I did some calculations and got different results to you. Where have I gone wrong?

Cheers
Adam

 

[BobK]

I don't see any disagreement. I said the actual power was 3W which is the same as you came up with.

bob