Sunday, 21 July 2013

Master your Multi Meter - A basic tutorial

A multi-meter is one of the most versatile pieces of kit in your workshop toolbox. The question is though, how do you use it productively, and what can it tell you? Well, consider this blog a beginners guide to finding your way round the most common features of a typical digital multi-meter, We’ll look at how to make some typical measurements and how to interpret the readings. So, read on and master your multi-meter.

Multi-meter basics – what is a multi-meter?
A simple enough question, but worth answering in a bit of detail. For electrical system measurement applications there are a number of different aspects of a system that you may want to measure or monitor. For example, to measure the voltage of an electrical system is often of interest as this effectively shows the electrical ‘pressure’ in the system that pushes the current around (to do the work – for example, heating a bulb filament to become white hot and emit light).
In addition, you may want to know the current itself, as current is effectively the amount of flow in the system – more flow, more work done. So the pressure and flow are linked, more pressure, more flow, more work done. So voltage as well as current measurement is often important.
But you can probably appreciate that a flow meter is a completely different measurement device and measuring principle than a pressure gauge – and it’s the same with electrical measurements – a voltmeter is a different type meter to an ammeter, or in fact, an ohm meter (measuring resistance to current flow in a part of the circuit). So you need different instruments for each – that is the beauty of a multi-meter! It’s a single meter, that is capable of measuring more than one thing in an electrical circuit – it is effectively more than a single meter, it’s several devices in one (hence the name multi). Typically, multi-meters will always be able to measure amps, voltage and ohms (resistance), but often they can incorporate other features that allow the user to do much more analysis and measurement of a circuit.
Originally, multi-meters were analogue meters, with a needle and dial (like a speedometer). However, these have been almost universally superseded by the digital multi-meter, also known as a DMM. Note that analogue meters are often known as AVO’s or AVO meters (Amps-Volts-Ohms). DMM’s are much more robust than an analogue meter as they don’t need a sensitive needle/dial, however, some users prefer to look at a dial as it is easier to process visually and the detect trends (that’s why digital speedometers haven’t really caught on).

Fig 1 - Analogue and Digital type multi-meters (Draper)

What can you measure with a multi-meter
Let’s concentrate on the basics – how do you connect a meter to the circuit to measure volts (pressure), amps (flow) and resistance (resistance to flow). As we mentioned, the measurement principle is different for each, and so is the way that the meter is connected in the circuit. Let us study each case:

To measure the voltage, you need to connect the meter ‘across’ the component or circuit section of interest, effectively in parallel. The meter then ‘sees’ the same pressure as the component and can measure and display the value. Note that the volt meter has a very high resistance; this ensures that no additional load is applied to the circuit by the meter, and hence the circuit itself is not disturbed by the meter whilst performing a reading. Take a look at the circuit diagram:

Fig 2 - Connection of the DMM for voltage measurement 

Note that most vehicle circuits will be earth return, hence one side of the voltmeter may often be connected to a convenient earth point when taking a reading. Another common, but perhaps underused technique when measuring voltage, that is particularly useful on vehicle circuits where voltage is low but current is high, is too measure the voltage ‘drop’ across part of the circuit. Particularly if a high resistance is suspected to be causing a problem – for example, across a switch or connector. Measuring the voltage drop highlights a resistance in a working, loaded circuit and can easily show up bad connections, the diagram below shows the meter connection – a typical value is that the voltage drop should be no more than 10% on any part of the wiring circuit to the component.

Fig 3 – Connection of the DMM for voltage drop measurement

To measure current, the meter has to be connect into the circuit – in series. Note that the meter is connected in circuit so it can measure the flow around the circuit when in operation. When using a multi-meter to measure current, the resistance of the meter circuit is very low. This prevents the meter from creating an additional circuit resistance and affecting the accuracy of the reading. The diagram below shows how the meter is connected:

Fig 4 – DMM connected for current measurement

The important thing to remember is that the meter itself will have a limit to the amount of current it can measure, most DMM are limited to 10 amps maximum. So, you must be careful not to overload the meter circuit, which is often protected by a fuse inside the meter.

Apart from voltage and current, it’s often useful to be able to measure the resistance of a circuit, or a component– that is, the restriction that the circuit/component provides to the flow of current. According to Ohms law, the resistance (in ohms) multiplied by the current (in amps) equals volts. So by knowing the current and voltage across a component, you can calculate its resistance (resistance equals volts divided by amps). However, it’s not always convenient to measure both voltage and current in a circuit (as you need 2 meters), also, you may want to establish resistance without powering up the circuit (for example, measuring a component out of the circuit).

For this application, you can use an ohmmeter (ohms are the unit of resistance). An ohmmeter measures resistance by applying a small current through the component/circuit and establishing resistance by measuring voltage and current. The current is tiny, supplied by a dry cell battery inside the meter, so there is no anger of damage to the unit under test, also, it is removed from the circuit completely for the measurement so the circuit does not need to be activated. The circuit for measuring resistance is shown below and always involves complete removal of the component, or isolation of the circuit, in order to make a measurement.

