# Calculating the current limit

Okey, sorry, I can not wrap my head around this…

I have the KL17H248-15-4A that I bought from Ryan, and some A4988 drivers, and now I’m about to setup the current limit with the potentiometer on the driver. I use the 12v PSU from V1 too.

The A4988 has 1A continuous current per phase and the stepper is rated for 1.5A.

If the formula is this:
Current limit = Vref / (8 * Rs)

…what value would I get? Can someone explain please? Which values should be for Rs?

I see a 2A limit for the A4988s. You can find info on setting vRef here: https://www.pololu.com/product/1182

Even if you can max out the driver and the steopers, those numbers usually only work in antartica, with water cooling.

Set them to 0.7V. That is usually about as high as you can go without things overheating and it’s strong enough if you wire the motors in series.

Jeff, I’ve bought a ton of 4988’s lately and they all seem to have 0.1 ohm Rs resistors. They seem to come stock at .7V and the low rider would miss steps and hardly had enough power to keep the gantry up. You could easily push it down by hand when running.

I run a cooling fan on mine at 1.5A and with ambient of over 90F have not had any problems.

Current limit = Vref / (8 * Rs)

1.5*(.1*8)=1.2V

(I do run .8 Volts on the MPCNC with no problems and no cooling)

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Yeah I have the same 0.1 ohm Rs resistors as you. And actually got the same values as you do (1.2) but they seemed of since there are so many other tutorials showing ones with 0.05ohm Rs.

Thank you very much for clearification!

Do you think heatsink will he sufficient for 1.2V as Vref?

I understand the frustration. It took me a while to figure out the Rs resistor value was different then every tutorial out there and I was only running at half power.

The stock heat sinks seem to work just fine if you put a fan on them. If they over heat the stepper just turns off.