Need Help!- Current Set Resistors

[dpgoldberg]

Please forgive this question but I am a newbie at dealing with electronics.

I need to install 3.5K current set resistors on my stepper motor connectors but I can't find that exact value. Can I pair a 3.3K and a .2K in either series or parallel? I don't want to fry either the motor or driver.

Thanks.

[Mariss Freimanis]

You can do that or just use a 3.3K resistor. A 3.3K will set the current to within 10% of the required value. Little is gained by setting to exactly 3.5A.

Mariss

[harryn]

Hi, if you put them in series, the resistances add. This approach is fine with the two resistors you have proposed.

If you put them in parallel, then the formula is:

1/R1 + 1/R2 + 1/R3 = 1/ R total

If you are doing it by hand, just make sure to find the common demoninator and then add it up. On a calculator, or course it is just pushing the buttons to add it, then using the 1/x inversion key.

Electrical engineers do this all of the time to end up with the resistances they need, sometimes even making some in parallel, and some in series.

When resistors start to warm up, the resistance usually rises. You can buy special "current sense resistors" that are very stable with temperature.

Also, most resistors have a tolerance, and are sorted out by the precision level.

If you buy a 100 ohm resistor with 1% resistance tolerance, then it will be anywhere from 99 - 101 ohms.

The 10% tolerance packages will tend to have anything that is within 5% removed, and only the "at least 5% off, but not more than 10%" in those packages.

The resistors are also rated by "power". Take "the current that will run though the resistor" x "voltage drop in the resistor" = power. You select the resistors power rating by picking one that has AT LEAST as high of rating as your calculation, ideally 3- 5 x higher. Usually, physically larger resistors have higher current ratings.

Common suppliers are digikey, mouser, newark, and in a pinch, radio shack.

I see that Mariss answered faster, but I will just leave this here anyway.