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01 February 2011

Finding an appropriate digital multimeter

Extech MN36 digital multimeter
I'm introducing my general physics class to electric circuits with an experiment.  I generally find that the idea of voltage and current doesn't really sink in until students have had a chance to measure these quantities in circuits with which they can play.  (I also find that there is little correlation between my best problem solvers and my best circuit hooker-uppers.  Circuits experiments give an opportunity for success to some otherwise weak students.)  So this week, my class will do the simplest circuit experiment one could ever devise:  keeping a constant battery voltage, graph the current through a variable resistor as a function of the resistance. 

For years, though, I could not do this experiment effectively.  The ammeters -- really multimeters -- in my classroom wouldn't measure a wide range of currents, and would blow a fuse if they measured too much current.  You can certainly trust introductory students to blow an ammeter's fuse, no matter how much preventitive instruction and warning you give.

I really wanted a meter which would measure currents as small as a few microamps, and as large as a few milliamps.  Then we can use the standard available power supplies that provide 5-15V with tens of kilohms of resistance.

Why are these values important?  Most cheap resistors are rated at about 1/4 watt... so to keep the resistor from getting hot and ruined, the voltage squared divided by the resistance for any individual resistor must be less than 0.25 watt.  Even with as much as 15 V across as small as a 1 k resistor, the power is acceptable.  That calculates to a maximum current of 15 milliamps... and my preference isn't to come too close to this maximum. 

The meters I bought 10-15 years ago weren't this sensitive to current.  Their sensitivity was in the 1 mA range.  But last summer I searched google shopping under "digital multimeter."  I found a number of reasonably priced meters that would measure currents as small as 1 μA!  I've pictured one, the Extech MN36, which was listed at $17 on Amazon.  No price guarantees, and I've never used this particular meter -- I'm just showing an example of what I found.  Look around, there are gazillions of meters, many of which will serve your needs.

When you're looking for a class set of multimeters, DON'T look at science supply stores!  Their prices are inflated, and they don't necessarily give you a good selection.  Look around at electronics retailers.  Make sure to check the specs to see that the resolution for DC current is in the 1-10 microamp range.  If you have to, buy four this year, and four more next year, etc. 

Once you have the meter, all you need is a huge pile of 5-200 kilohm resistors -- which can be found from electronics stores in bulk for 1-2 cents apiece -- and some battery holders.  You can add more expensive power supplies, breadboards, or "resistance substitution boxes" later if you want.  But don't let cost or procurement be an obstacle to simple and fun electronics experiments.

2 comments:

  1. For student labs, I actually prefer analog meters: ammeters and voltmeters. "Old school," with needles! I also prefer power resistors (3-ohm, 5-ohm, 8-ohm, 10-ohm for example, rated at 10 watts or so.) And I like to use well-regulated, 0-6 V variable DC power supplies.

    Together, they allow for an Ohm's law lab whose data is rife with values between 0-10. Easy numbers to deal with when the concepts are difficult enough.

    All those things cost money. But amortized out across 100+ students over 10 years, it's pennies per kid. Cheap analog meters might not last ten years in the lab, but good power supplies will, and the power resistors (though ceramic) are sturdy enough.

    It's rare for a student to hurt a meter in my class (unless they knock it off the table). I like a needle lunging the wrong way to tell them they need to reverse the connections. And I like the sweep of the deflection indicating more or less current or voltage.

    I actually skip past DMMs and use a current/voltage sensor for the AP (second-year) Ohm's law and resistivity labs. We also use those sensors for our capacitor labs, where they generate nice charging and discharging graphs.

    I guess I was just never a big fan of multimeters in the classroom. But that's one more reason why Baskin-Robbind makes 31 flavors.

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  2. I'm just teaching Junior High kids... What's the simplist and most reliable meter to get to show conductivity?

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