The meter used in the Bird 43 wattmeter, for instance, has a full-scale deflection current of 30 microamperes. The more sensitive a meter is, the more options it offers when designing it into a circuit. This will be demonstrated later. To configure a meter to measure voltage, a resistance is usually placed in series with it. If a meter has a full-scale reading of one milliampere, then the series resistance is calculated at 1, ohms per volt minus the internal resistance.
One volt across a 1K ohm resistor equals one milliampere. For instance, if a full-scale voltage reading of 10 volts is required and the internal resistance of the meter is 50 ohms, then the series resistance equals:.
For the same voltage range, if the meter has a full-scale reading of 50 microamperes, then the series resistance is calculated at 20K ohms per volt one volt across a 20K ohm resistor equals 50 microamperes.
If the internal resistance is 2, ohms, then the series resistance equals:. When higher voltage ranges are required — especially when greater than a few hundred volts — it is always a good idea to place the series resistor on the higher voltage side of the meter and connect the opposite terminal to ground as shown in Figure 5. A better solution is to use a voltage divider network with the meter connected across an additional small-value series resistor connected to ground Figure 6.
It is also important not to exceed the wattage rating of the series resistor. As an example, if a one milliampere movement is being used and the meter range of volts is desired, then the series resistance would be approximately K ohms.
If a single resistor is used to drop the voltage to the meter, a resistor of at least milliwatts is required. Most high voltage power supplies used in tube-type amplifiers have a built-in bleeder-resistor network consisting of one or several high wattage resistors. This network is used to discharge the high voltage capacitor bank when the amplifier is turned off to prevent possible electrocution when the covers are removed. The resistance value of the bleeder-resistor network is usually a compromise between the amount of time that is required to completely discharge the high voltage capacitor at least five time constants, typically 30 seconds and the power draw of the bleeder-resistor given off as heat when the amplifier is in operation.
The meter shown in Figure 6 can be configured to accurately measure the high voltage without subjecting the meter to what could be a lethal potential. The meter is connected through a series resistor at the bottom resistor in the chain.
If the voltage across the bottom resistor is greater than a few hundred volts, then an additional small-value resistor can be placed in series at the ground side of the network to limit this to some reasonable level. As an example: If the voltage to be measured is 3, volts and the high voltage filter capacitance is 30 microfarads, the bleeder-resistor should discharge the power supply to near zero in five time constants:. If the bleeder-resistor network consists of 10 x K resistors to make up the required one megaohm total, then the bottom resistor will have volts across it during operation.
The dropping resistor Rs in series with the meter added to the meter series resistance is now in parallel with the bottom resistor in the chain and — depending on the sensitivity of the meter — these must be taken into account if an accurate voltage reading is to be realized.
This problem can be mitigated by reducing the value of series resistance Rs and adding a series potentiometer R1 to adjust for the correct voltage reading. In this example, the meter is actually reading volts, which represents one tenth of the actual voltage being measured.
To configure a meter to measure current, a resistance called a shunt is placed in parallel with it. Full-scale current measurements ranging from microamperes to tens of thousands of amperes are possible depending on the full-scale current range of the meter.
For higher current measurements, three shunt standard full-scale measurements are typically used: 50, 75, and millivolts. The current and millivolt rating of the shunt is usually marked on the side of the connection posts.
The meter being used in parallel must be sensitive enough to accommodate the millivolt range of the shunt. Many panel meters are already designed to connect directly to the shunt, and the full-scale of the meter in millivolts is often shown in small letters on the meter face. Refer to Figure 7. Electrical Academia. An analog multimeter is a meter that can measure two or more electrical properties and display the measured properties along calibrated scales using a pointer.
As shown in Figure 1 , most analog multimeters have several calibrated scales for the single meter movement as shown in Figure 1. For the DC amperage scales, only the red test lead is moved to the appropriate jack. Linear or Nonlinear Scale. A linear scale is a scale that is divided into equally-spaced segments.
