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12Vdc to 24Vac inverter, sine wave

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New Member
I have a motorhome whose refrigerator uses a 24Vac "swing motor". A swing motor is basically a solenoid connected to the piston so it vibrates with the AC waveform in a linear motion rather than rotary. Here is a link on the swing motor for your info:
Swing motor | Technology

The unit had a transformer with 120V, 24V and 12VCT windings. When running on 12Vdc, it used a square-wave multivibrator driving the 12V. This circuit is burned out but the transformer is Ok. I've made up a PIC circuit with several temperature sensors to make it more modern. The PIC also drives a couple power FETs into the 12Vct winding to get 24Vac to run the swing motor. Everything works fine but the 60Hz square wave (stepped square ware) makes a buzz noise that is very irritating. I'd like to modify it to get the wave closer to a sine wave.

I have increased the switching frequency and tried my hand at PWM.

Anyone have any ideas?

Before everyone tells me to run out and buy a commercial sine wave inverter, let me say I have plenty of them lying around in my shop. We have used them (attempted more like it) for battery backup systems in our work and they have many drawbacks. example:
-Intermittently or constantly refusing to start with a load connected.
-shutting down well before the battery is low.
in other words we use them as door stops now.

Also the original inverter is no longer available (it likely buzzed too) and a new fridge is 1500.00



Most Helpful Member
I cant say for certain if this would help allot but I would try using a simple AC capacitor on the motor side and or the 120 volt side of the inverter transformer.

IF it is near the correct size it will greatly round off the square wave that is presently making all if the noise. Also by changing the PWM ratios this will also help fine tune the transformers outputs to work more like actual LC tanks with a fairly smooth sine wave as a result.


New Member
I played with the PWM on the weekend, upped the PWM frequency and added a capacitor. I have a decent looking sine wave now but the output voltage dropped off so I might have to scrap the old 60Hz transformer for a high frequency one. Trouble is I just threw out a bunch of toasted switching power supplies at work, now i'll have to wait to lightning to take out some more :)

I might also use a dedicated 8 pin PIC for the PWM as my LCD does not seem to like getting its updates interrupted so often to service the PWM.

I am not using the PWM in the PIC as there is only one PWM, so I just use timer1 interrupt.


Boost the 12V to 35VDC.

Use a class D inverter to convert the 35VDC to 24VAC.

Do you need isolation?

If so, the DC-DC converter must be isolated but it doesn't have to be regulated - that can happen in the class D inverter stage.
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