I got steppers...I need help adjusting the current on the SMC board

[robertcarpenter.net]

I got my stepper motors super cheap on ebay. They work just fine as far as I can tell. I wired them up to the Stepper Motor Controller board and they all spin. I pull the Full step jumper off and they spin slower...cool!

I didn't leave the controller on any one motor for long because I haven't adjusted the trimpot to cap to the correct Amperage.

I only know *basic* electronics theory but I can handle my math well.

The coil resistance is measured @ 3.0Ohms. The packaging says they are made for 4V. I=E/R=1.3Amps which makes sense because they are labeled 1.4Amps. Now I figure I want to back these off about 30% to keep them from running so hot. .3 * 1.3A is 0.4Amps.

Now what? If my target is 0.4Amps, how do I adjust R3, the trimpot on the Stepper Motor Driver board?

Edit: Also, I noticed that if I run these in half-step mode it doesn't take long at all to get that heat sink on the H bridge to the point that I don't want to touch it and I can smell the heat (if that makes any sense...). How hot should that thing be getting? Do I need thermal grease under the heat sink?

Edited 1 time(s). Last edit at 09/30/2008 04:20PM by Robert L. Carpenter.

---

Robert Carpenter
[www.robertcarpenter.net]

You adjust it so that the voltage on the test point gives the correct current through the sense resistor. E.g. if the sense resistor is 0.5R you want 0.2V at the Vref test point for 0.4A.

You should always use thermal grease with a heatsink. Depending on the current the chips can run very hot ~ 80C.

---

[www.hydraraptor.blogspot.com]

Hey thanks nophead!

I was just sitting down to calculate that target voltage value and I realized the error in my math above:

Quote

The coil resistance is measured @ 3.0Ohms. The packaging says they are made for 4V. I=E/R=1.3Amps which makes sense because they are labeled 1.4Amps. Now I figure I want to back these off about 30% to keep them from running so hot. .3 * 1.3A is 0.4Amps.

That bold sentence is wrong...really wrong. If I clamp it down to 0.4Amps, I will be reducing it by 60%, not thirty. I need to **reduce** the 1.4Amps by 0.4Amps...my target is 1Amp even.

That being said: The sense resistor on my board is 2.7Kohms. With my target amperage being 1.0Amps. (http://www.flickr.com/photos/hoeken/2487806993/sizes/o/)

E=IR
E=(1.0A)(2700Ohm)=2700V

Why does this not seem to fit? Something tells me that I should be getting numbers more around 2.7V. Where am I missing the divide-by-a-thousand?

Thanks again

---

Robert Carpenter
[www.robertcarpenter.net]

... you have to calculate with the current through the coils - this is 3.0 Ohms, so you would have a measured voltage over the coils of 3 Volts ...

Viktor

Viktor,
It's a chopper circuit so measuring the voltage across the coils is tricky. Much easier to measure the voltage on the Vref test point and double it to get current.

Robert,
The sense resistors are R1 and R2 and they are 0.5 Ohm on the schematic here: [www.flickr.com]

Edited 2 time(s). Last edit at 10/01/2008 04:40AM by nophead.

---

[www.hydraraptor.blogspot.com]

Okay. I am sorry but now I am totally lost.

Some questions:

1) Why are R1 and R2 the sense resistors? I see that they are hooked up to the sense circuits, and if that is what make it then so be it.

2) If R1 and R2 are the sense resistors, how does the test circuit (labeled 'stepper adjust') measure the current or voltage across those? Is it being faked by the rest of the 'stepper adjust' circuitry? As far as I can tell from that schematic we are pulling straight from Vcc, through R7(2.7k), through R3(2k trim), and to the test point. I don't know for sure what that capacitor is doing but it isn't going to effect the voltage by any order of magnitude...its probably just keeping the RF down or something.

3) If R1 and R2 are the sense resistors do I need to calculate the total resistance of the parallel circuit R1-R2? That would be 0.25Ohms. But they are each connected to a different sense pin and never together so that doesn't make sense to me. In my mind it would have to be that my sense resistance is just 0.5Ohms.

Regardless of the answers to my questions, with the sense resistor value of 0.5Ohms and my Amperage of 1.0A, I should be looking for E=1.0A*0.5Ohms=0.5V at my test point. I am going to set that for now.

---

Robert Carpenter
[www.robertcarpenter.net]

Your calculating the voltage at the sense point correctly, but it's highly unlikely that your sense resistor is 2.7k. As your maths showed that would need 2.7kV to drive one amp through it, not to mention give off 2.7KW of heat ;-}>

Assuming a 0.5 Ohm sense resistor

1A = 0.5V, with the sense resistors generating 0.5W of heat.

There are three separate inputs to the L297 (for current limiting), one from between R1 and one of the stepper motor's coils, one from between R2 and stepper motor's other coil and one from the potential divider formed by R3 and R7.

The L297 controls the current through a coil by rapidly turning it off and on to keep the Voltage from its sense resistor below the reference voltage created by R3 and R7.

There is more to it in terms of why using a higher voltage than necessary and cutting the power is better than just using the voltage your steppers are rated for but the key point is that they are entirely independent inputs to the L297 which then acts based on them.

Robert,
Yes if you set the pot so that vRef is 0.5V you will get 1A through each coil.

The current that flows in the motor coils goes through the sense resistors to ground, generating a proportional voltage drop. If you look at the internal circuit of the L298 the sense pin is just the emitters of the lower pair of transistors in each H bridge.

The L297 compares this voltage with vRef to decide when to turn each coil on and off to keep it at the required current.

---

[www.hydraraptor.blogspot.com]

A couple of quick questions.

1. You measure DC across the 0.5 Ohm sense resisters?
2. Do you messure while it is moving or holding station?

I ask this as I set it per the instructions of backing off till it doesn't move and then forward to started moving again.

Then I listened and advanced until the steps crisp sounded rather then soft.

My heat sinks and motors are cool to the touch and I can't really messure anything across the resisters with my current settings.

You measure the voltage on the Vref test point where it is constant, not on the sense resistors. It then does not matter what the motor is doing.

---

[www.hydraraptor.blogspot.com]