Multimeter

Uses

• Finding faults on electrical circuits & appliances
• Checking remaining battery capacity
• Finding voltage, polarity & regulation of wallwarts
• Much use in electronics too

Also imperfect for, but still pretty useful:

• Finding faults causing RCD nuisance trips
• Checking earth continuity

With additional equipment:

• Remotely locating cable & circuit breaks

Checking batteries

By Voltage

Measuring battery voltage is a very inaccurate but still useful way to get some idea of remaining capacity.

• Cheap carbon zinc cells may discharge from 1.5v down to around 1.1v before they're completely dead.
• Alkaline cells are dead by around 1.3v
• Lead acid cells are dead by 1.8v (10.7v for a 12v battery), below which they should not be discharged.
• NiCd & NiMH batteries' charge state can't be tested with a multimeter until they're close to flat.

Terminal voltage vs remaining capacity is not in any way linear, terminal voltage remains steady for a large part of the cell's life before deteriorating. The voltage/capacity relationship differs for the various battery types.

Current

Measuring max current delivery gives a better indication of remaining battery life, but is less convenient. It is only practically applicable to zinc carbon and alkaline cells (using a multimeter). Never try this with NiCd, NiMH or lead acid, the huge currents delivered can kill a multimeter.

Low current batteries (AA, 9v, coin cells) may be tested by reading their max current delivery for a second or two on the 10A range.

For analogue meters only having a 1A range, and for large cells on the 10A range (eg D cells), its ok to just touch the probe to the battery for a fraction of a second, and note how quickly the needle moves rather than what offscale value it is heading to. This is a simple trick to read currents of above the meter's max capacity. Probes must not be connected for more than a small fraction of a second, and this technique is not workable with digital meters. It probably voids warranties too, and can cause loss of calibration accuracy, so best use a low cost meter.

Finding faults causing RCD nuisance trips

RCD nuisance trips are normally caused by earth leakage. A megger is the best instrument to test for mains leakage, as it tests insulation at mains voltage or above, whereas a multimeter will typically test it at 1.5v - 12v. Despite this, a multimeter still picks up the majority of insulation failures, and thus is useful for finding the cause of nuisance trips.

One meter probe is connected to both L&N of the appliance, and the other is conected to the earth pin or the metal case. This gives a resistance reading, or ideally an open circuit reading. Keep fingers off during testing, as a finger will pass more current than the appliance.

Ensure that every power switch on the appliance is on. If testing a (disconnected) oven for example, all heating elements should be switched to on.

Fault resistance at 1.5v can be very different to resistance at 240v, so it is not possible to give a pass/fail threshold value. Any item that passes noticeable current is a possible insulation fail, and the 1 or 2 items with the lowest resistance readings are the prime suspects.

Appliances with line to earth filter capacitors (eg TVs, computers) conduct a little momentarily then go open circuit. Slight momentary conduction does not indicate failure, its the steady resistance after that that counts.

Appliances must always be disconnected from the mains when doing this test. Switching off but leaving connected is no good, any readings obtained that way are useless.

A few items are impossible to fully test using this method, namely anything with an electronic controller that switches mains power. Washing machines and touchpad type microwaves are the main examples of this.

Fault finding

Multimeters show

• where power is getting to (voltage range) (item powered up)
• what wire is connected where (resistance range) (disconnected from power)
• heater function test (resistance range) (disconnected from power)
• insulation failures (resistance range) (disconnected from power)
• fuses & filament bulbs dead or alive (resistance range) (disconnected from power)

and various other jobs.

Wallwarts

Applying the probes to the dc jack on the voltage scale indicates output voltage and polarity. It will also pick up on the small number of ac output warts: these will give no reading on dc range, but read on ac voltage range.

Regulated warts give the same voltage on or off load, so the voltage measured will match the voltage shown on the label. With unregulated warts, output voltage rises significantly with no load, so measured V will be higher than indicated on the wart's label by anything from 25% to 50%.

Cable break location

A capacitance range can be used to estimate the position of a break in a cable or circuit. Plug-in leads tend to break at one end; knowing which end means one end can be cut off and the lead repaired for re-use.

