Measuring Voltage

[Martin Hegedus]

Hi,

On another thread, Michael Wimble brought up detailed sensor logs.

Going off on a tangent with respect to that, what is the BKM for measuring voltage?

The straight forward way is to use an MCU ADC to measure the voltage from a simple resistor voltage divider (and maybe an analog MUX).

But from the experience of the group, is that the best way to go?  Other options, out of many I guess, is to use a unity gain op-amp to increase impedance and/or something to galvanically isolate the measured circuit and/or separate ADC.  Or is the simple way good enough to monitor the system?

If there are better ways, what chips/circuits are recommended?  Or any other electronic components, other than two resistors, which should be added to the mix?

Thanks

Martin

[Chris Albertson]

I have always figured that two resisters is good enough, and even 8 bits of resolution is overkill for tracking battery voltage.
I would bet that even if you just use the ADC pins on a common uP, you are actually using an op-amp.

[Martin Hegedus]

Thanks.

In part I'm curious how isolated the MCU needs to be from the high amp motor/servo circuit when measuring voltage.

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[Chris Albertson]

I think the only danger is that the motor is braking and generating back EMF into the battery.  Then your nominal 48V battery might have a few more volts than 48 across it.   It needs a fairly heavy robot before this matters much.

OK, let’s assume you are using a 48 volt battery and the MCU runs on 3.3 volts.     So the ADC will have a 0 — 3.3 volt range.      

I would apply a safety margin to the 48 volts and assume it might go to 60 volts.   So I want to divide the power supply volts by 20.

To keep the load low, I want the total resistance to be maybe close to 200K Ohms.   Using common parts, I might pick an 190K and a 10 k resistor.   This is close enough.

This provides a huge amount of isolation.  There is a 190K resistor between the voltage rail and the MCU.   You get this automatically by the nature of voltage dividers.

Measuring current is harder.   You can buy a hall effect current sensor chip on a breakout board or use a low value sense resistor.   If using a sense resistor, I always place a resistor of a few K in series with the MCU pin.  It acts as a current limiter and with a near zero current going to the pin does not drop volts.    I’d go with the hall effect device if the current were more than a few amps, mainly just to reduce losses in the sense resistor.