# COMMUNITY : FORUMS : TECH

## Trying to create a circuit to allow me to adjust the frequency of AC power

I'm trying to design an EM field generator with adjustable frequency. I've got most of it figured out, but I can't seem to find anything about how to create adjustable frequency AC power. I've got two designs I'm working on, one that is DC powered and one that is AC powered, but the best I can do on either ends up with a 60Hz EM field. I'm looking for adjustable frequency, ideally from 0.1Hz to 50kHz, although I'll be pretty happy to even get any range of frequencies at all. The range from 0.1Hz to 500Hz is the most useful and interesting in particular for my purposes, but any help on figuring this out at all would be much appreciated.

LargeMouthBass6 years ago
The sine wave will have to be artificially generated. The circuit to do it is an H-Bridge, (google for details) which allows the direction and amplitude of the current through the load to be controlled. The direction of the current is controlled by which two of the four transistors are turned on, and the amplitude and sine wave shape are created by driving the switches at different PWM duty cycles.

H bridge circuits are most commonly used to control the direction and speed of DC motors, but your application is another where it would be appropriate.

If I were doing it, I would drive the H bridge direction and PWM controls via a microcontroller. The microcontroller could also have an analog input to read a potentiometer to determine the frequency.

If you are trying to generate a field, I assume that your load is some kind of coil.

There are H bridges that come as a single part, that looks like a very large transistor. The benefit of using such a part is that you would just have to apply power, ground, and a the digital control siganls for PWM and direction, and not have to design the circuit itself, which can be tricky. These self contained H bridge parts may be able to output a current up to a few amps. Depending on how strong of a field you need,this may suffice.
NachoMahma6 years ago
. http://en.wikipedia.org/wiki/Variable-frequency_drive
mightywombat6 years ago
According to the information I found after a quick Google search on my phone, the easiest and cheapest (some say the ONLY) way to do this is to convert your wall power from AC to DC with a rectifier and then use an oscillator to create the frequency you want from the DC voltage. Hope this helps!