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Registered Member #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
Hello,
Well, there are so many sites about homemade CNC routers and they provides much driver schemes. The problem is that those schemes are complex for my electronic knowledge and doesn't make sense (in context I don't understand them).
I'm going to make a simple CNC router for milling PCBs. I need a driver which would control unipolar stepper motors. The most powerful I have are 1.7A and 4.08 DCV. They have to be controlled by CNC programs like Mach3, Kcam, TurboCNC etc. through LPT port. I want something simple and understable for me. As so far, there are two options for me and they're very similiar. The first one is using optocouplers and power transistors. There's line, which describes how I imagine:
LPT signal -> current limiting resistor -> optocoupler -> gate current limiting resistor -> IRF820 -> stepper motor coil
Second option is pretty same, only one transistor is added between optocoupler and IRF820 and it's role is amplifing signal, so I would use more powerful power transistors. The idea line:
LPT signal -> current limiting resistor -> optocoupler -> amplifing transistor (for example, BD139) -> gate current limiting resistor -> IRF820 -> stepper motor coil
Why I can't decide which options suits best is caused by that I don't know if first option will work. From what I've readed, LPT can supply only few milliamps. But that's enough for optocouplers to switch. Optocoupler's transistors powered by, let's say, +12V, should provide something aroung 50mA RMS current. IRF820's gate capacitance is 360pF. According to datasheet, the charge needed to open gate at +10V and Vds 100V is equal to 10nC. Rise time is 11nS. So, a amount of ampers needed to open gate at +10V in 11 nanoseconds, is calculated by i = Q/t formula. i = 10nC/11ns gives almost 1A. If I've calculated properly, then the next problem is that I can't figure out if optocoupler can provide this peak amperage. There's something as pulse current in optocoupler's datasheets, but under different conditions (for example, pulse width is 300uS and duty cycle 2%).
The second option (with amplifing transistor between optocoupler and IRF820) would guarante that it will work, but requires more messing with soldering, buying etc., so I prefer the first option. Of course, only if it would work, that's why I'm asking you.
There's site, wchich describes better the options I've said above:
("Transistor amplified optocoupler circuit" and "Optoisolated high power control circuit")
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
11ns rise/fall would give 45 MHz, your motors will not work at 45 Mhz so it makes little sense to use such high gate drive currents. Since a motor is an inductor (among other things) it is usually not a great idea to use such a fast rise time.
I would have used the simplest method and adjusted the gate resistor so there was little heating in the transistors and nice looking waveforms.
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Simple unipolar stepper motor driver - amplifing LPT signals
