Category Archives: TTL

Electronics TTL

Simple 3 Phase Power Circuit

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.

The simple TTL three phase signal generator with 400Hz output.
The simple TTL three phase signal generator with 400Hz output.
The three phase signal generator schematic and layout on a breadboard using 74LS TTL logic.
The three phase signal generator schematic and layout on a breadboard using 74LS TTL logic.