A close-look at analog panel meters and how to use them in your next project.
Everything today is digital. So why would you bother to use an analog meter these days? There are many good reasons.
Analog meters can be easier to read. A needle a bit more than half way across a 20V scale is around 12 Volts, and if you need more precision, look closer. On a digital meter saying 1234, is that 1.234 volts or 12.34 volts? Where’s the decimal point? This can be especially confusing on an autoranging display.
Analog meters have a certain look about them which digital meters can’t give. And if you want an old-fashioned or “Steampunk” look, an old round-faced analog meter will give you that in spades. And analog meters have one big advantage over their digital cousins – they don’t need a power supply. No more fiddling with +/- 5V supplies, sometimes isolated…
Whatever your needs or reasons, a knowledge of how to use analog meters can be a very useful thing to have. All you need is some very basic maths and a knowledge of Ohms Law.
The Joy of Movement
The main part of an analog meter is called the ‘Movement’. As this is the bit that actually moves, this is fair enough. A movement consists of a needle, attached to something that will move it across a scale to indicate a reading.
Magnetism is usually (but not always) involved. Moving coil meters are probably the most common type – a coil is suspended in a magnetic field, and when current passes through the coil it generates a force that is arranged to move the coil and the needle on an axis. Certainly most analog multimeters will use this type of movement. They can be made very sensitive – 50 μA or less for full scale deflection of the needle. This is a fairly crucial point – no matter what the meter actually reads – volts, ohms, temperature – the primary driving force of an analog meter is the current through it.
In a practical meter, the components are very small and delicate. The springs are akin to watch springs, the wires are very fine, and the clearances between the magnetic pole pieces and the core are very small. So, one of the problems with this kind of movement is that any foreign matter, especially magnetic particles, can seriously impede the movement of the coil in the magnetic field. This can lead to erratic operation or the needle “sticking” in some positions. Poking around with a pair of tweezers can do more damage than it solves. Meter coil wires are usually very fine and if overloaded may burn out. If any of this happens the meter is usually beyond repair. But if you don’t abuse or open them, they will give years of good service.
At the ends of the axle, above and below the springs, there are usually low-friction bearings, and as in high quality watches, these may be “jeweled” – made of semi-precious stones. This can make a good meter movement expensive. But they can be very sensitive, and they have very linear scales. There is generally a screw adjustment on the face of the meter which adjusts the spring above the needle to “zero” the needle in the right place. High quality meters may also have a mirror on the scale – this lets you make sure that you are looking at the needle at exactly 90º to avoid “parallax” error. In this way, you can get an accuracy of around 1% on a good quality meter.
There are various other names for moving coil meters – Galvanometers (after Luigi Galvani, who was one of the first people to play around with electricity) and D’Arsonval movements or Weston movements, after the scientists who invented and developed this type of meter.
Pumping Iron
Another common type of movement is the Moving Iron. In this one, the coil is fixed, usually wound around a fixed piece of iron, and another a piece of iron is suspended in the magnetic field that is generated when current passes through the coil, which magnetises both pieces of iron, which repel each other, causing the moving one to move and deflect the indicator needle. This type of meter can be made very rugged, and they are consequently not as delicate as the moving coil types. They are a lot less sensitive, less linear (especially at low currents) and less accurate. But, the other nice thing about this type of meter is that they are just as happy with AC as with DC. So they are well suited to things like battery chargers, mains voltage and current meters and the like, where great accuracy is not needed and ruggedness may be of great advantage.
There are other types of meter but they’re so rare that we won’t cover them here - the above two are the types of meter that you are most likely to encounter. This is pretty universally the symbol used for a meter:
Apart from what type of meter you have, the most important thing you need to know about a meter is its Full Scale Deflection, or FSD – the current needed to deflect the needle to the end of the scale. On a bare meter this will usually be given as a DC current, though on a meter which has already been adapted for a specific purpose it may be in another unit – Volts DC or AC, or even something like temperature. Generally, the most sensitive meter you can get is 50μA FSD, and they are getting rarer, though D’Arsonval made one that could detect 10 μA! As above, meters basically respond to current, but you can get them to read different parameters by using shunts, multipliers, rectifiers and the like. This is covered further below.