A nonlinear scale is a scale that is divided into unequally-spaced segments. Normally, the voltage and amperage scales on an analog multimeter are linear, whereas the ohmic scales are nonlinear. When using an analog multimeter, the correct scale must be used to obtain either a voltage, current, or ohmic reading.
On the scales display, only the volt scale, the volt scale, and the volt scale are shown. Figure 1. The measured resistance value on the scale readout is the R or resistance value that must be multiplied by either 1K , 10, or 1, according to which resistance range was selected. The DC voltage settings include the volt scale; the volt scale; the volt scale; and the volt scale.
This gives a good approximation for a sine waveform, but for others there can be large errors. True RMS voltage measurement requires a considerably more complex circuit, and this is unlikely to be available in an analogue multimeter.
Some high end digital multimeters have this capability. It may also be found that there is a separate scale for low AC voltage ranges. This results from the forward 'turn-on' voltage needed for the diodes which makes the low end of the scale non-linear. For higher voltage ranges, typically with a full scale deflection of above 10V, the standard scales are applicable. The way the analogue multimeters works to measure resistance is a little different tot hat of the current and voltage because battery is needed along with a few additional resistors.
In order to provide the resistance measurement capability a battery is needed to drive current through the external resistor being tested. The amount of current flowing provides an indication of the resistance. When making resistance measurements using an analogue multimeter, it is found that the high resistance indications are at the left hand section of the meter, i.
This may be a little confusing at first, but one quickly becomes accustomed to this. When using a resistance measurement on an analogue multimeter or analogue VOA meter, it is first necessary to "zero" the meter. This is needed to calibrate out any variations in the battery voltage. It is achieved simply by sorting out the two analogue multimeter probes and adjusting the control normally labelled "Zero" for zero ohms.
Once this has been achieved the meter can be used accurately. A further point to note is that the negative terminal of the analogue multimeter is positive to the positive terminal, i. For most measurements this is not of any consequence, although for some measurements of semiconductors it will have a bearing.
It can be seen that by adding the shunt and series resistors as well as a resistor network and battery, for resistance, it is possible to provide a considerable amount of additional capability for the basic analogue moving coil meter. It is important to use resistors with a close tolerance for the series and shunt resistors used within analogue multimeters. Any errors in the values of these resistors will reflect into the accuracy of the measurement.
Some may even be far more accurate than this. It is also worth remembering that the resistance of the meter itself must be known very accurately as this will also have an impact on the readings. Often the resistors may be wire wound to provide higher stability, especially over the operating temperature range of the test equipment. These resistors are also able to accommodate the higher current levels needed for the higher current ranges.
Three Types of Digital Multimeters. Your typical array of multimeter options at any hardware store, whether in -person or online, consists of three basic digital multimeter types. The reason they are broken into three is due to the range of functionality these devices provide.
Typically, dc voltage measurements use the full count capability of the ADC, since signal conditioning is rather straight forward: it uses resistive dividers and filters. Voltage , also called electromotive force, is a quantitative expression of the potential difference in charge between two points in an electrical field.
Voltage can be direct or alternating. A direct voltage maintains the same polarity at all times. Advantages of Digital Multimeters They are more accurate than analog multimeters. They reduce reading and interpolation errors.
The 'auto-polarity' function can prevent problems from connecting the meter to a test circuit with the wrong polarity. Digital multimeter displays have no moving parts. These meters provide secure data that can show peak usage and isolate outages. Working of the Energy Meter. The energy meter has the aluminium disc whose rotation determines the power consumption of the load. The disc is placed between the air gap of the series and shunt electromagnet.
The shunt magnet has the pressure coil, and the series magnet has the current coil. Digital instruments are becoming more popular because of their advantages over analog instruments , such as, greater speed, increased accuracy, better resolution, reduction in operator errors and the ability to provide automatic measurements in system application.
Multimeters indicate the presence of, and measure the quantity of, electrical properties such as voltage, current, and resistance. If your multimeter is digital, it will require a small battery to operate.
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