- Voltmeters measure voltage.
- Voltage is measured in volts, V.
- Voltmeters are connected in parallel across components.
- Voltmeters have a very high resistance.
Next Page: Multimeters
Also See: Voltage and Current
Analogue displays have a pointer which moves over a graduated scale. They can be difficult to read because of the need to work out the value of the smallest scale division. For example the scale in the picture has 10 small divisions between 0 and 1 so each small division represents 0.1. The reading is therefore 1.25V (the pointer is estimated to be halfway between 1.2 and 1.3).
The maximum reading of an analogue meter is called full-scale deflection or FSD (it is 5V in the example shown).
Analogue meters must be connected the correct way round to prevent them being damaged when the pointer tries to move in the wrong direction. They are useful for monitoring continously changing values (such as the voltage across a capacitor discharging) and they can be good for quick rough readings because the movement of the pointer can be seen without looking away from the circuit under test.
To take an accurate reading from an analogue scale you must have your eye in line with the pointer. Avoid looking at an angle from the left or right because you will see a reading which is a little too high or too low. Many analogue meters have a small strip of mirror along the scale to help you. When your eye is in the correct position the reflection of the pointer is hidden behind the pointer itself. If you can see the reflection you are looking at an angle.
Instead of a mirror, some meters have a twisted pointer to aid accurate readings. The end of the pointer is turned through 90° so it appears very thin when viewed correctly. The meter shown in the galvanometers section has a twisted pointer although it is too small to see in the picture.
Values can be read directly from digital displays so they are easy to read accurately. It is normal for the least significant digit (on the right) to continually change between two or three values, this is a feature of the way digital meters work, not an error. Normally you will not need great precision and this digit can be ignored or rounded up.
Digital meters may be connected either way round without damage, they will show a minus sign (-) when connected in reverse. If you exceed the maximum reading most digital meters show an almost blank display with just a 1 on the left-hand side.
All digital meters contain a battery to power the display so they use virtually no power from the circuit under test. This means that digital voltmeters have a very high resistance (usually called input impedance) of at least 1M (often 10M) and they are very unlikely to affect the circuit under test.
For general use digital meters are the best type. They are easy to read, they may be connected in reverse and they are unlikely to affect the circuit under test.
It is important to connect meters the correct way round:
Connecting a voltmeter in parallel
When testing circuits you often need to find the voltages at various points, for example the voltage at pin 2 of a 555 timer IC. This can seem confusing - where should you connect the second voltmeter lead?
Voltage at a point really means the voltage difference between that point and 0V (zero volts) which is normally the negative terminal of the battery or power supply. Usually 0V will be labelled on the circuit diagram as a reminder.
Analogue meters take a little power from the circuit under test to operate their pointer. This may upset the circuit and give an incorrect reading. To avoid this voltmeters should have a resistance of at least 10 times the circuit resistance (take this to be the highest resistor value near where the meter is connected).
Most analogue voltmeters used in school science are not suitable for electronics because their resistance is too low, typically a few k. 100k or more is required for most electronics circuits.
The need to break the circuit to connect in series means that ammeters are difficult to use on soldered circuits. Most testing in electronics is done with voltmeters which can be easily connected without disturbing circuits.
Connecting an ammeter in series
Galvanometers are very sensitive meters which are used to measure tiny currents, usually 1mA or less. They are used to make all types of analogue meters by adding suitable resistors as shown in the diagrams below.
Making a Voltmeter
A galvanometer with a high resistance
multiplier in series to make a voltmeter.
Making an Ammeter
A galvanometer with a low resistance
shunt in parallel to make an ammeter.
The photograph shows an educational 100µA galvanometer with multiplier and shunt. This meter is unusual in allowing small reverse readings to be shown: the maximum meter current is 100µA (or 20µA reverse).
An ohmmeter is used to measure resistance in ohms ().
Ohmmeters are rarely found as separate meters but all standard multimeters have an ohmmeter setting.
1 is quite small so k and M are often used.
1M = 1000k = 1000000
Multimeters are very useful test instruments. By operating a multi-position switch on the meter they can be quickly and easily set to be a voltmeter, an ammeter or an ohmmeter. They have several settings (called 'ranges') for each type of meter and the choice of AC or DC.
Some multimeters have additional features such as transistor testing and ranges for measuring capacitance and frequency.
A digital multimeter is the best choice for your first multimeter, even the cheapest will be suitable for testing simple projects and I recommend this one from Rapid Electronics: Digital Multimeter (basic)
For further information please see the Multimeters page.
Multimeter photograph © Rapid Electronics.