Fig 5 – DMM connected across component for resistance measurement

Note that it often makes sense to check or calibrate the meter before the resistance measurement is made. To do this you connect the test leads of the meter together, and check that the reading is zero ohms. Also, note that the component or circuit must be completely isolated or you will get false readings

What else can you measure?
Now we have covered the basic measurements, it’s worth noting that most multi-meters have additional measurement modes or features, some of which can be quite useful and are worth understanding. Typical extra measurements that you may see, depending on the meter are:

Continuity test:
Very similar to a resistance test, often incorporating an audible signal to give a ‘go’ – ‘no go’ indication. Very useful for testing bulbs, fuses, switches etc. – components that are generally either open or closed circuit – this mode gives a quick indication if OK or not. The meter generally gives an audible signal (from a buzzer) if the resistance is below a certain value. Can also be used on wiring and connectors to detect open circuits, as long as the circuit is not live, and is isolated.

Diode test:
Another type of resistance check but specifically for diodes (which are an electrical one way valve). They are semiconductor junctions and need a minimum voltage applied across them before they will ‘switch on’ and conduct. Most multi-meters will not provide this minimum voltage in resistance test mode, as they tend to use very low voltages to prevent circuit or component damage. In diode test mode, a small current is supplied by the meter. This is used to test the diode in forward and reverse direction (by reversing the lead connections manually). For a healthy diode, one way should conduct, the other should block. When the diode conducts, the voltage drop across the diode is shown on the display, generally about 0.5 -0.7 volts depending on the diode type. If the diode is blocking, no current flows and the display shows zero. 

In addition, special features are often added to multi-meters aimed at specific applications, for example, Electronics laboratory test meters may include:

An additional connector to allow temperature measurement via a thermocouple that is generally supplied with the meter. The tip of this is the sensing element and the temperature is displayed on the meter display – not that thermocouples are not incredibly accurate at approx. +/- 1 degrees centigrade

Transistor Test
This is an electronic component test feature, similar to diode test, but to test the gain factor of a transistor (i.e. the amplification ability of the transistor, also known as hfe). Unless you’re an electronics engineer or technician, you won’t need this.

Capacitor Test
As above, specifically for testing the capacitance value of a capacitor – again, unless you are into electronics, you won’t need this!

Multi-meters aimed at Automotive Diagnostics are also popular, these may include the following features:

Electronics Tacho
This mode allows measurement of engine speed via an inductive clamp generally supplied with the meter. The clamp picks up ignition pulses from the HT lead, the meter calculates the time difference between the pulses and converts this to engine speed. Sometimes the meter can be adjusted for 4 stroke or 2 stroke engines to give the correct reading, otherwise the reading has to be halved for a 2-stroke or wasted spark ignition system.

Dwell/Pulse width
This mode allows pulse evaluation, to be able to understand the width of a pulse, or the duty factor of a pulse (i.e. how long the pulse is active within a switching cycle). Typical automotive applications would be – points dwell measurement (how long the points are closed for - normally given as an angle in degrees or %); injector opening time (in milliseconds); idle speed control valve (% on/off of the driver circuit), there are also others…

Practical Measuring Tips
When using a multi-meters, bear in mind the following:
  • Note that most multi-meters will measure AC (Alternating Current) and DC (Direct Current). For vehicle applications, you will be measuring DC almost exclusively! 
  • Make sure that when you are measuring that the meter is set correctly before you connect to the test point. Select the correct mode (AC volts, DC volts etc.). 
  • Some meters are ‘auto ranging’ so they are able to automatically detect the correct range according to the input. However, with some meters you have to select the range manually – be careful when doing this, if you are not sure what range you need, start at the highest and work your way down the scale!
  • Make sure that you connect the test leads correctly for the mode you are in, there are normally several jack sockets, different ones for current and voltage. If you don’t get this right you will get no reading (best case) or worst case, you will damage the meter!
  • Most digital multi-meters will read a maximum of 10 amps current, and most are fitted with an internal fuse so that if they are overloaded the fuse blows before any damage occurs to the cables or the meter.

Typical Measurements
Let’s look at a typical measurement application - measuring voltage. Voltage is a measure of the system ‘pressure’, and you need this to push the current around. No volts no current flow! To measure voltage, simply connect the leads to the appropriate jack sockets 

Picture1 – Connecting leads into jack sockets

Use the dial to select the correct measurement range, for a 12 volt system it’s between 10 – 20 volts, for this meter we can select the 30 volt range, or, if the meter is capable of auto ranging, just select DC volts

Picture 2 – Selecting correct range on the meter

Now connect the leads, for negative earth cars, black lead to a good earth, red lead to the test point (positive earth cars, other way round)

Picture 3 – Connecting the leads

You’re now connected, so you can observe the reading, the display will show the voltage potential at the test point. The test point should show a voltage reading similar to battery volts.

Picture 4  – Meter reading battery volts from light switch supply

Before starting any voltage measurements, connect your meter across the vehicle battery, this shows that the meter is working, and that the battery is not flat! Note that in voltage range setting the resistance of the meter is ‘high’ so that it doesn't affect the circuit being tested by being a significant additional current path.

A digital multi-meter is a useful piece of kit, particularly one with the useful extras for automotive use. However, you don’t need to spend a fortune, all meters will read volts, ohms and amps and those are the basic functions you need for electrical system fault finding. The most important things to look for are a good quality, durable unit with a protective case or holder that will stand up to the working environment. Long leads are essential for use around the vehicle (look for leads of approx. 1 metre), in addition to a large clear display (display back lighting is also useful).

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