First things first: What is it good for? The short answer is measuring voltage (for our immediate purposes, DC voltage) and resistance and, hopefully, amperage. The longer answer is that, if you don't make your multi-meter part of your tinkering/debugging, you are (in the immortal phrasing of South Park) gonna have a bad time.
Regardless of what kinds of widgets you intend to tinker with, your first purchase should be a multi-meter. I'm serious here. Like, Global Thermonuclear Armageddon levels of serious. When you're first tinkering, a multi-meter is mainly useful for checking voltage levels and verifying whether current is flowing where you intend it to flow...and in the direction you intend it to flow. A short-circuit can be an, ahem!, highly unpleasant surprise -- even without damaged equipment/widgets. A multi-meter will ultimately, easily pay for itself in visits that you don't receive from The Magic Smoke Fairy.
In addition to its money-saving graces, a multi-meter will not only give you more time to "play," it will also make that time more enjoyable. What do I mean by that? Well, I'm a programmer by trade. So my first instinct is to blame my own code when I connect all the wires and [sad trombone] the project doesn't work. I've wasted entire evenings on wonky jumper-wires.
But in this hobby, I'm usually working with inexpensive, no-name widgets -- the kind shipped by the cargo-container-load. These are most certainly not UL-rated, much less ranked in Consumer Reports. (This includes power supplies, which is a whole 'nuther subject for a whole 'nuther post.)
Also, unlike programming for full-size computers, debugging options are pretty thin on the ground. I'm not at a point where I can justify the cost of an oscilloscope. Often, a voltage level (or lack thereof) is my only way of seeing what's going on inside the wires.
I was naive when I bought my first multi-meter; I was casing out the "Electrical" aisle at the small Home Hardware franchise, saw this model, and thought, "Oh, hey! I'm gonna want one of those!" Turns out, it doesn't measure amperage. That's mostly an inconvenience; rarely do I need anything besides DC voltage:
- "Is this supposedly-recharged battery ready for the 'special' recycling box?"
- "Is this jumper-wire borked?"
- "Did I screw up my voltage-divider math (again)?"
That being said...get yourself one that measures amperage alongside DC voltage, AC voltage, and resistance.
Also make sure that you have more than one way to attach the red and black tips of your multimeter to whatever it's measuring. Like I said, I was pretty naive; my model only came with the probe-style leads. For checking battery voltage, that'll do ya:
One important note about the colour-coding here:
- Red == Positive voltage, noted by the "plus-sign" on your battery or DC power source.
- Black == Ground voltage, noted as the "minus-sign" on your battery or DC power-source. There's technically a difference between "negative" and "ground" voltage. That's (once again) another topic for a later time.
"What is it good for?" Verdict: This battery is freshly charged. (Note: AA and AAA Alkaline batteries -- i.e. the throwaway non-rechargeable ones -- are expected to put out 1.5 volts in their prime. Their rechargeable counterparts? Only 1.2 volts. And I will go to my heckin' grave wanting to wedgie every last corporate moron responsible for that travesty...)
But for most other purposes, probe-type leads are useless. Even if you had enough hands to hold the leads to the positive and negative wires in your project's circuitry, we all can understand what a spectacularly, gawdawfully BAD IDEA that would be. Even with the relatively low voltages/currents of battery packs.
That's where "alligator cords" come in. Checking the voltage of bunch of batteries in a pack looks like this:
"What is it good for" Verdict? We have three AA rechargeable batteries in series, with the expectation that they will add up to ~3.6V. So, yeah, these are pretty freshly charged.
My Gentle Reader will notice that, again, colour-coding comes into play. When you buy a handful of alligator-cords, they are colour-coded. Stick to the formula: Red is for positive voltages; black is for ground/negative. We'll talk about other colours at a later date.
But what if you're measuring voltage/current/resistance in-circuit? For instance, how can we objectively know that the 3mm LED in this circuit is operating at something close to the optimal voltage:
Even accounting for the artificial lighting under which this photo was taken, that green LED looks a tad pale. Going by experience, that is usually a symptom of low voltage. But, checking the LED's datasheet, we know that the LED's operating-range voltage is 3.0V-3.6V -- a tad under the 3.72 voltage of the battery-pack.
Now, because the "Intro to Electronics" tutorials say that we're supposed to add a resistor to the LED's circuit to keep the voltage under the 3.06V maximum, and also the operating current under 20mA (a.k.a. 0.02 Amperes). The kit came with 220-Ohm resistors for this kind of circuit, and I popped in one of those to make sure that the LED doesn't pop because -- dude! -- those things can be LOUD when they blow out. Ask me how I know. :~/
So let's see what the multi-meter has to say about the voltage. We do that by splicing the multi-meter into the circuit with the alligator cords and a couple extra jumper-wires (the latter of which we'll address in a future blog-post):
To splice the multi-meter into the circuit, notice how we add a red ("positive") jumper-wire ahead of the LED and a black ("ground") jumper-wire behind the LED. And how we patch each jumper-wire into the appropriately-coloured multi-meter lead via jumper-wire. We are essentially making the multi-meter the "middle-man" in the circuit to measure the total amount of voltage. We could do the same for resistance. Or -- with a more sophisticated multi-meter -- the amperage. (But, with Ohm's Law, you only need two out of the three values to compute the other.)
"What is it good for?" Verdict: Turns out that we were right about the voltage (and by the math of Ohm's Law, current): That LED is, basically, running too cool. (The upshot -- again, something that will be covered shortly -- is that we can afford to adjust the value of the resistor used in the circuit. Downward, in fact.
That's a good place to start/end with multi-meters. When "Widget Wednesday" covers trimpots and rheostats and the like, measuring resistance will come back into play. But for now, the intent is to give My Gentle Reader enough tooling to figure out whether or not the magic electron-juice is flowing through a given set of wires...and whether it's too little or (worse!) too much for the widgets in its path.
Until next time, Tchein ton siault d'beluets!
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Credits where they're due:
- Dennis, for taking the first photograph while my hands were busy keeping the battery from rolling away from the multi-meter's leads.
- Nicole, for "rehoming" her electonics supplies and gear with me, including the alligator-cords. One can never have too many alligator-cords. :~)
- BJW Electronics, for soldering leads onto the wire of the battery-pack to make it breadboard-friendly.
