DIY VFD Update

Nice PCB - did you use a CNC router to do it? - if so, was it homebrew

- did a good job...

Andrew VK3BFA.

Thanks, the circuit was drawn out with a freeware PCB program. The image was printed out with a laser printer on some enameled paper, the image was then transfered from the paper to the copper clad using a regular clothes iron and the paper disoleved in water. The copper clad was then etched in a DIY etchant consisting of muratic acid and hydrogen peroxide. There are several articles on the web that describe this methode of making DIY PCBs on the web in greater detail.

Best Regards Tom.

I'll look these up.

My original instinct was that SCRs were better suited than IGBTs for this application, but I have not really dug into the issue. I'll get the datasheet for the Toshiba IGBT and think about it.

Good point. The price differential probably makes it worthwhile to eat the expense of a few bad devices, if it comes to that. But I would buy more units than strictly needed. And to cover lab accidents.

Joe Gwinn

Joe is right, IGBTs are not well suited to high current pulses. You have to keep them fully saturated, or the current hogs to the hottest part of the die, in a positive-feedback death spiral. The design of the FET to BJT drive scheme built into the transistor only allows so much base drive. SCRs are better at this, as increasing current increases drive to the internal transistor-like structures it is built from. You could also use a bank of parallel MOSFETs, although the 350 V supply requires using more of them. MOSFETs are so cheap, you might be able to buy a couple dozen new for what a used monster SCR or BJT would run you surplus. MOSFETs are quite tolerant of pulse overloads, too.

Jon

That board is far better than I've ever achieved with toner transfer, what paper and printer did you use? Martin

The printer is a HP laserjet 3200m, paper is HP presentation paper which is a gloss paper.

Best Regards Tom.

Does that mean that you just use a variable freq square wave, or is it like the commercial VFDs & use PWM on a carrier?

Bob

Filter Caps do have a problem with ripple currents. You should also consider stored energy in the motor magnetics (1/2 * I * I * L) versus the capacitor (1/2 * V * V *C). Inductive kickback into the supply caps in a PWM system can cause some really spectacular capacitor failures. I highly recommend you have that cap in a stout box before you test at more than an amp or 2. If the cap blows you will be releasing 500+ joules explosively.

I think your rectifier bridge looks too small for the rest of the circuit.

Best of luck Carl

The rectifier is only rated at 600V @ 25A. As soon as i find a deal on something bigger i will be upgrading. For the time being i am limiting the power during the current testing phase. Eventually i will build a box out of perferated metal to house the finished unit before i test at full power at 220V.

Thanks for the heads up on the caps.

Best Regards Tom.

Any progress on drawing up a circuit diagram?

RWL