Multimeter voltage question

[Njguy]

If you use a multimeter to test the voltage of a battery, or whatever else, does this create a short circuit or does the multimeter only allow a small amount of current to go through? Assuming there is no other loads in the circuit. This is a really noob question but i couldn't find a straight forward answer on it.

 

[crutschow]

Obviously if it applied a short circuit, the battery current would be very high (limited by its internal resistance) and there would be no voltage to measure since the voltage across a short is zero. So voltmeters are designed to have as high an impedance as possible so they don't disturb the voltage due to any source impedance. The manual for the meter will tell you what that value is. 10 megohms is a typical for a multimeter when measuring DC voltages.

Alternately, if you place a multimeter in the current measurement mode, then is exhibits a very low impedance, since it is connected in series with the load and you don't want any significant voltage drop across the meter which would disturb the measurement accuracy (the maximum drop is typically a couple tenths of a volt full scale).

 

[KeepItSimpleStupid]

Some analog voltmeters are rated in Ohms/volt. e.g. 50K ohms/volt. So if you were measuring 2 V, the equivalent resistance would be 100K ohms.

Some, meters have a different equivalent resistances based on the range used. The high value of resistance can be troublesome for Electricians because leakage currents create phantom voltages or voltages that will drop to zero with a light load. There are special digital meter that have a low Z scale.

 

[audioguru]

A 9V battery in a smoke alarm uses an extremely low current sitting there doing almost nothing for one year. If you measure the voltage of a dead one it will be pretty high at about 8V or more BUT IT IS DEAD! To measure a battery voltage it must have a load. A load for a 9V battery is usually 10mA to 25mA (360 ohms to 900 ohms) which is MUCH lower than the 10M of a multimeter. Then a dead 9V battery from a smoke alarm will measure only a few volts.

I HAVE the old 9V alkaline battery from my smoke alarm. Its voltage with no load is 8.06V. With a 620 ohm load (14.5mA when the battery is 9V) its voltage is only 5.5V and is dropping fast. The current is also dropping to 8.9mA.