The technique is to measure the cable's capacitance at each end. Capacitance is proportional to the length of unbroken connected cable, thus the approximate position of the break is easily found.

Meters with self contained capacitance ranges are easy to use, requiring only to touch the probes to the cable without touching the connections with one's hands (which would alter the readings).

Issues

Wrong range destruction

Applying power to the meter probes when switched to the current or resistance ranges can cause instant destruction. Analogue meters are usually very vulnerable in this respect, one mistake can kill them.

Live probe plug

A probe lead coming out when reaching the probes to mains causes a free swinging live connector, and can cause shock. Probe lead plugs should be a stiff fit to avoid this. Loose fitting plugs are common on old meters, and are a safety issue. They can be fixed by very slightly bending the metal pin of the plug.

Explosion

There have been occasional cases of multimeters exploding due to

• application of excess current (eg conecting to mains when on current range) and
• application of excess voltage (eg due to lightning strike when attached to mains wiring)

However this is a fairly rare phenomenon a long way down the list of DIY safety issues.

Many new meters meet certain IEC use categories to improve safety in this respect. Meters without such certification are best not used on high energy circuits, ie those where high voltage and high current are both available.

Non-rms readings

On ac ranges, most multimeters don't read rms values, they translate to rms value assuming a sine wave. This works very well for sinusoidal waveforms (eg mains voltage), but measuring non-sine waveforms will give wrong reading values. Some often encountered non-sine ac waveforms include:

• TV CRT (tube) heaters
• rectified unsmoothed waveforms
• output from dimmers
• output from invertors & UPSes
• often output from generators under load
• output from electronic lighting transformers

RMS reading meters are available, but their price makes them rarely used for DIY work.

Buying

£10 in 2010

£2.44 (2010)

For DIY use, check the meter has:

• low dc voltage range to measure batteries
• 250v ac or higher range for mains voltages,
• dc current range

and preferably:

• resistance ranges able to measure from under 1 ohm to megohms
• ac current range(s), many lack this
• a current range of 10A or more, not all have this

There are also other ranges and features available, but those are more useful for electronics than DIY.

New

Most new multimeters are now digital. Few don't have the necessary ranges for DIY use.

Where meters will be used for trade, Fluke are well known for their robustness & reliability.

New meters cost anything from a couple of pounds to over £1000. Cheap digital meters sometimes suffer from an accuracy issue, in that when the battery gets low they may read a bit low without giving any other indication of a problem. Analogues are immune to this by design. If you buy cheap, its not often encountered but worth bearing in mind.

Used

Used multimeters meters span the technology range from 1930s Avometers to modern digital instruments. Despite the differing appearances, the job these all do is much the same.

Risky rarities

Pre-war metal cased multimeters turn up rarely. These are dangerous, and you can expect a particularly nasty shock if you take them anywhere near mains. However meters that look like they came from a previous civilisation are hardly difficult to spot. They're styled like old pocket watches.

More modern home made meters occasionally turn up. These were usually constructed by hobbyists short of pocket money. Such meters typically have no protection or safety features. Metal cased ones too often lack adequate basic insulation, and usually have unsecured wiring prone to coming adrift and touching the case.

For both the above types, the main risk is shock. With most shocks the victim withdraws their hand very fast, but when you're holding the live item in your hand, as one does with meters, you're in real trouble. There's also the issue of explosion risk due to no IEC certification or design compliance, a risk specific to their use on high energy circuits, for which they're unsuitable.

Making your own

Making analogue multimeters is easy, but with new ones so cheap now its pointless.

Features & specs

Digital or Analogue

The noticeable differences between digital and analogue moving pointer display are:

• ease of reading (digitals display the number, analogues point to marks on a scale with numbers by it)
• robustness (analogue mechanisms are fragile, and the meters should never be dropped)
• Analogue meters can be killed instantly by passing heavy overcurrent through them, eg by connecting to a battery when on the 10mA range

Other differences are either trivial for DIY work (eg needle displays show changing values semi-well, whereas digital ones become unreadable) or minor and inconsistent (eg accuracy, where figures for both types overlap over a mostly similar range). In terms of required functions for DIY, both do all the things required.

