Help with current limiting resistors

Hello !

I'm having a bit of a problem with my current limiting resistors, I'm using Oatley electronics driver board with Oatley electronics constant current supply for steppers and 20V 40A power supply.
Constant current supply is rated at 2.5A with current limiting resistors to suit a motor. Value is worked out using formula: 1.25/stepper current rating. They are two 5W resistors that can be connected in parallel or series to get the correct rating.
Now, the motor that I want to use is rated at 50V 3.5A. I spoke to the designer of the board and I was told that extra amperage is ok if I provide adequate heatsink. Using the formula, I worked out that current limiting resistors should be 0.357 Ohm (1.25/3.5=0.357).
I managed to get the motor turning, driver transistors are cool, but current limiting resistors are getting so hot that they melt the solder on the pcb.
What am I doing wrong ? Should I use larger resistors (more wattage) ?

Thanks for help !

Did you set the current using VR1 on the constant current supply? Selection of the resistors should set the current below what is needed so that the adjustment will allow you to go above and below the desired current. Did you remember the insulating washer on IC1 attaching it to the heat sink?

Steve

I have fitted insulating washers on both ic's, as per kit instructions. If there was short circuit through the heatsink, ic's would probably go up in smoke as soon as I turned the power on...
Vr 1 has very little effect, so I guess that I did not choose correct resistors.
Maybe I should go for 0.45 or 0.41 ohm resistors (1.25/2.75 or 1.25/3)

Zed

Zed,

Calculating for power...
3.5 Amps with a voltage drop of 1.25 across the resistor gives you 4.375 Watts.
3 Amps gives you 3.75 Watts
2.75 Amps is 3.4375 Watts

Using 5 watt resistors in series you have an overall 5 watt rating. If you put 2 5 watt resistors in parallel, you get a 10 watt rating.

assuming 5 watt resistors....
You could do:
3 1.3 Ohm in parallel for 0.43 ohms 15 Watts for 2.8 amps
2 0.82 Ohm resistors in parallel for 0.41 ohms 10 Watts for 3.04 amps
2 0.75 Ohm in parallel for 0.375 ohms 10 Watts for 3.33 amps

I would be tempted to try the 2 0.75 Ohm in parallel. With a 10 watt rating they should stay relativly cool, even when mounted right against each other.

Steve

Another trick is to not mount them near the PCB or each other. Leave the resistor leads long and let them sit up in the air, and apart from each other so the heat has room to dissipate. A fan will also make a huge difference.

Thank you Steve !
I managed to get things under control by fitting 2 resistors in parallel (0.47 and 2.7 - total of 0.40 ). Current can be adjusted on vr1 now. Maybe I will fine tune it further because I get correct current right at the end of adjustment on vr1, but at least I know I'm on right track.
My other problem is that my steppers are rated at 50V, bench supply can deliver 20V, but after constant current module (on its output) I get only 3.5V. Motors are turning, but only about 100 rpm or so.
Is it possible to modify this module to get more voltage on its output ?

Thank you
Zed

If it's a constant current source, then it would (correctly) reduce the voltage once the correct current is flowing through the system if the motor is not spinning. The instantaneous voltage as the coils come on and off during motion may be much more but you would only see that with a scope or other peak measurement device.

2.5 amps * 20 volts is only 50 watts power. The motors are rated for 3.5 amps * 50 volts = 175 watts power. So you are getting less than a third of the power out that the motors can manage. You need a much bigger supply and a larger driver. For those motors, you need something like a low end Gecko and a serious power supply.

Thanks for explaining it to me. I think I will try Gecko G250 driver board. At least it is standard board that a lot of people use... And power supply, well I will see what comes up.

Zed