# A4988 I can't get Vref higher than ~0.25V

Hi,
I am using
this board: https://www.pololu.com/product/1183
And everything was working perfectly. After changing the input voltage I tried to readjust the Vref through the potentiometer and I can’t get it any higher than about 0.25V. But I have calculated, that the maximum for my stepper is 0.96V. (2.8V and 1.68A | 1.68/0.7=2.4 | 2.4/2.5=0.96V) It is possible, that the driver got too hot in the meantime. Could a low maximum Vref be a sign that the chip has overheated at some point?

Hello,

That maximum VREF voltage might be an indication that you have damaged your A4988 by setting the current limit too high. You should not set the current limit on an A4988 carrier that high. As the product page states, the maximum current per phase should be 1A without a heat sink or forced air flow. You indicated that you are trying to set the current limit to 2.4A per phase, which none of our stepper motor driver carriers are rated for.

If your stepper motor is rated for 1.68A per phase, you can get close to the stepper motor’s rated current per phase with the DRV8825 stepper motor driver carrier, which can do about 1.5A per phase without additional cooling.

-Derrill

Thank you for the quick reply. I will go with the DRV8825 then. I am a bit confused over the current limit. I know the 1.68A current limit from the datasheet of the stepper. Can I still apply the 30% extra current in full step mode then or does that only apply to the value I measure from the stepper? (1.68 / 0.7) / 2?
I had a heatsink on it and a small fan blowing on it.

You can increase the VREF voltage by 30% if you are using the driver exclusively in full-step mode. However, you will need to use the lower of the current per phase ratings of those two devices, in this case the DRV8825 carrier is the lower of the two at 1.5A per phase. So your formula would be Vref=(1.5A/0.7) / 2 making your VREF setting about 1V.

Your heat sink and fan might have some affect on the current handling of the DRV8825, but it is probably best to make small upward adjustments until you see signs the driver is starting to overheat, then back down from there.

-Derrill

A post was merged into an existing topic: Testing A4988 to determine if it’s working properly