Voltage

Just about all multimeters of all ages have the required voltage ranges. Its unusual to find ones that don't, though they do exist.

Current

Older models often have only low dc current ranges up to 1A, with no ac current range. This means they won't measure current drawn by mains appliances, but this is rarely used so can't be considered very significant. Low end meters tend to have only current ranges that are too low to be useful for diy.

Resistance

The resistance ranges offered depend on the meter's position in the market place rather than its age. 1930s meters were pretty much as well specced as modern ones in this respect. For both modern and old meters, low cost models may lack megohm ranges and a range able to measure below 1 ohm.

A diode resistance range is common on multimeters. This doesn't add anything useful for DIY work, and can be regarded as just another resistance range.

Continuity

A continuity range is also sometimes found. This is a simple go/ no-go test for cables. It will often have the wrong resistance threshold for DIY work, and the resistance ranges do the job already, making this range of little use. If it also has an audible indicator it becomes very handy, as its then not necessary to look at the meter to get a go/ no-go check, or even to position the meter within view. A lot of meters lack audible indication.

Capacitance range

Measuring capacitance is not often useful for DIY. However it can be used to estimate the position of breaks in leads and ring circuits.

There are 2 types of capacitance ranges on multimeters.

• Self contained capacitance measuring - just touch the probes and it reads the capacitance.
• Ranges that will read capacitance value if connected to the mains via the capacitance under test.

Unfortunately the latter are much more common on used meters, and much less useful.

Using the basic type of capacitance ranges

Lower spec meters that can only read capacitance by connecting to the mains via the capacitance can not safely be used at mains voltage for such tasks. However a lower voltage higher frequency supply is just as effective, so for example a 12v (240v/20) 1kHz (240v x20) source will do the same job. Admittedly not many DIYers have such things, but a few do.

Where no such signal generator is to hand, one could use

• a small mains transformer with a harmonic resonant capacitor.
• a PC with freeware signal generator software to produce a suitable voltage and frequency,
  • Voltages below a few volts become increasingly inaccurate, with voltages below 1.2v often not working at all.

Lastly one can use 50Hz low voltage and scale the resulting readings up to get the correct value.

Buying a new meter is usually easier though!

Other ranges

Some meters also have:

• transistor tester
• dB scale is common on analogue meters
• Range doubler or tripler switch (improves readability and accuracy on analogue meters)

None of these are much use for DIY work.

An ignition timing range is also sometimes seen on meters intended primarily for car work. This can be of use to diy car mechanics, but its also not hard to make an ignition timing reading circuit for multimeters without this range (if you can do basic electronics).

Convenience

1930s meters sometimes used a plethora of sockets, with either some range changes being achieved by plugging the probes into the right socket, and some with the range dial, or sometimes all selections made with the sockets. This makes changing range slower, but the overall effect on work rate is trivial. Occasionally this mixed approach is seen on meters from less distant decades too.

The real issue with these meters is that range selection mistakes are easier to make, and analogue meters are vulnerable to range selection mistakes. In practice they seem to be ok, but care is necessary with range selection on these.

Autoranging

Autoranging digital meters are available. These switch range by themselves as needed. This can improve work rate a little in some situations, but is of limited use for DIY, since the quantities measured don't span multiple decade ranges. 240v is either there or not, batteries don't stray outside of 1-15v, etc.

If buying an autoranger, the ability to be range set manually is sometimes an advantage for more demanding work.

Probes

Meters may come with prod, croc clip and hook probes. If not, these can be added later if wished.

Safety

Old meters mostly lack shrouded probe plugs, making them a bit less goof-proof. Old meters often have probe lead plugs that pull out easily if you overstretch with the leads. This is a safety issue, but is easily fixed by bending the plug very slightly.

Many old meters lack any current range fusing, increasing the (tiny) risk of injury if misused.

Old meters mostly lack category certification, making them a bit more vulnerable to explosion due to indirect lightning strike, internal faults etc. However old meters are still widely used in industry, including many Avometers from the 1930s.

• example causing fatalities

Rubber covers

A rubber jacket makes the meter more likely to survive a drop.

See Also

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