Going Ballistic
Meter movements have what is called “Ballistics”. This is how the meter responds to a step current applied to it. A big meter with a long needle will have a lot of inertia, the movement of the needle may encounter air resistance, and the inductance of the coil may have some effect, so it will not respond very fast and may take some time to deflect to the correct reading. Smaller movements will rapidly move the needle to the desired point and may even overshoot and oscillate slightly before coming to rest. Meters may have vanes or even dash pots to damp the movement if this is undesirable, but there were “ballistic meters” where the deflection was proportional to the total charge applied to it. These were used for testing telephone lines. Analog phones had a capacitor in them – usually around 2 μF – and when a voltage is applied to the line you got a “kick” of current; these meters were designed to maximise the kick to indicate the health or otherwise of the line, and a skilled operator could tell how many phones you had connected as well! Some meters intended for broadcast audio use have specified ballistics, so that momentary levels that would overmodulate a transmission can easily be seen, and there are standards for this too, notably from the British Broadcasting Corporation (BBC) but many other international standards organisations as well. Audio meters are usually calibrated in decibels (dB) or the closely related volume units (VU) – here is a typical example:
You also get Mirror Galvanometers – where the movement moves a small, light mirror rather than a long, heavy needle. In days of old, this would deflect a light beam onto a scale, which could be large, giving the effect of a long needle without the weight or inertia. These days they are used to deflect laser beams for bar code scanners, micro marking, welding and cutting, effects in discos and the like. Scanlab www.scanlab.de/en is a company that specialises in mirror galvanometers and scan systems that use them, and their website is instructive if these interest you.
Shunts and multipliers
A bare meter (if it is of the moving coil type) has a fairly low DC resistance and an FSD that will be anywhere from 50μA to 10mA or higher. At FSD it will typically have a fair bit less than a volt across it. You can make it read a
different value, which may be a higher current or a voltage, by using shunts, which are resistances used to divert, or “Shunt” current away from the meter, and multipliers, which are resistors used to drop voltage, or limit the current to FSD at a certain voltage.
The Jaycar and Altronics catalogues helpfully provide some data on the range of analog meters they sell.
Here's an example from the Altronics catalogue:
There may not appear to be a lot of information here, but, in fact, it is precisely what we need to know. Take the Q0500A, first on the list. It is a 1 mA meter with a coil resistance of 210 Ω. That means that when it is passing 1 mA, at Full Scale Deflection (FSD) it will have 210 Ω x
1 mA = 210 mV across it. (As an aside, notice how that is the same as the basic sensitivity of most Digital Panel Meters?)
If a meter directly measures the current that you want it to measure, you’re home and dry. If not, you have to adapt it to do so. You can’t get a meter to measure less at FSD than its basic sensitivity without using some electronics (for example the 5A meter above cannot be adapted to measure 50 mA). But you can get a meter to measure more than its base FSD current, or indicate voltage instead of current, by using shunts and multipliers. So let’s look at those.
Shunts
Let’s say we have a Q0500A 1 mA meter from the table above. But we want this to give us an FSD of 500 milliamps – 500 times as much. We can do this by putting a shunt across the meter terminals so that 499 mA flows through the shunt, and only 1 mA flows through the meter, like this:
The shunt is obviously going to have a very low resistance – 1/499 of the meter resistance in fact. With the 1mA meter above, this works out to around 0.4 Ω. In this case, you could probably find a 0.47 Ω resistor and a 27 Ω resistor in parallel – or use a 47 Ω or 100 Ω trim pot in parallel for pinpoint accuracy. In a case like this, you can probably use conventional components, but where your shunt resistance needs to be less than about 0.1 Ω, you can probably just use a piece of copper or resistance wire, though you’d have to experiment with the length and gauge of the wire. If you are doing this, you can measure a long piece of your shunt wire (a metre or two) and then guesstimate how much you are going to need for your shunt. Another trick is to make your shunt a bit longer than you need, and move one of the meter wires along the shunt until you get the correct reading (again, use a known good DMM to check) . You can use a screw connector or a soldered connection to fix the meter wire in place, and maybe to adjust the length as well. If your shunt winds up a bit too short, you can often file or saw your shunt a bit to increase its resistance and get the FSD just right. Bear in mind that a moving coil meter will only give 1-2 % accuracy, so if your shunt is 1 or 2 % off, it’s not going to matter a great deal.
