Analogue displayAnalogue 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 half way 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.
Taking accurate readingsTo 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.
Digital displayValues 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 the least significant 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 1M or more, usually 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.
Connecting metersIt is important to connect meters the correct way round:
- The positive terminal of the meter, marked + or coloured red should be connected nearest to + on the battery or power supply.
- The negative terminal of the meter, marked - or coloured black should be connected nearest to - on the battery or power supply.
|Connecting a voltmeter in parallel|
- Voltmeters measure voltage.
- Voltage is measured in volts, V.
- Voltmeters are connected in parallel across components.
- Voltmeters have a very high resistance.
Measuring voltage at a pointWhen 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?
- Connect the black (negative -) voltmeter lead to 0V, normally the negative terminal of the battery or power supply.
- Connect the red (positive +) voltmeter lead to the point you where you need to measure the voltage.
- The black lead can be left permanently connected to 0V while you use the red lead as a probe to measure voltages at various points.
- You may wish to use a crocodile clip on the black lead to hold it in place.
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.
|Connecting an ammeter in series|
- Ammeters measure current.
- Current is measured in amps (amperes), A.
1A is quite large, so mA (milliamps) and µA (microamps) are often used. 1000mA = 1A, 1000µA = 1mA, 1000000µA = 1A.
- Ammeters are connected in series.
To connect in series you must break the circuit and put the ammeter across the gap, as shown in the diagram.
- Ammeters have a very low resistance.
GalvanometersGalvanometers 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. The photograph shows an educational 100µA galvanometer for which various multipliers and shunts are available.
|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.
|Galvanometer with multiplier and shunt|
Maximum meter current 100µA (or 20µA reverse).
This meter is unusual in allowing small
reverse readings to be shown.
OhmmetersAn 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. 1k = 1000, 1M = 1000k = 1000000.
|Analogue Multimeter||Digital Multimeter|
|Multimeter Photographs © Rapid Electronics|
MultimetersMultimeters 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.
Analogue multimeters consist of a galvanometer with various resistors which can be switched in as multipliers (voltmeter ranges) and shunts (ammeter ranges).