Maplin Multimeter: how to use?

[Andy Mabbett]

I recently purchased a cheap digital multimeter from Maplin:

(search for the product reference, "QT18U")

Picture:

The instruction manual is not helpful :-(

Obviously translated from another language, by someone whose first language is not English, it also assumes a level of knowledge beyond mine.

Can anyone recommend a web page which explains the basics, including the meanings of the various symbols?

Thank you.

[Dave Plowman]

Might be easier if you asked specifically what you don't understand?

Think Maplin do a book called 'Getting the most from your multi-meter' or somesuch, but it may not cover the very basic basics.

Two simple things to remember - always start on the highest range if you're not sure what you're measuring. Never measure resistance or continuity on a 'live' component.

And the most common gotcha is trying to measure voltage after you've measured current. When measuring current, the meter goes in series with the source and load, and has a very low resistance. As such, it usually has a different terminal for current measurement. Leave the leads in this state and try to measure voltage where the device goes in parallel, and that low resistance either blows the meter fuse - hopefully - or the meter itself. So get into the habit of always putting the leads back to the normal socket(s) after measuring current, as trying to measure current with the leads in the wrong sockets will do no damage.

[Martin Warby]

ok working clockwise

1st 2 to are AC volt ranges - use the 600V range for UK mains

next we have DC (current)ampere ranges from 200 microamps to 200 milliamps - probably not useful for a DIYer

the next one is a 10A DC curreent range agin probaly not to much use to the DIYer

next one hFE is for measuring transistor gain - I'm a 4th year electronics student and have never felt the need to use that, make of that what you will

Next we have didode test - agian never used my self

Next 5 are resistance ranges - used to test continuity in a circuit if the display reads '1' on the left then you have an open circuit (ie a broken wire) if it reads a low value or 0 it means there is a path for current to flow.

next 5 are DC voltage ranges - from 200 millivolt to 600V most useful range here is the 20V range,this is useful to test batteries and battery powered appliances.

the connectors on bottom right are from bottom

COM - common - plug the black lead into this one V OHM ma - use this if you are measuring Volts,Resistance or DC current (200ua -200ma)

10ADC - use this when you are using the 10A range as described above

some definitions AC -alternating current - mains voltage as always AC as are the inputs or outputs of transformer(unless DC is stated on casing)

DC - Direct Current - This is the type of power you get from batteries or some wall warts

OHM(the symbol which looks like haedphones) - unit of resistance

Resistance - how difficult it is to pass current through something.

hope this is some help any probs just ask

Martin Warby [e-mail]

[Jason]

for translations try systran a google search should find it.

-- Yours Jason

[e-mail]

[a_mcbride]

This site make give an introduction :

[Lee Blaver]

snip

Snipped due to just replying to this point;

The diode test feature (on my model) is actually very useful, it uses a constant voltage, current-limted supply of 1mA to measure the diode's forward voltage drop. I use it mainly to check transisitor junctions for quick go/no go fault finding.

Lee

[derek]

Spot the deliberate mistake.

Here goes.

It is a class of instrument called a V.O.M. (Volt-Ohm-Milliammeter), see the small writing adjacent to the red test plug/socket.

It has two wires terminated in test probes. With the test leads plugged into the sockets as per the photograph..

It can measure DC voltages or intermittent DC voltages presented across the test probes, from 200 millivolts to 600 volts in 5 ranges.

1) V (dc) Legend ----------

2) Intermittent V (dc), Legend - - - - - (Result is the average)

It can measure AC voltages

3) V (ac) Legend ^v^v (Result is approximate) 2 ranges 200 VAC and 600 VAC

It can also measure the DC current or intermittent DC current flowing through the test leads and hence through the meter, you must break into the circuit under measurement to do this. It can be dangerous, even with modest currents (a few mA) flowing high voltages can and will appear across a break in the circuit, and the point at which you are measuring the current might anyway be at an elevated potential, don't for instance measure the anode current of a power output valve by breaking into the anode circuit and have the whole bag of mashings up at 275 volts, measure the volts across the cathode resistor instead, after checking it! Anyway.. 4 ranges from 200 microamps to

200 milliamps.

4) A (dc) Legend ----------

5) Intermittent A (dc) Legend - - - - - - (Result is the average)

6) A single additional 10 amp DC range with a switch position of it's own, use of this range requires that the red positive lead is transferred to the other socket indicated by the 10A switch position, as well as the 10A range selected on the switch.

