low power voltage dividers - what am I missing? [solved]

[gregcope]

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.

[Felix]

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?

[gregcope]

Its a solar powered battery.

Vcc - 27M - 3.3M - GND

Yes to get 30V down below 3.3v

[TomWS]

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

[gregcope]

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?

[TomWS]

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.

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

[gregcope]

TomWS

Many thanks.  That explains it!