help with power resistors

I just hacked an Epson C80 printer to get the motors to learn how to drive them. There is a Stepper for the paper advance and a 2 wire dc motor for the inkjet carriage. These were actualy driven by 35v and 45v. I want to learn how to drive these and have been reading the archives.

For the 2 Wire DC motor. The winding resistance is 23.8 ohms. If I try to run this at the TTL level I get 5/3.8= 210 mA current. I undersand that the max curent the pins can handle is 20 mA. So I assume I need to put in a Power Resistor in series to reduce the current? 5V/.02A = 250 ohms. Minus the 23.8 give me about a 220 ohm resistor. The power would be 5v*.02A = .1W
Will this work? How do I determine if I need put the resistor on the ground or +5v side?

As for the Stepper Motor.
It is a 4 wire bipolar motor. There is 3 ohms between each winding. 5V/3ohm = 1.66 amps. Using the same logic would I need a 248.33 resistor in each leg to be able to drive it directly with the output pins?
Power would also be 0.1 W

Are these assumptions correct?

Thanks for the help
Steve

Putting an 220R in series with the output will limit the current, but then you'll form a voltage divider with that resistor and the resistance of the motor and end up with less than 0.5V across the motor which probably won't turn it.

You need to use a transistor or FET to switch a larger amount of current through the motor. Take a look at "PICAXE Manual 3 - Interfacing".

Here is some other stuff you need to consider:

- Measuring the resistance of a DC motor does not give you the full picture. The effective resistance gets higher when the motor is running. Maybe 10 times higher when unloaded. What you are measuring is more like the STALL RESISTANCE.

- A DC motor with (say) 100 ohm effective resistance, when supplied with 45V will consume approximately 20 WATTS. Rising to nearly 90 WATTS when stalled (23 ohms). Driving the same motor from 5v (that is, a 5v supply capable of supplying sufficient current) would use approx 0.25 WATTS running and 1 WATT stalled. The power supplied to the motor would prolly be insufficient to turn it at all.

- To boost the current output of the PICAXE from 20mA up to the level required by the motor coil requires a transistor, not a resistor. Using a 220 ohm resistor will increase the overall resistance of the motor circuit, reducing the power delivered to approx 80 MILLIWATTS. Certainly not enough to turn a 20 WATT motor.

You prolly need to use the PICAXE to switch a separate, higher voltage and higher current supply for the motor.

OOPS. Hippy posted while I was typing.

Ok, that makes sense. I am trying to get back the basics as I made a mistake with this driver: <A href='http://www.weedtech.com/wtsmd-m.pdf ' Target=_Blank>External Web Link</a>
and this motor: <A href='http://www.kelinginc.net/KL23H51-24-8B.pdf' Target=_Blank>External Web Link</a>. I have a 24V 4.5 amp power supply.
When hooked up the hexfet stated to smoke. The manufacture said I put too much current on it. He said I put 24/1.8 = 13 amps and smoked it. How if the supply is only rated at 4.5 amps? I figured that the supply will only give UP to 4.5 amps. Since the motor would only pull 2.4 amps I would be at the high end of the driver rating 2 amps.

I dont understand how to size up the driver / power supply for the motor.
Thanks
Steve

The spec sheet does show 1.8 ohms for that motor - so the mfr is correct - the power supply you have eveidentlyhas not got current limiting or a fast acting fuse and so would put out a large current before it too was smoked... best to use ohms law figures before you connect! A 'simple' curent limiter can be made with an LM317T on a heat sink - look at the spec sheet - it only takes a resistor between output and sense pin and then you take the output from the sense pin... then you can smoke the LM317 instead of an expensive motor... good luck and keep calculating power and currents with E=IR and P=IE = I*I*R = V*V / R ... pretty soon you'll get a feel for it and smell less burning...

Premelec is correct. To provide sufficient power (WATTS) to drive a motor, you need to work out how many amps you need. Motors are generally supplied with voltage ratings. This is usually enough information, assuming that your supply can provide enough current. (A PICAXE can control many things, but not supply much current)

You will definitely need a transistor to amplify the power provided by the PICAXE by a factor of at least 100, maybe more.

To power up the motor use an IRF530 or IRF540 Mosfet, they can handle 28A / 540 from memory.
Look at manual3.pdf, interfacing etc.
(look at PWM for speed control also, man2)
The Motor spec sheets may say 30V or 24V but
then you see the duty rating is less than
100%, in reality they are run at 18V to 20V.
Most will even perform reasonably well at 12V.
If your motor gets very hot after a couple of minutes try a lower voltage, too much
load on the motor will make it hot also.
may