voltage too low in a voltage divider

[JohnKElectroTech]

Hello,

When I find the voltage difference between ground and a point between the two resistors, I get about 2.2V, when I expect 2.5V. I switched the resistors in case the resistances were far from their stated values of 2 MOhm, but I got the same voltage. I also tried new batteries and two different bread boards, but got the same voltage. Can you tell me why the measured voltage is low? Do bread boards have resistance large enough to create the voltage drop? Thanks.

 

[MikeMl]

What kind of volt meter are you using?

 

[crutschow]

It's a classic case of the measurement affecting the measured value. You forgot to consider the impedance of your measuring device.

Your voltage divider has an output impedance of 1 megohm (two 2 megohm resistors in parallel). A typical multimeter has an input impedance of 10 megohm. Thus the measured voltage is reduced to a value of 10/(10+1) or 90.9% of the open circuit voltage. For 2.5V this gives a voltage of 2.27V, close to what you measured.

 

[JohnKElectroTech]

Thanks, Mike and Carl. I'm using a USB oscilloscope, which probably has an impedance as Carl said. When I replaced the 2 megaohm resistors with 100K resistors, the voltage went up to 2.5V.