how do multimeters work? voltage divider circuit

[monster]

hi

i was testing my data logger today and making a voltage divider for it. the Velleman pcs10 is really good data logger!

anyway, i used ohms law and realised that when putting resistors across a 81v battery -you better use high ohms or its gonna go bang. so i picked the biggest resistors out and made the voltage divider out of them. when i came to measure the volts drop across the different resistors some thing really weird happened. it was like ohms law didn't work any more and i had to check and re-check to make sure i wasn't seeing things. the volts dropped across the individual resistors did not add up to the total battery voltage.

eventually i tried another meter and the two meters gave different readings? suspecting the battery, i tried again with a fresh meter battery -no change. things got weirder when i tried connecting the meters at the same time, and one meter started to affect the others reading. i figured it must have been something to do with how the multimeters work? and the really high resistors i was using were messing with the meters. i reduced the resistance of the voltage divider and all was well after that.

 

[fechter]

The voltmeter has a certain input impedance (resistance). For a good meter, this will be quite high, like 1 meg ohm or more. If the voltage divider resistors are on the same order of magnitude as the meter's resistance, the meter will "load" the circuit and cause an error. You can calculate the requred value of divider resistors and take into account the meter's input resistance if you know (or measure) what it is. If the divider resistors are much lower resistance than the meter input, you can pretty much ignore the meter's effect.

 

[ngocthach1130]

Yeah an ideal voltage meter would have infinite resistance so no current will travel through the meter and you are only reading the voltage off the circuit and not cause any voltage drop. Of course it is easier to use very high value resistance in real life. So i guess your resistance circuit is too large.

Current meter( Amp meter) is the opposite, ideal current meter must have 0 resistance so it does not alter the actual amount of current traveling through.

 

[Johnbear]

I Just picked up this, it seems to be a great little pocket meter!

DMR-1500


3-3/4 Digit LCD Display
Auto Ranging
Resistance to 40M Ohms
Capacitance to 100uF
Frequency to 100kHz
Duty Cycle Measurement
Continuity Buzzer
Diode Check
Auto Power Shut off
Two Year Warranty

 

[Link]

How much?

Sounds like my old meter minus the oscilloscope.

 

[Johnbear]

Thats the best part $37.00!

 

[Link]

Hmm, not bad.

Still annoyed that I never got to try out the oscilloscope on mine (something like 40MHz). It used a serial plug, and I didn't have any on my computer. They really needed to use USB. :?

Oh, I also remembered that mine also tested transistors.

Cost me like $60, though.

Interesting feature yours has that mine didn't: Duty cycle? I've never actually come across a situation where I'd actually need that . Yours also went higher on the capacitance. Mine went from 40µF down to like a few nanofarads.

 

[rkosiorek]

The PCS10 has an input impedance of 1Mohm. so it is not the highest. It already has an input divider made up of a 1Mohm resistor (R6,R8,R17,R16) in series with a 91Kohm resistor (R5, R7, R15, R14).

to scale your voltage just connect the proper resistor in series with the input.

using ohms law the divider formed by 1Mohm in series with 91Kohm with an input of 30V means i have 2.5V across the 91Kohm resistor. if i put a 2.18Mohm resistor in series with the existing divider chain a 90V input would keep the same 2.5V across the 91K resistor.

2.18Mohm is not a common value but 2M and 180K are, so it is not hard to come up with the required value.

rick

 

[Patrick]

Duty cycle is very handy. It's the same as dwell (in older cars) - it's also the same as PWM in DC controllers.

 

[Link]

Yeah, but I can use voltage to do the same thing for PWM controllers. Since the meter averages out the voltage for like 0.5 second or whatever, the voltage is proportional to the duty cycle. :?

Or I can just use the oscilloscope function. Which my new cheap meter doesn't have :x.