Problem- Power resistors,Pls help

[sa1]

What value of resistors would I use to limit the current to the steppers?
my specs:
Power supply: 24v 15amp
Motors: Nema 23 6-wire. 3.5v 1.4amp/ phase. 1.8deg. (3 axis)

According to my calculations:
3.5/1.4 = 2.5ohms (motor coil resistance).

Resistor
24v-3.5/1.4 = 14.64 ohms
1.4*1.4*14.64 = 28.69 watts (min.)

Is this correct?
Thks

[Zig]

Yes.

However You will need to make sure that the resistor while rated at the correct dissipation will not be glowing red hot while doing it.

Inother words .. at what surface temperature will the resitor be operating while dissipating the power.

[sa1]

Do different resistors operate at different temperatures?
If so, how will I know which one to use??

[Zig]

It is a game of specmanship.

In these days of miniaturisation some manufacturers specify their product to be able to dissipate a certain amount of power at a particular surface temperature.

The higher the temperature the more likely the component or its surounds will be affected and have shortened life.

You dont want to have components which might cause inadvertent failure of adjacent components ( say excessive temperature rise of an electrolytic capacitor) due to heat generated next to it.


So read up on the specs and select. You might be offered two 15 ohm 30W resistors. One will dissipate 30W and reach a temperature of say 50C the other might dissipate 30W and reach a temperature of 70C. Both will do it happily but one might have deleterious effect on the printed circuit board say.

Dont get fooled by specmanship.

[sa1]

Thanks Zig,

Now it makes sense. Just for information,, I plan to install the resistors on a separate board and have jumper wires connected from my controller card to the resistors. Also have a serious cooling fan blowing against these resistors.
In this way there is no heat what so ever directly on the driver board. The two PCB's are separate from one another, just the jumper connecting them.
..Now thats left is to find a supplier that actually specifies the operating temperature of their resistors...(ha,ha,ha).

[pminmo]

Originally Posted by sa1 What value of resistors would I use to limit the current to the steppers? my specs: Power supply: 24v 15amp Motors: Nema 23 6-wire. 3.5v 1.4amp/ phase. 1.8deg. (3 axis) According to my calculations: 3.5/1.4 = 2.5ohms (motor coil resistance). Resistor 24v-3.5/1.4 = 14.64 ohms 1.4*1.4*14.64 = 28.69 watts (min.) Is this correct? Thks

Your calculations are correct. You will either need one or two per motor depending on drive method, wave drive can use one, two phase uses two.