Newb question - Could someone tell me the point of these resistors?

Thread Starter

kenzo42

Joined Feb 26, 2014
45
Hi, I am building this circuit and was wondering what the point of the two 1.4ohm resistors (circled in a red box - R5 R7) are used for.
Also, R8 is for "current sense". Does this need to be a special type of resistor?
Thanks.

102cd990d98d437eaa37aa6cd53ff1b3.png
 

Ian0

Joined Aug 7, 2020
7,041
Current monitor. (hence the label "current sense") The voltage across the resistor connects to A0 and A1 so that the microcontroller can measure it.
The diodes are protection diodes for the input. Note that they are in the direction that current doesn't normally go through them, so post #2 is not relevant.
 

WBahn

Joined Mar 31, 2012
28,192
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.
 

Thread Starter

kenzo42

Joined Feb 26, 2014
45
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.
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?
 

WBahn

Joined Mar 31, 2012
28,192
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?
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.

How well it has to be known depends on how accurate you want the voltage measurement to be.

As to the type of resistor, about the only thing that you might need to consider is how stable the value is, particularly with temperature. But depending on how well you need to know the voltage, that may not even be a factor.
 

MisterBill2

Joined Jan 23, 2018
14,280
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.
 

Thread Starter

kenzo42

Joined Feb 26, 2014
45
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.
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.
 

Ian0

Joined Aug 7, 2020
7,041
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.
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.
 
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