i wonder at what 'watt' rating should the two resistors be on this circuit below , the motor i am using is 800ma 10v. Am I likely to burn anything up if I use .25w resistors? perhaps the .5 ohm should be of a higher watt rating?
Assume the 0R5 resistor can get 5V at most, it will drat 5^2/0.5 = 12.5W. But I don't know that circuit, nor how long time the resistor is supposed to having voltage over it since that is kinda poor information concerning the circuit. What part number is the ic?
If the running motor current is 1A, then since the motor current flows though the 0.5Ω current-sensing resistor, the steady-state dissipation would be I*R = 1*0.5 = 1/2W.
However, if your steady-state running current for the motor is ~1A, I'll bet the stall (starting) current is like 5 to 10A, so the resistor should be much more than 1/2W.
i have just been testing using a .47 ohm 0.75 watt resistor, but i could buy something larger like you say, how high should i go ? can i go too high and ruin the performance? maybe a 2 watt resistor would be o.k?
You can make low ohm resistors from calibrated lengths of copper wire. If you use a piece of common #14 house wiring you'll easily be able to handle the stall current of your motor. Total cost will be as close to zero as you can get. I bet there's even a website that addresses this kind of thing.