# Problema con ramps

buenas tardes compañeros, quisiera retomar el proyecto de mi MPCNC; pero eh tenido problemas, me dijieron que para poder calibrar mis drivers a4988, debo tener los motores conectados y con un Vref de 1.1V, como mencionar, son motores nema 17 (son los que venden aqui en la pagina, creo que son de 1.5A), me habian comentado que teniaque poner el Vref a 0.7V, mi duda es, tengo que ajustar a que voltaje, y si tienen que estar conectados o desconectados los motores, muchas gracias!!

The 0.7V is the right starting voltage for most A4988 stepper drivers. From my reading, for the vast majority of the A4988 drivers produced today, you can use the equation:

Current Limit = Vref x 2.5

So, 0.7V works out to be about 1.75A max current for the motor. There are a few boards on the market that have different sensing resistors. The equation for these boards is a bit more complicated, and you will have to read the resistance values from the board. Here is a website that outlines how to use the sensing resistors values in the reference voltage calculation. But I think you are pretty safe assuming you have the mass-produced version where 0.7V will work just fine.

As for having the steppers motors plugged in, I set up a Ramps 1.4 board recently, and I had the steppers plugged in, but I see no reason why that would be required.

y si se puede saber, que tipo de motores paso a paso configuró? de cuantos Amp, el motor y cuando Vref, colocó en los a4988?

My build was different since I was using DRV8825 stepper drivers with my Ramps 1.4 board, so the Vref calculation is different. I just mentioned it because I made the adjustment while the steppers were plugged in, though as I mentioned, I cannot see why it would make a difference either way.

The calculation you need:

Vref = Imot x 8 x Rsen

Where Rsen is the value you read off your resistor, and Imot is the current you want to use with your stepper motor. 'Here is a link on how to read SMD resistor values.

Common values for the Rsen resistor on the A4988 are 0.05, 0.068, and 0.1 ohms, so the equation I wrote in my previous post was for an A4988 with a Rsen resistor 0f 0.05 ohms. ’

If it were me, I’d set the value at about 80% of the rated motor current to start. If you had 0.05 ohm Rsen resistor on the board, I’d set it as:

Vref = (1.5 * 0.8) * 8 * .05 = 0.48V.

If you had a 0.068 ohm Rsen resistor, the calculation:

Vref = (1.5 * 0.8) * 8 * .068 = 0.48V. = 0.65V.

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El motor que pones es el KL17H248-15-4A, es de 1,5A, encontrado aquí:

Añado esta copia de pantalla para localiza el valor de la resistencia para después convertirlo como te muestra Robert. La copia de pantalla es de este video, te será útil verlo.

Por último la fórmula viene de la hoja de datos del fabricante del Driver, pag 9:
https://www.allegromicro.com/~/media/Files/Datasheets/A4988-Datasheet.ashx

Saludos.

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