# Thread: Little lost calculating current limiting resistors

1. ## Little lost calculating current limiting resistors

Well ive spent the last two nights trying to figure out what i need for current limiting resistors for our router build and im throwing in the towl and asking for help

We are building a jgro style router with a 97" long (in Y) table and using nema 34 framed motors

spec sheet's here http://www.kelinginc.net/KL34H280-45-8A.pdf

The powersupply we are going to go with is giving us 36v DC with more than enough avaiable amps (borrowed it from an old 3axis cnc with massive ac servo's and built a rectifier for it)

Now from my math, i think i need 3.28ohm 330watt resistors or if split them up and run two resistors per motor (wired unipoler) i can go with 6.5ohm 165watt resistors

Is this correct? or am i wrong?

And either way, what do i need and where can i buy them? Digikey is showing \$80 for resistors large enough by math, im hoping im wrong

Thanks

-Jacob

2. The motor is 4.5A and 0.71 Ohms unipolar. You have a 36VDC supply.

1) The motor "takes care" of 3.2V (4.5A times 0.71 Ohms) so that leaves 32.8V (36V - 3.2V) you have to drop.

2) 32.8V divided by 4.5A is 7.3 Ohms. You will need two of them.

3) 32.8V times 4.5A is 148 Watts. Two resistors are needed per motor so you will be dissipating nearly 300 Watts of heat per motor in these resistors.

4) Big resistors are expensive and \$80 doesn't sound unreasonable for the size parts necessary to dissipate this much heat. For your trouble you'll get fairly poor performance (10:1 voltage overdrive ratio, full or half-step resolution, resonances, etc.).

Mariss

3. For the size of the machine you are building, I would recommend skipping the resistors and buying a set of Gecko drives. The extra cost will be worth it and you won't be disappointed.

John