Please help, current limit resistor calculations

[scratch_6057]

I have some small Vexta steppers I want to try working with
5 wire 4.8V 340 mA. Too small to run off my HobbyCNC
driver board. I was looking at the TurboCNC site
( http://www.dakeng.com/turbo.html ) and under the ‘Hardware’
link he shows a 2 stepper driver based on a Microchip PIC16F84
(code available on that site).
( http://www.dakeng.com/stepdrive.gif ) He uses 2N2222
(500 mA) transistors which would be suitable for these 340mA
motors, but I would like to use TIP120s (5.0 A) so that the driver
would be a little more general purpose (useful).

I’m thinking the circuit should include current limiting resistors
in order to prevent burning out coils in the event of motor stall.

I’ve gotten myself confused on how to calculate the correct
resistor values. The way I’m thinking right now is listed in the
next few lines

Motor Supply Voltage _________________ 12 V
Subtracting Motor Rated Voltage _______ 4.8 V
Equals _____________________________ 7.2 V
Subtracting Transistor BE saturation ______ 2 V
Equals _____________________________ 5.2 V
Divided by 340 mA ________________ = 15.30 ohms (rounded up)

To be on the safe side figure I go with 18 ohms for 288 mA.

(Yea, I know I’m loosing torque)

Common wire to 12 volts one resistor between each coil and
transistors collector.

Is my method of calculating anywhere near correct?
What am I missing here?

[kreutz]

Motor Supply Voltage _________________ 12 V Subtracting Motor Rated Voltage _______ 4.8 V Equals _____________________________ 7.2 V Subtracting Transistor CE saturation ______ 2 V Equals _____________________________ 5.2 V Divided by 340 mA ________________ = 15.30 ohms (rounded up)

Calculations are correct. You can use a 15 ohm Resistor without problems. Use a 3 watts resistor between each coil and the transistor.

Why?

Because you might want to drive it with the "2 phases on" sequence, and the resistor in series with the common lead will drop too much voltage when that happens.

[scratch_6057]

Thanks SO MUCH for the confirmation, sometimes after looking
at something for far too long my brain starts to get foggy and I start
to doubt my own logic / reasoning.

AH, you caught my fat finger typo that WAS supposed to be
CE saturation and not BE

That’s what I had in mind, 4 coils = 4 resistors for the exact reason
you mentioned ‘half stepping'. I guess I failed to express my thoughts clearly.

[kreutz]

you mentioned ‘half stepping'

I mentioned Full step with "Two phases on" which gives you more torque than "wave stepping" with only on coil energized at the time. Also for half and micro-stepping, if you ever want to try them, you will need the one resistor per phase approach.

[scratch_6057]

Kreutz,

I had a couple of more questions about things I needed clarification
on i.e. coil sequencing but then I found the other thread that you seem
to be currently active in
(http://www.cnczone.com/forums/showthread.php?t=25361 )
which looks like it has already covered most everything so there is really
no point (as far as I can see) in my asking, and you answering the same
things in this thread. I'll just play ‘CATCH UP’ and if anything comes up that I
can’t get clear in my head I’ll let you know. How’s that work for you?

Thanks for the help.

[kreutz]

OK, perfect.