Computing stuff tied to the physical world

TK – Multimeter

In Hardware on Mar 22, 2012 at 00:01

Welcome to the Thursday Toolkit series, about tools for building Physical Computing projects.

One of the tools you don’t strictly need, but which I very strongly recommend getting, is a multimeter.

A multimeter measures stuff. I picked the Voltcraft VC170, Conrad’s own (re-)brand (item 124403):

DSC 2947

Actually, my suggestion for this series would be to get item 046027, which includes a whole set of additional tools for only €12 extra. It won’t break the bank, and it gets you various screwdrivers, tweezers, a simple loupe, a lamp, and a few more items.

Anyway, back to the multimeter. Trust me – this is one of those lab instruments which will enable you to learn more about electricity than anything else. And this is one of those cases where a small amount of money will go a huge way – this particular unit lets you measure voltage, current, resistance, frequency, and more. The VC170 even does non-contact AC mains sensing, to detect live wires from a short distance.

I’ve got over half a dozen multimeters by now. Low-cost as well as expensive / more accurate ones. My favorite one is this VC170 (or rather, its predecessor, the VC160 which I’ve been using for several years now). Why? Because it’s very small, it’s fast and responsive, and it offers an excellent set of trade-offs.

Some more expensive ones are very sluggish (but also produce considerably more accurate 5-digit readings), some beep very annoyingly all the time, and some don’t have the sensitivity you need. Of all the multimeters I have, I end up using my trusty VC160 most of the time. It does what I need, and it doesn’t fill up my desk.

You can’t really go wrong with this. You’ll want more than one multimeter if you really get into electronics. Here’s a not-too-contrived example: measuring incoming and outgoing voltages of a power regulator at the same time, as well as incoming and outgoing currents – that’s 4 multimeters! So by the time you want a more advanced one, this first unit will still come in handy in certain use cases.

The good news is that one is fine for a huge range of situations. This one will measure up to 230 VAC mains (with a small caveat, see below), and all the way down to fractions of a µA of current (ultra-low power, anyone?).

Learning how to make the most of a multimeter is a story far beyond this initial Thursday Toolkit series. But it’s really easy to get started and learn along the way. Even just fiddling with a resistor, or a capacitor and a resistor, and measuring what happens in various hookups can be a great way to understand Ohm’s law, and all the basics of electronic circuits. Do two resistors in series draw more or less current? What is the resistance of two resistors in parallel? How much voltage are my near-dead batteries giving out, and how are they performing under load? Is that power supply doing what it’s supposed to do? And perhaps most important of all: are the proper voltages being applied to the different parts of my circuit? Trivial stuff with a multimeter – you can simply measure it!

Multimeters are very robust, especially auto-ranging ones like this, which can take any voltage and figure out all by themselves whether it’s over 100 V or in the millivolt range. But there are ways to break things. Big currents always tend to cause trouble, and even the best multimeter won’t be pleased if you push a few amps through it while it’s trying to measure microamps. Which is why the above set of input jacks is actually quite nice: voltage and current are very different quantities, and you have to hook up the measuring cables in specific ways to measure the different types of units. But mess-ups do happen… I’ve blown fuses inside my multimeters a few times – fortunately, they are easy to replace.

All multimeters have trade-offs. This one gets many of them right though, and does auto-ranging.

Then again, this multimeter seems to be at its limit when asked to measure 230 VAC, i.e. AC mains around here. It displays “OL” (overload). But it can measure 230 VAC just fine when using the “Select” button to fix it to the maximum range before doing the measurement.

The other thing is not to get carried away by the 4-digit display. You’ll be able to measure 3999 vs 4000, but that’s not an absolute accuracy, i.e. you shouldn’t expect to be spot on when measuring 3.999 V versus 4.000 V – the accuracy is only about 1.5 %, so it might well be 3.940 V, or 4.060 V. The only purpose this serves, is to show you slight fluctuations – fairly accurately. So it might be off a bit, but you will be able to see small dips and increases in voltage, current, resistance, etc.

And to be honest: 1.5 % accuracy is actually pretty amazing for such a low-cost instrument, if you compare it to the old analog multimeters which you had to read out by estimating the position of their needle!

The VC170 added a function I’ve dearly missed on the VC160: frequency measurements. Its specs says that it works up to 10 MHz, but a quick test here tells me that it’ll work up to at least 25 MHz with a 1 Vpp signal (wait for tomorrow’s post to find out how I tested that). The frequency range is in fact very convenient for microcontroller debugging of timing loops, for example – I’ll go into this in a future post.

So much for the multimeter. If you solder electronic circuits together, all I can say is: get one!

  1. “Then again, this multimeter seems to be at its limit when asked to measure 230 VAC, i.e. AC mains around here. It displays “OL” (overload). But it can measure 230 VAC just fine when using the “Select” button to fix it to the maximum range before doing the measurement.” This is probably due to its CAT III 250v rating, it means you should not use it on a 230v mains circuit as it could be unsafe due to peaks above 250v. This rating has to do with the safety of the device at certain voltages and current available or short circuit current. This meter is rated for electronics and low current testing. A multimeter rated for mains testing, say at a recepticle, would be rated at CAT III or CAT IV at 300v or 600v. If you are working on a breaker panel on a three phase system with large fault current available you would want a meter with a Cat IV at 600v or 1000v rating. In fact I am not aware of a rating at 250v, the ratings are usually 150v, 300v, 600v, 1000v.

    • Good points – I’m definitely not advocating this basic multimeter for serious use with AC mains circuits, but it can handle an occasional check.

  2. You don’t strictly need a multimeter? Maybe, but it seems to me the use of a meter (and other more advanced tools) is almost what defines “doing electronics” (creating, modifying, and troubleshooting) as opposed to merely using electronics.

    • I agree that it’s definitely the first instrument to get – but IMO someone strapped for cash and eager to try out stuff in Physical Computing can get a decent amount of mileage out of a few LEDs + resistors + clever use of an ATmega as measuring tool. In the digital domain, go/no-go is often easily determined w.r.t. voltages and currents. I sometimes use a JeeNode + Bridge Board to keep track of a bunch of voltages and signals while trying out things on a breadboard.

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