You can work out the shunt resistance with the following formulae. You have to know either the meter resistance or its voltage at FSD for this, and how to find this is covered under “Testing Meters”.
So the rule of thumb is to divide your desired FSD sensitivity by the meter FSD sensitivity, then divide your meter resistance by (the result minus 1), which will give you the resistance of your shunt, or if you know the voltage at FSD of your meter, just use Ohm’s Law to find the shunt resistance, as in the second formula above.
If there is a big difference between your actual and desired FSD - in the above case it is 500 times – then you can leave out the -1 in the brackets of the first formula without going too far wrong. In general, use a meter that is the nearest you can find to your desired FSD.
A professional shunt will be made of material with a low resistance / temperature constant, so the reading will not vary with temperature, but home-made ones are almost as good unless you’re really finicky. You can often buy shunts for the standard meter brands and sizes, they can be mounted externally or sometimes on the meter terminals. Current sensing resistors often make good shunts, but current sensing usually involves low voltages (10 mV or so) so check that your resistor can take the power of dropping 100-200mV for your meter.
A word of warning. Where possible, connect the meter and the main current to the shunt separately. If they use the same screws for connection, and a screw comes loose, the main current may try to flow through the meter. If this happens, exit one meter, stage left….. Shunts with separate connections like this are called “4-terminal Shunts” for obvious reasons (2 terminals for the current, 2 for the meter).
Even using a conventional resistor on a PCB you can do this: mount your shunt resistor high off the board, which is good practice anyway, and take the meter connections off the resistor wires, not off the PCB. So, if you get a dry joint and no current flows in the shunt, it won’t go through the meter.
You can get 4-terminal shunts. For example, these ones are available from Jaycar:
But take care here. These shunts are often designed as current sensing shunts and may only generate 50 or 100 mV at their rated current. This may only give 1/2 or less FSD on your meter. And if you use a 200 A 50 mV shunt for your QP0800A 1 mA meter, It would need about 800A to give FSD on your meter, and this is way beyond the specifications for this shunt and would probably burn it out (and then probably burn out your meter as well!). So, if you need one of these, get one with the largest possible voltage for the rated current, or get a meter with 50 mV FSD - they do exist.
For currents under about 5 – 10 A, a shunt will often just be a length of suitable wire mounted in the back of the meter, above that something beefier is needed. But I came across some rather nice 4-terminal resistors recently that would be ideal for meter shunts. They are made by the company Rhopoint (https://www.rhopointcomponents.com/?s=4-terminal&lang=en) and are PCB and/or heatsink-mountable. They are available in various power ratings, and come in a range from 1Ω to 1mΩ. They are high precision resistors (1%) intended for precision current sensing, but would make great meter shunts. They are difficult to get, though, and quite expensive. Here’s an example:
Multipliers
If you want to use a meter to measure voltage, the trick is to put a resistor in series with the meter coil and arrange it so that when the desired voltage is put across the combination, a current will flow to make the meter indicate FSD. You would usually try to keep this current as small as possible, to limit the loading on what you are trying to measure. So let's use a 50 μA meter.
Jaycar offer the QP5012 meter with an FSD of 50 μA, and a coil resistance of 3000 Ω. If we want this meter to indicate 25 Volts FSD, the entire resistance (i.e. resistor plus meter coil) will have to be 25V / 50 μA = 500KΩ.
Now the meter resistance is 3KΩ, which is about 0.6 % of 500 KΩ – so we could just use a 500 KΩ resistor and our readings would only be 0.67 % out – the precise resistance needed is 497 KΩ.
But, if you can’t find a preferred value, or you are finicky, or your meter resistance is more than 1% of the total resistance you need, you can use a preset resistor – preferably a multi-turn one – or better still a combination of a fixed resistor (use the nearest preferred value lower that what you calculated – say 470 KΩ here) and a preset (say 50 KΩ) in series to get to the exact value required.
Your calculated resistor will often, as here, turn out to be a non-standard value, so this is a good way to go. The average DMM these days is more accurate than most analog meters, so you can use your DMM to compare and calibrate your meter.
