Question on resistor divider network for DC bus voltage sensing

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
728
Hi all,

The picture attached is from this User Manual. You can see four resistors of 500k and R7 which is not physically on the board. However, they claim that if you put R7 = 13.3k on either board, you will be sensing 0-3.3V from an input voltage range of 0-420V.

If I say Vsense = (420*13.3k)/(500k*4 + 13.3k) = 2.77V, which is not the range they stated. For 420V I should read 3.3V. Maybe I'm confused or there's some implicit data?
 

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dendad

Joined Feb 20, 2016
3,885
Without looking into it, I would think the output of 2.77V for 420V in is the correct way to go. You do not want full scale voltage readings at the "normal" voltage input. If you do, then you cannot read an over volts condition. When I design an input for say, 4-20mA in my industrial control stuff, it actually can read 0 to 24mA. That way, errors out of range can be sensed. So I think the same applies here.
 

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
728
Without looking into it, I would think the output of 2.77V for 420V in is the correct way to go. You do not want full scale voltage readings at the "normal" voltage input. If you do, then you cannot read an over volts condition. When I design an input for say, 4-20mA in my industrial control stuff, it actually can read 0 to 24mA. That way, errors out of range can be sensed. So I think the same applies here.
Makes sense. Another question, what would be the advantage of using 4 resistors of 500k instead of one 2M resistor?
 

dendad

Joined Feb 20, 2016
3,885
Makes sense. Another question, what would be the advantage of using 4 resistors of 500k instead of one 2M resistor?
Using a number of resistors divides the voltage across the resistors. Having 400V or so across one resistor is asking for it to break down.
I have seen high value resistors in power supplied "mysteriously" go open. The original Mac Plus computers often died as the start up resistor in the power supply would go open. Too many volts across one resistor. The fix was to replace it with a couple in series.
 
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