# AC current for coil

Discussion in 'The Projects Forum' started by kuba777, Feb 16, 2011.

1. ### kuba777 Thread Starter New Member

Feb 16, 2011
2
0
Hello,

I need to excite big coil with 250 mA, 400 Hz AC current. Impedance of coil at 400 Hz is 85 ohm, DC R is 5 ohm. Do I need special AC power supply for this?
What would be best way to make sure the current has right value?
I would appreciate any comments or suggestions.

Regards

2. ### wayneh Expert

Sep 9, 2010
15,219
5,568
Yes. A DC motor spun at 400 rps might work.
Easiest way is to measure AC voltage across a low ohms resistor in series with your coil. Not all meters will work at 400Hz, though. Measuring peak voltage could be achieved with a diode (or a bridge rectifier) and a capacitor configured as a peak detector. The best way would be an oscilloscope that would allow you to see the waveform.

3. ### CDRIVE AAC Fanatic!

Jul 1, 2008
2,223
105
Unless I'm missing something that calculates to 22.5VAC (Sine) input V.

4. ### CDRIVE AAC Fanatic!

Jul 1, 2008
2,223
105
Wouldn't the output freq be dependent on RPM x Pn where Pn is the number of poles? Also, wouldn't the output from a DC motor be pulsating DC?

5. ### wayneh Expert

Sep 9, 2010
15,219
5,568
Yes, the number of poles matters. Not sure how many poles are in a small DC motor?
I think without a rectifier, the current goes back and forth, meaning it is true AC. I suppose this depends on the internal configuration. The ones I play with make AC.

Dec 26, 2010
2,147
302
A DC motor may not give useful AC if it is a commutator type, as a lot of small and medium sized DC motors are. A permanent-magnet commutator motor would definitely give (rough) DC.

Permanent-magnet stepper motors should give AC, but I would not like to make any bets about their efficiency used in this way.

7. ### CDRIVE AAC Fanatic!

Jul 1, 2008
2,223
105
Using a motor as a generator for this application doesn't seem like a practical solution to me. A more realistic solution would be a 400Hz audio oscillator feeding an audio amplifier. 400Hz is well within the passband of same and a 10W amp will do the job with some power to spare.

What is this going to be used for? If my memory isn't escaping me I remember 400Hz as being the freq for the HV power in military aircraft.

8. ### kuba777 Thread Starter New Member

Feb 16, 2011
2
0
The coil is used to check sensitivity of microphone to an external ac magnetic field. I have to compare voltage from microphone and from calibrating coil after they are placed inside big coil.

This microphone is going to be used in aircraft. From what I know 400 Hz is used in military aircraft.

Thanks for suggesting audio amplifier, I don't now why but I haven't thought about it before.

9. ### wayneh Expert

Sep 9, 2010
15,219
5,568
+1
I just threw it out there as an "outside the box" idea. Audio amp is a good idea and you can buy pre-made circuits or kits for just about any level of mono or stereo power you want.