I have (had?) a desire to power a three phase motor from a disc drive earlier this afternoon. I don’t have a handy off the shelf inverter/3 phase signal generator so I decided to make one from what I had lying around. Using two 74LS74 Dual D Flip-Flops and any old square wave generator for a clock ( a LM555 will work ) I built a circuit in the configuration in the photos shown. The the 3 flip-flops are set up as a Johnson Counter. I started my circuit with a low-frequency and some LEDs on the outputs (don’t forget a couple hundred ohm current limiting resistor) because I don’t have a quad input scope. No real gotchas here other than making sure you tie /CLR and /PRE high. Don’t mind the third chip on the board, It was a AND gate from something else.
[Dear Tektronix, Please hook me up with a sweet quad input MSO and I’ll forever brag and show it off, I swear! No shipping needed I’ll pick it up in Everett, WA]
Next, and not shown, I’ve used TI’s FilterPro to design a low pass filter to get a sine wave. FilterPro is free software, good stuff. Now all you need to do is feed your circuit 6x the desired output frequency to get your three-phase output. What I love about this circuit is the two 74LS74 chips have date codes that make them 2 months older than I am (circa 1976). No big deal because of the low frequencies. I will probably set up a PIC with a LCD to generate a 50% duty cycle PWM output to vary the frequency and then design some amplification on the back-end of the filters. I read somewhere the drives run 43kHz? and around 9V or so…
Like that sweet bread board? I won it free from Digikey on their Facebook page last winter, I hope they do that contest again this year!
If you’re reading this blog historically oldest to newest you’ve noticed I’ve benched the programmable load.; I’ll tinker with that some more later on this weekend and make mention of it in a post later on.
