# Dont believe vref readings for mp6500

Hi
I recently bought 4 mp6500 stepper controller boards (potentiometer control, unsoldered). Have tried to set the vref based on your very good video. The problem is that I’m seeing current values from - XXXmV to a max of about 350mV.
Since my steppers have a continuous current rating of 2A, and I have heatsinks on the ICs, I figured I’d go for a 1V vref. Am surprised to see negative mV vref values, and dont understand why the max is so far below 1V.

I have the stepper boards plugged into a CNCShield board that I’ve been using for several months, and that is plugged into an Arduino Uno running GRBL.
The stepper power is 20VDC, the arduino is 5VDC.

One additional concern/note. Initially I plugged just one of the stepper boards into the CNCShield. That is where the 350mV numbers came from. As I added additional boards, the vref numbers went dramatically lower. Namely, w/ 4 boards plugged in, they were (all 4) < 10mV max. This almost sounds like I need to invest in a new CNCShield board? Or possibly in a new 20V supply? the thing is, both of these have been working just great for months, and the CNCShield really just connects these pins with those pins…

Hello.

It looks like you are using the equation relating the current limit to VREF from our video about setting the current limit. The equation in the video is for the DRV8825 and does not apply to the MP6500. As explained in the “Current limiting” section of the MP6500 product page, the equation for that driver is `current limit=VREF×3.5 A/V`, so if you want to set the current limit to 2A, VREF should be about 570mV.

It is quite strange that VREF changes as you connect more drivers to your CNC shield and that you are ever seeing negative voltages. Does the motor supply voltage (VM) change depending on how many drivers are connected? Please post some pictures of your boards and your setup (including ones that show how you are measuring VREF) along with information about your CNC shield and power supply, like datasheets or links to where you got them.

Also, can you try testing a driver without the CNC shield using the minimal wiring diagram on the product page?

- Patrick