NB leaving the lead in this low impedance socket will very likely cause damage if the instrument is subsequently used to measure volts. especially *The Mains* ! Most likely the 10 Amps is not routed through the switch and a low resistance shunt is present across this 10 Amp input at all times. This socket is only *Ever* used to make measurements on the 10 A (dc) scale.

7) It can also measure resistances presented to it across the two test leads on 5 ranges from 200 to 2,000,000 OHMS the Legend for the Ohms ranges is the greek letter Omega. (The Ohm [the unit of resistance] was called after George Simon Ohm who discovered Ohm's law. Note that measuring components mounted on printed boards with other components present is very likely to yield results that need careful interpretation. Note also that despite appearances to the contrary the Ohms range actually works by measuring voltages, so if the circuit in question is powered up it is highly likely your measurements will be up the shoot and/or the operation of your circuit may be altered, (Permanently even!). Sort of Schrodinger's Cat/Heisenberg's Uncertainty Principle at work here.

8) It appears it can test transistors inserted into the appropriate transistor socket, there is a range for measuring "hFE", one of the "h" parameters which indicates the (F)orward current transfer ratio in common (E)mitter mode. AKA "Current Gain" a vulgar term.

9) It appears to be able to test diodes which are presumably cunningly inserted in the transistor socket, a bit too small to see, poss. the NPN socket which seems to have some additional inscription on it. This is where the book comes in handy. You can also adequately test diodes by measuring the ratio of their Forward/Backward resistance on the Ohms range, high is good.

HTH

DG

[Grimly Curmudgeon]

Christ. Next we'll have engineers who've never used a spanner.

What; we already have?

Bugger me.

[Alan]

In message , Grimly Curmudgeon wrote

Many engineers never have to leave their keyboards although a basic grounding in the 'hands dirty' part of the appropriate industry does help a lot in design

[Grimly Curmudgeon]

Agreed. I've got a lot of respect for those hardy souls who've done a bit of graft then got the degree afterwards.

I have no respect for engineers who've never used tools in their appropriate discipline.

The rot started with the Americans, imo.

[mike ring]

My diode function is also good as a continuity tester, you could use a low resistance range to see if a circuit is broken, or shorting to another, or if a fuse is good.

However the diode range has a buzzer to give an an audio indication of continuity - very handy if yours does

mike r

[Lee Blaver]

The 400 Ohm resistance range on mine has a buzzer, but what would be really useful is an ESR test function ;)

Lee

[geoff]

In message , Grimly Curmudgeon writes

Fdisk is your friend

[derek]

Scarcely good enough for monodes, in fact ;-).

Hey c'mon many an engineer has never used a yeller plastic 5 quid "Diode tester".

Real engineers would use an AVO and you wouldn't like that!

Trust me!

DG

[Martin Warby]

There are plently of other ways to test a diode apart from a £5 multimeter.

As for hFE this varys from transistor to transistor (even the same type/model).We are taught to design amplifiers based on feedback which makes the amplifier less dependant of hFE.To find the value of hFE we consult the data sheet for that particular transistor

Martin Warby

[James Salisbury]

A small gotha

the polarity of the supply is usaly opostite to the lead colours on a VOM such as yours, but correct on a DVM when reading OHMS

I would also at the first opertuinity replace the glass fuse with a ceramic fuse of same size and fast rateing these can be obtained from RS or farnell.

I would suggest NEVER takeing that meter near mains, the problem is that the mains can send several thousands of amps through it and the arc when you make contact can liberate molteln metal etc. Even a 12V battery under certain circumstances can cause more current to flow than can be dealtwith by a glass fuse.

[geoff]

99 times out of 100 someone testing a transistor doesn't give an IMM about the hFE, they're jut looking to see if the diode junction is IMM'd

[geoff]

In message , James Salisbury writes

What a stupid thing to say.

A voltmeter is designed, and should be able, to test voltages up to the rated maximum voltage (look on the back, it's normally 1000 volts) - set to the correct voltage range of course.

The input impedance of a voltmeter wouldn't let thousands of amps flow. Go home and put a paper bag over your head, there's an ICBM coming

[Chris Oates]

The cat conundrum is about it being both dead and alive at the same time - like the 'many worlds' theory Heisenberg's more applicable - the act of measurement affects what you are measuring

'I think'

[geoff]

In message , Chris Oates writes

It's about not being able to know which. Of course, dead or alive, they still want feeding