# Thread: Calculating the correct Voltage & Current

1. ## Calculating the correct Voltage & Current

Hi,

I am at the final stages of building a 3 axis router and I am having trouble determining the correct power supply requirements.

I have three Nema 23 stepper motors with rated voltage of 4.0 V and current of 2.0 Amps per phase (bipolar serial).

I also have three SGS-Thompson L297 and L298 based driver boards. Voltage can be between 8-26V. Maximum current output 2.0 Amps.

The three driver boards are connected to a PC via a parallel port breakout board which requires 12V.

My question is what is the optimum power supply in Volts and Amps to power this set-up? I would be very interested to know why.

Secondly, if I were to add a fourth axis with same motor and driver board, what then would be the optimum supply voltage/current?

2. In general you want to have a suplly voltage much higher than what you are looking at. Here is some further explanation of that point:
http://www.pminmo.com/wiki/index.php?title=Performance

Supply current is a debatable topic. On full and half step (which your L297-8 arrangement is) you will have instaneous demands for 4A per motor per your description, but that doesn't mean you need a 12A supply for 3 motors. As the supply voltage increases the duty cycle of the chopper will decrease. So the average DC current value will decrease. So first decide what voltage you want to run.

3. Like Phil said, with a voltage limit of 26 volts you won't be able to get much speed out of your steppers. Speed is pretty much directly proportional to voltage, and for best speed you want a supply at least 10 times and preferably 20 times the rated voltage of the motors.

You might try running the motors parallel instead of series. In parallel your motors would be rated 4 amps and 2 volts. Running parallel at 2 amps, the static torque would be half the series rating, but torque at higher speeds would probably be better. It should be easy to try, and may give you improved performance over series connection.

Anyway, use the highest voltage you can with your drivers. Depending on how the drivers are rated, you may need to leave some voltage margin for back EMF.

Jeff

Ps Don't know why L298 based drivers would only be rated at 26 volts. I thought the L298 chips were rated for 50 volts. Even allowing margin for back EMF, the 26v rating sounds rather low.

4. The L298 has a operating voltage up to 46V with a max limit of 50V, is there something else limiting your driver voltage to 26V?

5. Hi,

Yes, you are right regarding the L297/L298 configuration, it does have higher maximum voltage rating. Yes, you are right regarding the limiting factor (you guys are good!). My existing driver boards don't need a secondary power supply to produce the 5 Volt for the logic if the supply voltage is no more than 26 Volts.

Following your given advice I can however modify my existing wiring to remove this constraint and draw the 5 Volts from the break-out board and increase the voltage to the motors. Problem solved (hopefully). I just need to power the break-out board with 12 Volts (to get a regulated 5 Volt output)and supply the drivers with =>40 Volts. I was thinking of a PC power supply but read in previous posts that this type of power supply is not ideal. Is this correct?

6. You can use pc supplies in series, but use a nice filter cap in conjunction with them.

7. You could wire the +12 from multiple PC supplies in series to get higher voltage, but there are a few things you need to watch for.

First, many PC supplies have the DC ground connected to the case. Connecting supplies like this in series would short the supplies through the green wire ground in the power cord. You need to measure between the case and the DC ground to make sure there is no connectivity. If they are connected, it may be possible to open the case and disconnect the them.

Second, most PC supplies need a fairly substantial load on the +5v line to work properly. People use either a fairly hefty resistor, or an automobile tail light bulb.

Third, like Phil said, you will probably need an external filter cap.