Here is the formula you should use to work out your multiplier:
Knowing the meter resistance is not as important for calculating multipliers as it is for shunts, as the meter resistance is often a pretty negligible term in this formula and can often be omitted. And if you do drop this term, RMULT is directly proportional to the desired VFSD, leading us to the Ω / V figure that was essential to know in the days of analog multimeters. Using a 50μA movement, you will need a total resistance of 20KΩ for every volt of FSD.
This is good for “Quick and Dirty” guesstimates of the resistor you will need, you can then use a preferred value a bit less than this, and use a preset to make up the difference and get your FSD exactly right.
For your convenience I have included a table of the Ω / V for the most common meter sensitivities below.
|
Meter Current |
Ohms Per Volt |
|
50 μa |
20 KΩ |
|
100 μA |
10KΩ |
|
200 μA |
5 KΩ |
|
500 μA |
2 KΩ |
|
1 mA |
1 KΩ |
|
2 mA |
500 Ω |
|
5 mA |
200 Ω |
|
10 mA |
100 Ω |
One thing to note, if you’re adapting your meter to read a very high voltage, make sure your multiplier resistance can take that voltage (as it will have most of the voltage across it). You can get special large resistors for this purpose. And take care with your insulation and clearances too!
Here is a typical resistor for this sort of thing – you can get resistors like this in values up to 500GΩ or so. This one is about 2 inches long, rated at 20kV:
Centre-Zero meters
You can get centre-zero meters with the needle at rest in the centre of the scale – which obviously indicate both positive and negative values. They are useful for things like battery backup or solar power systems, so you can see whether your battery is charging or discharging, and by how much.
They were common in the old teleprinter days, as teleprinters used either 20 mA “double current” - negative for a mark and positive for a space, or 40 or 60 mA “single current” which was either on (mark) or off (space). So old teleprinter distribution systems had meters that read +/- 60 mA.
I looked after a few of these, and got some meters out of them when the internet came in and they went out. This is a really nice one which I have adapted – using the above techniques - to read +/- 60 mA, 600 mA and 6 A – very useful for doing charge / discharge tests on batteries. Note the mirror on the scale (just below it) as mentioned above. This is from Metrix – a well-known and respected French meter maker.
Moving Iron Meters
Moving Iron meters are notoriously insensitive compared to moving coil meters. Here are some figures for a couple I have:
|
No. |
Type |
FSD current |
FSD Voltage |
Resistance |
|
1 |
Moving iron |
5 A |
143 mV |
0.0285Ω |
|
2 |
Moving iron |
(30V) 131 mA |
6V |
45Ω |
The number 2 meter above is a 30V moving iron meter. The coil resistance is 45Ω and at FSD the coil has 6V across it, and a current of around 131mA in it. It has a 180Ω multiplier resistance which drops the additional 24V. That’s 7.5 Ohms per Volt!
The whole assembly, at 30V, dissipates 4 watts! I’ve seen a 15V version of this meter used in radio sites, and after a few years, the plastic faceplates would go brown. Now I know why…… Here are a couple of pictures of these meters:
ABOVE: Moving Iron 5A meter coil (the back of the one whose face is above) – few turns of thick wire, 0.028 Ω resistance. Note the high quality terminals :-). Cheap as chips and not super-accurate, but very rugged and reliable. The coil bobbin is about 1¼ inches in diameter.
ABOVE: Moving iron 30V meter coil and multiplier resistor. More turns of thinner wire, and due to the 180 Ω multiplier resistor they have used a PCB with half-decent terminals. But with 4W dissipation at FSD, it takes 131mA from your power source before you’ve even begun to power your load!
Measuring AC
If you want your meter to measure AC voltage, all you have to do is put a rectifier – preferably a full bridge rectifier - in series with it like this:
However, there are some considerations here. The diodes you use will have a certain voltage under which they will not conduct (around 0.6V for Silicon, 0.3 for Schottky and 0.2V for germanium). So in the diagram above, if you use silicon diodes you will need at least 1.2 volts in your measured voltage – or about 0.85 V RMS – before your meter will indicate anything. With Germanium diodes it will only be around 0.4V, or 0.28 V RMS – so there is a use for those old germanium diodes in your spares box! Because of this, the bottom end of the scale will be decidedly non-linear. On a meter reading 300 VAC FSD this will not be a huge problem, but if the FSD is 10 VAC the effect on the scale will be very marked.
