Creating Your Own Meters

If you ever wanted to install meters into projects, or be able to measure values that exceed that of your current VTVM, VOM or DMM, one thing you could do is create your own.

First, you need to get a meter and find the voltage it takes for a full scale deflection, current it takes for a full scale deflection and the internal resistance of the meter. If you do not have one of these measurements, you could use Ohm's law to find the missing measurement (I=E/R; R=E/I; E=I*R). But if you are missing two or more, you will need to find them yourself.

DC Voltmeters

Assume you have a meter with an internal resistance of 150 Ohms, a full scale deflection current of 0.0001 amps and a full scale deflection voltage of 0.015 volts.

First decide what the range of your voltmeter will be (lets say 0-1 volts). Subtract that voltage by the voltage it takes for a full scale deflection on the meter (1-0.015=0.985). You will need to place a multiplier resistor in series with the meter to create a voltage drop that equals that number (0.985) so that your meter can function up to that voltage. You will then use Ohm's law to find the value of that resistor, by dividing the voltage drop you need by the full scale deflection current of the meter(0.985/0.0001=9850). That number is the value of the multiplier resistor you need to put in series with the meter to get the voltage range you wanted.

If you then add the resistance of the meter to the resistance of the multiplier resistor (9850+150=10000), you can see if the value of the multiplier resistor is correct by then multiplying the total resistance of the meter by the current it takes for the meter to make a full scale deflection (10000*0.0001=1). If the number you get is equal to the range of that setting, the value of the resistor is correct (1=1).

Ammeters

Assume you have a meter with an internal resistance of 150 Ohms, a full scale deflection current of 0.0001 amps and a full scale deflection voltage of 0.015 volts.

First decide what the range of your ammeter will be (lets say 0-1mA). Subtract that value by the current by the current it takes for a full scale deflection on the meter (0.001-0.0001=0.0009). You will need to place a resistor in shunt with the meter that will carry up to that much current, and send the remaining to the meter. You will then use Ohm's law to find the value of the resistor, by dividing the full scale deflection voltage by the current that resistor will carry (0.015/0.0009=16.6). That number is the value of the resistor you will place in shunt with the meter to get the current range you wanted.

AC Meters

To measure alternating currents in either of these two meters, you must first rectify the voltages by means of a bridge, and putting in a small capacitor to filter the pulsing DC output a little. After it is rectified it can then be fed directly to the meter circuit and read. But the reading will be the peak value of the AC wave. It will require a different scale than the DC, so that you can read the effective value (root mean square, or RMS value) of the wave. Or if you want you can read it of the DC scale, but you will then need to multiply that reading by 0.707 to get the effective value. But multiplying the reading by 0.707 works for sine waves only.

If you don't know what all this means, let me explain. The sine wave is constantly changing voltage and polarity. It goes through cycles of zero, then rising to its peak voltage, falling back towards zero, and rising in voltage again, but in the opposite polarity. 60Hz means it goes through 60 cycles per second (once was called 60cps until 1967). The peak value of the wave is the highest point in the wave, where it is the highest voltage. The AC sine wave is rarely measured by its peak value. 120VAC actually has a peak value of almost 170 volts, and thats what the meter would read if you stuck it in the wall. But because the voltage of the wave is constantly changing, 170VAC(peak value) does not produce the same amount of power in a resistive load as 170VDC. 170VAC(peak value) actually has the same effect as 120VDC. So what the effective value (RMS value) is, is a direct comparison of an alternating current's ability to do work to a direct current's. The effective value of an ac sine wave is equal to the square Root of the Mean value of a current Squared (RMS).

Below is a graph of the sine wave that is 120 volts alternating current at 60 hertz. In one cycle, the voltage starts at 0 volts after 0 seconds. After 1/240 sec. the voltage rises to its peak value of 170 volts. At 1/120 sec. the voltage falls back to zero. Then at 1/80 sec. the voltage rises to 170 volts at the opposite polarity. And the cycle ends at 1/60 sec. where the voltage falls back to 0.

Back to Electrical Projects & Schematics