Hi,
I want to measure the voltage of a boat 12v circuit in a lowpower way!
I brought some 27M ohm (±5% 0.5W) and 3.3M ohm (±1% 0.6W) resistors to cut down on the current draw. I had wanted to measure a 24V circuit (up to 30V charging). However measuring a 12V battery at 13V the divider gives 1.17 volts instead of the expected 1.41. Tried different set of resistors, same result.
Am I missing something about using higher mega ohm resistors in this use case?
What resistors (through hole) would people suggest? Want to KISS and just use a voltage divider, no mosfets.
Boat, 12V, 24V, 30V does not sound like really low power :)
Anyway, you forgot to mention how exactly do you wire the divider? I think the higher value at VCC but can you confirm?
Is this to bring it down below 3.3V for a Moteino or similar?
Its a solar powered battery.
Vcc - 27M - 3.3M - GND
Yes to get 30V down below 3.3v
How are you measuring the resulting voltage? With a 3M ohm source resistance, you need a very high impedance measuring device, otherwise you'll load the signal below its actual value.
Tom
Am using a UT-61E multimeter. Suppose to be high impedance, from what I read... but I have no clue.
What would more sensible numbers be?
10M+1M? Or lower?
Quote from: gregcope on July 24, 2018, 02:08:21 AM
Am using a UT-61E multimeter. Suppose to be high impedance, from what I read... but I have no clue.
Specs for that MM are >10Mohm resistance for DCV measurement. This jives with the numbers you're reading. A 10M resistor in parallel with your 3.3M would be equivalent to about 2.5M and would yield about 1.1 V (1.094 to be exact) with 13V input.
Quote
What would more sensible numbers be?
10M+1M? Or lower?
The resistors you have are right on - just don't leave your multimeter attached ;)
Be sure to put a 0.001 cap across the 3.3M if you're feeding this into an analog input of the processor.
Tom
TomWS
Many thanks. That explains it!