If you only use one diode, as below, you get less voltage drop, but this halves the sensitivity of your meter, and you should put another diode in the opposite polarity across both the meter and the diode, unless you know the meter diode can take the full reverse voltage of the AC. But diodes are cheap, so why not go full-wave?
Bear in mind that a simple rectifier like this will not give you a true RMS AC voltage reading, but an average value. Depending on your waveshape you may have to recalibrate it, so making your RMULT partly a preset is a good idea, as is using an RMS DMM to calibrate it.
If you have a meter reading current, with a shunt, your options are to rectify all the measured current – which if it is reading 300A will involve buying some very expensive diodes – or just to rectify the small current through the meter – which will mean that your shunt has to have considerably more voltage drop than for a DC shunt. There are other, better ways round this – you can use a moving iron meter, and/or use current transformers for high currents.
The other solution that is widely used – notably in AC Millivoltmeters – is to include the meter and diodes in the feedback loop of an amplifier. This is called a precision rectifier circuit, and it will keep the scale linear within the limitations of the amplifier’s slew rate. But this is an article about meters, so I am not going to go any further there.
Meter Sizes
Most modern meters are square or rectangular, and there are some standards. The meter cases are usually rated by the diameter of the cylindrical protrusion at the rear that houses the movement. So you get MU-38, MU-45, MU-52 and MU-65 types. MU-45 are probably the most common. Here are the main dimensions for each size:
|
Meter |
A |
B |
C |
D |
E |
F |
G |
|
MU-38 |
38 |
45.3 |
50.5 |
31.8 |
31.8 |
19.5 |
4 mm |
|
MU-45 |
45 |
52.5 |
58.8 |
37.5 |
37.5 |
19.5 |
4 mm |
|
MU-52 |
52 |
60 |
64.5 |
48 |
48 |
25 |
4 mm |
|
MU-52E |
52 |
66.2 |
80.2 |
63.7 |
48 |
28.4 |
4 mm |
|
MU-65 |
65 |
82.5 |
101 |
79.5 |
63.8 |
33.9 |
4 mm |
All dimensions in mm. But there seems to be a bit of interpretation of these standards – for example, the fixing bolt size (G) is given as 4mm but I have definitely seen other sizes used.
Older meters tend to be round, like the one below. Some makers love them for the steampunk look they can give to your project.
Commercially bought meters
Many electronics suppliers still supply analog meters with a lot of different current and voltage FSDs. If you buy a Voltmeter that reads (say) 20V FSD, in practice this meter will usually have a multiplier resistor built into the back of the meter. If you take the meter apart you will see it is probably mounted on one of the terminals inside the case. The same goes for Ammeters. If you buy a 20A moving coil meter, it will probably have a shunt mounted between the terminals at the back of the case. These facts are useful to know, because if you have some spare meters knocking around in your “junk box” you can often modify one to do what you want, even if they do not have the desired FSD.
As an example, I got a couple of nice meters out of a dead battery charger some time ago. They read 60 Amps FSD, not much immediate use to me, but the basic meter sensitivity without the shunt was 4 mA FSD, which is a lot more useful. Here are some pics of the meters with the shunts attached. One of the Shunts reads “50 A / 50 mV” so the meter would have to have an FSD of 60 mV. Curious, I tested it and lo and behold FSD was at 5 mA at 60 mV. So you could use any 50 mV shunt with this meter. Nice!
Moving Iron meters are a different matter – they are generally built to their range, and are difficult to modify. You can use droppers with moving iron voltmeters, but as above they will take a lot of power! A Current Transformer is the right solution for moving iron meters (if you're measuring AC).
Edge reading meters
You can also get edge reading meters, where the movement sits on its side and the needle is bent through 90° to read on a curved scale like this:
They save a bit of space, but because of the curvature of the scale they are not easy to read accurately. They’re great for test equipment where high accuracy is not required, and you can get very small ones that you can use for battery or tuning indicators.
