Thanks for the reply. When you say well known value, does it just need to be your standard 1% tolerance resistor or is there another type of resistor that's even more precise?To provide a bit more detail, the motor current for the two motors (M2 and M4) must pass through R7. This results in a voltage difference across it. This voltage is then passed through a low-pass filter formed by R8 and the parallel combination of C4 and C6.
R8 is NOT the current sense resistor. That's R7. The value of R8 is not overly critical, but the value of R7 needs to be well known in order to make accurate measurements.
The circuit for the top two motors is the same.
There are certainly more precise resistors than 1%. Also, it doesn't have to be a precision resistor, as long as it's actual value is well known, such as by measuring it.Thanks for the reply. When you say well known value, does it just need to be your standard 1% tolerance resistor or is there another type of resistor that's even more precise?
Would it make more sense to be 1.4k ohms? The circuit design here is definitely 1.4ohms, not 1.4k ohms. That was one thing I was wondering about too.The values of the two resistors, R5 annd R7, are partly blocked by the squares drawn around them, so are they 1.4 ohms or 1.4 K ohms? Each of the resistors is shunting a 1N60 diode, which I think are germanium diodes with a low forward drop. So the resistors appear to be current sampling resistors with a clamp diode to limit the voltage applied to the analog input.
So a reasonable guess is that the purpose of the resistors is for stall detection of whatever motors are connected.
No, should be 1.4Ω. A motor which stalls at 500mA would give 0.7V, which is a convenient voltage for the processor to measure. 1.4k would limit the motor current to 3.5mA.Would it make more sense to be 1.4k ohms? The circuit design here is definitely 1.4ohms, not 1.4k ohms. That was one thing I was wondering about too.
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by Jake Hertz