How to find the correct resistor?

[jetijs]

Hello,
I'm new here, I have read, that increasing the voltage to the motors will increase speed. Currently I use 5V from a computer power suply, I my top speed with my hobbycnc.com 200oz, 3v, 3a motors is about 10 tpi. My driver board is fet3 from stepperworld.com here's the link:
http://70.85.87.12/stepper/fet3.htm
It's said in the controller manual, that this board can handle up to 45v with appropriate resistors. So, how can I calculate the correct resistor value that I need for say 12v or 24v? Now I'm using 0.33ohm 50w resistor for each motor running at 5v.

Sorry about my english, I'm from east Europa.
Thank you!

[jeffs555]

The ohm value for the resistor is the (supply volts - motor volts) divided by the motor current. For 3v, 3a motors, and a 24 volt supply, the resistor would be (24v-3v)/3a=7 ohms. The power rating of the resistor must be at least (supply volts - motor volts)*motor current, so (24v-3v)*3a=63 watts. You probably want a large safety margin for the power so use 100 watt resistors.

PS Your english is excellent. I could not tell that English was not your native language.

[jetijs]

Thank you very much!
You helped me alot

[xyz100]

Originally Posted by jeffs555 The ohm value for the resistor is the (supply volts - motor volts) divided by the motor current. For 3v, 3a motors, and a 24 volt supply, the resistor would be (24v-3v)/3a=7 ohms. The power rating of the resistor must be at least (supply volts - motor volts)*motor current, so (24v-3v)*3a=63 watts. You probably want a large safety margin for the power so use 100 watt resistors. PS Your english is excellent. I could not tell that English was not your native language.

Wow! 63W of heat to power just one phase of the motor that dissipates 9W per phase only! Unfortunately since stepperworld.com sells really outdated design based on discontinued UNC5804 without chopping capability your other choice would be to build a chopper board. Schematic, pcb, etc is available from http://pminmo.com/3axis/3axis.htm . I bet 100W resistor will cost more than the chopper. Plus NO HEAT!