As I said in the last instalment, I regard a Multimeter as pretty much essential in my modding toolbox. One of these should be on your shopping list _way_ higher than a Dremel Â…
I think itÂ’s a great pity that the badge of honour that ownership of a Multimeter, and more importantly, knowing how to use it has been almost completely usurped by that most overrated of tools, the Dremel

Links to the previous instalments, in case you missed them:
Part one: power tools;
Part two: metalworking tools;
Part three: wiring tools ;


Choosing a Multimeter

Frankly, it would take a long time just to shake a stick at the multitude of models available, so IÂ’m not going to attempt to compare them all.

Broadly speaking, Multimeters fall into three camps:

1 - Analogue: because of the difficulty in obtaining accurate readings (especially when measuring resistance) due to user error in interpreting the scales, IÂ’d suggest that nowadays, an analogue Multimeter is only really suitable for someone that knows what reading they are likely to obtain, and just needs that confirming.
Before there’s a chorus of boos from the old skool electronics whizzes, I’m fully aware that the analogue Multimeter used to be the _only_ type available, but that’s progress for you – it used to be an art, but with the advent of digital meters, it’s become a mere skill.

2 – Digital: much better for the beginner: assuming you have the scale set correctly, there’s no room for user error in interpreting the readings – it’s there in black & white … well, black and grey.

3 – Autoranging Digital: basically the same as the digital type, seemingly more simple due to the lower number of scales to switch between, but way more expensive because of all the electronics required to make it look simple…

What it comes down it the end is that I suggest anyone buying a Multimeter for the first time gets a digital one, and get one that is just a couple of quid/bucks more than the cheapest you can find. If you end up needing a better one, then you can buy it later on Â… this is much better than buying the most expensive, only to be intimidated by all the options, and end up not using it.


Once youÂ’ve got your Multimeter, you want to know to use it, so the rest of this article is going to be some note on basic tests.


Determining Output Voltages

You should already know what the 4 wires on the PSU side of the Molex connectors supply, if not, the pinout is:
1 – Yellow wire = + 12V DC
2 – Black wire = ground (0V DC)
3 – Black wire = ground (0V DC)
4 – Red wire = +5V DC

Let’s assume you’re sceptical about the “7 volts trick” (I know I was till I measured it – it still seems totally counterintuitive to me) & want to check it out. Your Multimeter should have a 10 V DC range, so set it to that (if your using an autoranging model, just set it to DC V). Poke the end of the red/positive test lead into pin one (yellow wire), and the black/negative/ground lead into pin 4 (red wire). Your Multimeter should now be reading 7 V. QED.


Continuity Checking

Any other tutorial is going to shy away from telling you itÂ’s ok to test high AC/mains voltages Â… and this is going to be no exception. In practice (and IÂ’m talking about the most fleeting accidental contact with a finger here), the 110 V DC US mains voltage will tend to burn rather than shock, UK 240V 2-phase mains will give you a shock but wonÂ’t injure you, and UK 3-phase mains (415V AC) is where it starts to get nasty, i.e. potentially fatal. There are always exceptions though, so itÂ’s safer just to regard mains and high ac voltages as a no-go area.

This can leave you with something of a problem though Â… imagine your rig wonÂ’t boot up at all .. youÂ’ve tested the power outlet by plugging in something else, it works, so whatÂ’s next ? You canÂ’t test whatÂ’s coming out of the end of the wall to PSU lead Â… but you can disconnect it from the supply & test continuity. This is not the same thing at testing the voltage, but will at least tell you if there is a break in the cable.

In this case, you’d disconnect the lead, set your Multimeter to the highest Ohms setting available, like X 1K, put one test lead on one of the pins of the mains plug, and the other in the corresponding PSU plug – a reading of zero or very low resistance means that the conductor is good, a reading of infinite or very high resistance means that the conductor has a break in it somewhere, and the cable needs replacing.

You can also test fuses the same way. Remember, zero resistance = good, infinite resistance = bad.



Determining Resistance

For example, if you have a resistor that you think my be useful, but have no idea what value it is (IÂ’ve never learned to the decipher the code myself).

As with continuity checking, set your Multimeter to its’ highest resistance setting, probably X 1K ohms. Hold the contact of one test lead to one of the resistor’s wires, then hold the other leads’ contact on the other wire. Whether or not you get a meaningful reading will depend on the value of the resistor – if it has high resistance, good – for example a reading of three means you have a 3K Ohm resistor. If you get a tiny reading, like 0.125 (125 ohms) or something, you need to try the next lowest scale on your meter, this will probably be X10 ohms. Try the same test again – if your first reading was accurate, you should now be seeing a reading of 12.5 (X10 = 125 ohms). Chances are that your first reading on the higher scale wasn’t that accurate (especially if you’re using an analogue Multimeter), so always go with the second measurement – I wouldn’t be surprised if your resistor turned out to be 250 or 75 ohms instead of the 125 ohms you would have first thought.


Determining Current Drain

Imagine you have some LEDs of a known forward voltage, but have no idea what current they draw. To find out, first youÂ’ll need a power supply of the correct voltage. This may be tricky if your LEDs are rated at 3.3v, so to make it easy on myself weÂ’ll say theyÂ’re 5v.

To find out what current they draw, you could set your Multimeter to the most likely range (probably 250mA), and connect the LED and Multimeter in series off a 5v line on a spare Molex connector.

Specifically, what youÂ’d need to do would be:
1 - connect the positive/red Multimeter test lead to the +5V line (red wire) on a Molex connector,
2 - connect the black test lead to the anode (positive terminal) on the LED (usually the longer wire),
3 - connect the LED’s cathode (negative terminal – usually identified by the short lead and/or a flat on the side of the LED) to the a ground on the Molex connector,
4 - read off the current draw in mA (milliamps).



Well, thatÂ’s about if for the basics on basic Multimeters ! ThatÂ’s just about all IÂ’ve ever used one for, although more expensive ones have further options such as diode & transistor tests.