Multimeters, a basic tutorial.

[NelsOn-30]

This is being written for the self admitted electrically challenged members and visitors.

A multi meter will be on most long time modelers list of essential tools.

The following assumes you will be working with safe voltages, less than 20 volts.

The multi meter consists of 3 major parts, test leads, selector switch and read out.

The TEST LEADS transport the electrical source into the meter to enable measurement.

The SELECTOR SWITCH adjusts the input to a small direct current proportionate to what you want to know about the electrical source.

The READ OUT converts the small current into a visual calibrated display, analog (Needle on a meter scale) or digital (numerical read out created by an analog to digital converter)

A multi meter is capable of providing measurements of voltage, current, and resistance.

VOLTAGE-(Electromotive force (Pressure)) The meter utilizes a low current to deflect the meter in proportion to the voltage drop across an internal resistor. By selecting the size of the resistor higher or lower currents are sent to the meter in proportion to range selected. When measuring alternating current a rectifier is inserted ahead of the meter.

It must be remembered that resistance alone does not cause a voltage drop without current flow. When looking for power loss in a track layout a load (such as a test lamp) at the affected location will create a detectable voltage drop across the problem.

CURRENT-Amperes (Volume or amount of electricity) To measure current the circuit must be opened and the test leads inserted so that all the current to be measured flows through the meter.  All current flows through an internal resistor creating a very small voltage drop and sending the resulting low current to the meter.

RESISTANCE-(Ohms) Unlike voltage and current that use power from the circuit, resistance measurement uses an internal battery as a power source.

TURN OFF POWER TO CIRCUIT BEFORE MEASURING RESISTANCE, external voltage/current could damage meter on Ohms scales.

Calibration is required to achieve accurate resistance measurement and is achieved by shorting the test leads and adjusting a rotary control to "zero" the meter. You should note the resistance scale reads backwards because the higher the current the lower the resistance. The meter should not be stored in the resistance mode to avoid draining the internal battery.

The resistance scale is most useful to trace wiring; by locating zero resistance between the ends of a wire you know both ends are of the same wire.

The above descriptions are generic and exclude many factors such as auto ranging.

OK it's your turn to question or?

[grumpy]

Just a note that not all multimeters include ammeters.
Don

[pdlethbridge]

Thanks guys. I was using it earlier to see if I had a short in my bowser mike. Its a great way to make sure that the drivers are installed correctly.