Meter deflection
The usual meter needle deflection is 90°, though types with 110° deflection are common and you get them up to 270°. Here’s one which is probably about 240°. This is a moving coil meter used in electricity substation instrumentation – measuring AC so it has a rectifier - note the non-linearity at the beginning of the scale. The non-linearity of the scale above 300 is probably due to the moving coil getting outside the magnetic pole-pieces.
Note also the 300/5A mark in the bottom left – this indicates that this meter is used with a current transformer, and its actual FSD would be 5 Amps.
Meter Markings
How did I know this was a moving coil meter? From the symbols at the bottom below the 300/5A marking.
From left, these mean:
- • AC only (An AC/DC meter will have the squiggle with a line above or below it)
- • Moving coil, with rectifier (see below)
- • 1.5 % accuracy
- • For use in the vertical position (the upside-down T shows this – if it is to be used horizontally it will have a symbol like a table)
- • The star indicates the class or high voltage specification – a 2 inside the star indicates it can take 2 KV for 1 minute, a 3 indicates 3KV (between the meter terminals and the grounded meter case).
The symbols are according to certain German DIN standards, and they are very difficult to find information on. The following is the best I could put together for the meter types.
Have a look at the moving iron 5A meter face and that on the centre-zero meter shown on earlier pages and compare the symbols on those.
This example is a moving coil meter, DC only, 1.5% accuracy, vertical mounting, class 2.
Mounting your meter
To fix your meter onto your case, you need to get the dimensions of the meter as above. For the main cylindrical part you can use a hole saw, chassis punch, nibbling cutter or jigsaw. You then need to make holes for the mounting bolts. You can put the meter in the hole and then roughly position the meter, rotating it back and forth slightly so the bolts leave marks on your case. Use a square to mark the holes and then centre-punch and drill them. Make sure you keep any metal shavings, especially steel or other magnetic ones, away from the meter – if they get inside your meter they can ruin it.
Meter terminals are usually substantial and quite able to hold a small PCB, so you can often mount your PCB directly on the meter terminals and dispense with other PCB mounting arrangements, if you design your PCB with the appropriate hole spacing.
Rescaling
If you adapt a meter and the scale on the faceplate is not what you need, you have a few choices. The Dymo-type tape labellers produce a very acceptable text quality and you can relabel the scale using numerals produced on these labellers, cut with a fine knife or pair of scissors and carefully positioned. This does not look really professional, but works fine, especially if you are just changing mA to V, or changing 0-1-2-3-4-5 to 0-5-10-15-20-25. But if you have a meter scaled 0-1-2-3 and you want to change it to one of the above scales, or to (say) a dB scale, you have a problem – the major divisions won’t line up.
The inner workings of a MU-45 meter and removable faceplate
You can then print a new scale as a label on a laser printer, or have it made as a plastic sticker (preferable, as paper discolors quite quickly) or have a new scale professionally printed – only economic if you’re doing it in quantity.
The custom label for the Retro Computer Dashboard in Issue 28
Driving your meter
Usually, your meter will be bought or adapted for a specific measurement and you will not have to make special efforts to get it to read what you want. Or your circuit diagram will specify the sensitivity of the meter required.
When designing yourself, you may design a circuit around a meter that you have. Bear in mind here that meters will not usually indicate changes at more than about 10 or 20 Hz, so you can use a higher frequency pulsed signal to drive them.
A good rule to follow when designing is to make sure that the maximum current through the meter – even under fault conditions – is not more than around 1.5 times the FSD – any more may damage the meter. The lower the better.
So, if you are using an Arduino or other microcontroller to drive your meter, you can use a PWM port. If you have a DAC port, you can use that.
Make sure it has enough drive capacity, or buffer it. Arrange it so that when your port is putting out full voltage, or 100% modulation, the meter reads full scale. You should then be able to drive it to 1% resolution or better, and you’ll be doing well if you can read differences less than that.
Voltmeter used in our Arduino-powered Geiger Counter from in Issue 63
TESTING meters
If you buy old meters or take them out of old equipment, you can’t assume that the markings on the scale on the meter is what it takes to actually deflect it. So you’d need to measure it to find out the FSD current and voltage, and if possible, its resistance as well, though this is not essential.
