Re: 5-axis machine controller

["D.F.S." <dfs-at-xmission.com>]

> 
> 
> If you have lots of power supply and big resistors, the original old l/r
> drive system is quite easy to implement for 6 lead stepper motors. You need
> a big resistor from each coil centertap to the positive supply. Example:
> 40vdc supply, 2v 4 amp stepper coils. You need 2 resistors/stepper and 4
> fets/stepper.
> the resistor needs to drop 38volts at 4 amps... r=v/i = 38/4 = 9.5ohms,
> now hold onto your hat... P = i^2R = 4 * 4 * 9.5 = 152 watts

OK, unless I'm missing something the only reason to raise to voltage and
add the resistor is to speed up the rise time but not toast the coils.
Right?

As long as I keep the power dissipated in the coil to 8 Watts I'm OK?

So I could also use 12V input to the coils, a 2.5 Ohm Resistor -at- 40 Watt
OR 5V .75 Ohm and 12 Watt?

As such the trade-off is one of torque and speed VS Input Voltage and 
power Dumped by the Resistor? 

How much of a disadvantage are we talking about?

Keeping in mind this Machine is designed for building exactly this
kind of circuit board. I'd much rather get a setup that works although
at 1/4 speed and 1/2 touque but is dirt simple and cheap THEN use it to
build a GOOD controller for itself.

I have some great PCB design software to plot out the boards, I have the
mount already built to hold the drill, I have the drill bits, I have
The etching process down for the boards. The only thing missing is the
PC->Machine interface.

You example clearly shows the advantage of chopper drives and the second
cut at this will surely be of such a design.

> The fets are connected from each of the 4 remaining stepper wires to ground.
> A standard fet will need about 10 volts gate drive to insure full turn on.
> Try something like a IRF540. These are pretty cheap $.60 to 1.95 depending
> on quantity. 
> I have seen several circuits that use an rs232 driver chip to
> drive the fet gates 
Now THAT is a good idea. The fact I have about 800 MC1488 RS-232 Line Drivers
I'll ever need, that is a killer use for them.

The max 232 has all the crap required for the charge pumps and run about 2 
bucks by the time all that is set up.
The 1488 when I will have the higher voltages handy on the board are a great
option, that is a REALLY good idea.

I think these little suckers are good to at least +/-30 volts, I'll just 
feed them whatever is required to fully enable the FETs.

> You will need a small diode from the
> gate to ground to prevent the negative drive from getting to the fet gate.
I assume with the 1488 that requires external sources for both voltages
I can simply ground the negative voltage input, end of problem.

> The resistors can be expensive... 

All the more reason to build that chopper drive ;-).

Thanks a LOT.

That RS-232 

Marc