Ground when separating Power supply and Logic Voltage supply

[luck09]

Hi all,
I am trying to control a stepper motor with L298. I have a question about GND pin.
L298 has two voltage supply pins for power and signal. But it has only one GND pin. So I suppose that the GND pin is common between power voltage and siganl voltage here. If I am using two external voltage for L298. One from a power supply, one from a Pin of microcontroller. Power supply has two pins, one is GND. So does the microcontroller. How should I connect the GND pin of power supple and GND pin of microcontroller to the GND pin of L298? Or should I connect three GND pins to be common? Does it affect the circuit?
Thank you very much!

 

[cdh7]

Re: Ground when separating Power supply and Logic Voltage su

Hi,

Yes, you have to connect all the grounds. But ideally only at one common point! So you will have a microcontroller/digital ground net and a motor/power ground net, and they joint together at one point at the power supplies.

It looks like the L298 does in fact have separate grounds. The datasheet says that current passing through the load comes out at the current sense pin. If you don't plan on doing current sensing, just connect that pin to power ground.

You have to keep digital ground currents separate from the motor ground currents. The reason is that high currents from the stepper motor will flow through motor ground traces causing voltage drops due to resistive losses. If some of your digital returns also use this same trace, you will see "ground bounce" in your digital circuitry, possibly leading to errors. So you have to bring your digital ground currents back to the ground point by a different path.

I'm attaching a basic diagram to try to clarify my point.

Regards,
Chris

 

[luck09]

Re: Ground when separating Power supply and Logic Voltage su

Thank you for your reply!
I think I got your point. I burned out and broke pin of two L298 (http://www.st.com/stonline/books/pdf/docs/1773.pdf). Well, Multiwatt package is so difficult for me to plug in breadboard. Therefore, now I am using power transistor for stepper motor driver. The motor run finely, but I want to ask you to get deeper understanding of how to design a drive circuit. (My schematic is attached)
The transistor I am using is TIP31A with the datasheet at http://www.datasheetcatalog.org/datasheet/stmicroelectronics/4137.pdf
From the datasheet, I get some information to choose resistance R1-R4. The voltage Vbe in ON-state is about 0.6V. In fact, I don't see this value in datasheet, I think I need to measure it next time to confirm it. Power transistor might have different value? From that I can find out the current controlled from microcontroller, Arduino in this case, based on the resistance R1-R4. I think the calculation here is fine.
As I read from (http://www.cs.uiowa.edu/~jones/step/circuits.html at figure 3.4), D1-D4 are used to deal with inductive kick, I think it's fine. But for D5-D8, I don't really understand. Could you explain more detail?
So D1-D8 will be chosen based on what information? I know that faster diode is better, but I don't have an exact value of rate of diode to make sure. Normally, I find they choose general diode 1N4001.
This schematic I used based on the schematic at http://hades.mech.northwestern.edu/index.php/Image:Unipolar_stepper_circuit_schematic.png .They may calculate to make sure that we don't need to have collector resistors. But I don't know how to calculate.
That's all of my questions. I hope that you could answer for me.
PS: By the way, did I connect the GND correctly in this schematic?

 

[cdh7]

Re: Ground when separating Power supply and Logic Voltage su

Hi luck09,

From the datasheet, I get some information to choose resistance R1-R4.

I haven't used the Arduino before, but make sure it's able to supply the base current you need. Looks like you calculated around 25 mA per transistor?

voltage Vbe in ON-state is about 0.6V. In fact, I don't see this value in datasheet, I think I need to measure it next time to confirm it.

It depends on the collector current but will almost certainly be higher than 0.6V. Check the graph in the datasheet.

But for D5-D8, I don't really understand. Could you explain more detail?

It's actually explained on the U-Iowa site at Figure 3.4. Due to the winding configuration, the inductive kick causes one end of the winding to go to a high positive voltage and the other end goes to a high negative voltage. So you need catch diodes for both cases.

So D1-D8 will be chosen based on what information? I know that faster diode is better, but I don't have an exact value of rate of diode to make sure. Normally, I find they choose general diode 1N4001.

The 1N4001 should work fine. You have to make sure the diode can handle the reverse voltage. I guess you're probably working with 12V or so, so that's no problem. The the diode must be able to handle the inductor current, which is also no problem if the stepper motor needs less than an ampere. For low efficiency applications, you don't have to be too picky about the diode.

PS: By the way, did I connect the GND correctly in this schematic?

Yes, I believe that's correct.

Regards,
Chris