The easiest way to do this is with a variable power supply. Let’s say you have at your disposal a 0-20V DC power supply. The smallest current a meter will measure is likely to be 50 μA. To get a current of 50μA at 10V DC, you’ll need a resistance of 200 KΩ. So, hook up your variable power supply in series with the meter and a 220 KΩ or 180 KΩ resistor, and your best DC current DMM like this:
Slowly turn up the voltage on the power supply and see what the meter does. If the meter does not deflect to FSD, reduce the resistor by a decade (to 22 KΩ) or even to 2K2 or 220Ω if the initial deflection was very small, and start again. Once you get it to FSD, read the current off your DMM.
If you don’t have a variable supply, use e.g. a 5V supply and a 100KΩ potentiometer in series with a 10KΩ resistor, and then a 10K pot +1K resistor and so on. Make sure the pot is initially at maximum resistance, and decrease the resistance until you get FSD.
You now know your meter’s FSD current. It’s also handy to know the meter’s resistance and/or the voltage across it at FSD – it will simplify working out shunts and multipliers later.
Remove your DMM from the meter current path, or use a second one (everyone’s got at least two DMMs, right?), set it to voltage and measure the voltage across the meter at FSD – it will usually be less than a volt. From this, you can work out the meter's resistance (RMETER = VFSD / IFSD).
A word of caution here. Don’t just stick your ohm-meter across a meter. If it’s a sensitive meter, the measuring current may be an order of magnitude or two greater than the meter is designed for, and you may bend the needle round the end-stop. If you know the meter sensitivity is 1mA or more, by all means use a multimeter to measure it, you’re not likely to do much damage.
Protecting your meter
Meters are occasionally subject to overloads and – especially in the case of sensitive moving coil types – these may damage them. As noted above, most meters will have under 200mV across their terminals at FSD. So, Germanium diodes (which start conducting at around 200 mV) are ideal for protecting sensitive meters – but make sure the FSD voltage is less than 200mV! Put a couple of diodes back to back across the meter, and it will not get more than about a 50% overload, which it should cope with. Germanium diodes may leak slightly (which will have the effect of shunting the meter) so make sure your calibration is still correct. When you are dealing with current meters and shunts, the diodes should be able to take more than the full shunt current. Schottky diodes, with their low voltage drop, can also be used for this, and they are available in higher current ratings. Be careful if you’re using pulses to drive your meter – the diodes may limit the pulses if the duty cycle is low and they are greater than the usual FSD voltage of the meter. If you are using 100% duty cycle = FSD, then you don’t need to worry.
There are those who say that you should put a backwards diode across your meter, especially if it is being driven with pulses, to limit back EMF, as you do with relays. I’ve never found this necessary – there is a resistance in series with the meter usually, and the inductance of a meter, and the current in it, are usually much smaller than those of a relay coil. But, it would not do any harm.
Lastly, if you ever have to ship a meter, especially a moving-coil one, or take it places where it may get jarred or jolted, you should short out the meter terminals – and by this I mean the meter coil connections, not after any resistors which may be in circuit. Doing this shorts out any back EMF generated when the coil moves, and this tends to oppose any motion and keep the coil from moving too much if it is jolted. If you design a meter into an equipment, consider putting a shorting switch on the connections to the movement in the “off” position for this reason. Most analog multimeters have an “off” position which does just this – and you should use it when the meter is not in use.
Conclusion
Now you have no excuse not to use an analog meter. They are easy to use, reasonably priced and will add a bit of class to your projects!
ABOUT THE AUTHOR: David Ashton was born in London, England, He was raised, trained, and worked in Rhodesia, then Zimbabwe and now lives in Australia. Based on this, he describes himself as a Pom-Rhodie-Zimbo-Aussie. Workwise, things are much the same. He has run electronics labs and managed telecommunications centers. He has run his own communications business, and for the last 20 years worked as a telecom specialist keeping a large communications network up and running. In his own words: "I'm a jack of all trades and, yes, I admit that I'm the master of none, but I kind of like it that way because it makes it difficult to get bored." He is now retired and still does not have enough hours in the day…