 # A4988 sense voltage limit

Hey,

I've been looking through the Datasheet to the a4988 and I noticed that there is a Max voltage for v sense of 0.5V. Now I imagine this is for the sense resistors. I know the pololu board uses .05ohm sense resistors and they are 0805 package. These would be 1/4 watt resistors but at 0.5V through 0.05ohm resistors is that not a draw of 10amps?

I know that the current is limited through the driver but how is that calculated? How does the sense resistor go against ohms law (I know the draw here has to be less here since that resistor can't handle dissipating that much)?

Max voltage is 0.5 volt. Resistor is 0.05 ohm. I call that margins. What's the problem with that?

I guess the problem I’m seeing there is 10 amps of current through the resistor at that voltage and resistance. 5 watts through a 1/4 watt resistor. I know this can’t be right but I’m not sure where I’m looking at this wrong.

I doubt a 4988 handles 10 amp.

No, this has nothing to do with the sense resistors. It is a limitation of the chip itself. And it really is a maximum value, not something that should occur in normal operation.
Because of the principle of operation of the chip, the voltage on the sense resistors will occur positively and negatively. And the negative voltage is dangerous for the chip.

Keep well clear of maximum values - they are not the same as recommended operating values.

I still think I’m confused on the sense resistors though with putting that low of a value resistor. What controls the current running through those sense resistors?

Motor current flows through those sense resistors, and the chip should keep that below ~1Amp.
That calculates to 50mV (0.05volt) across the resistor, and 50mW (0.05W) dissipation.
Leo..

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Hi,
Current flow through load and sense resistor.

Tom...    1 Like

Awesome. Thanks so pretty much what I was thinking because the current is limited.