WAY OT Accurate, but cheap amp meters

eric · Since 13 Jan 2006 1871 Posts XTreme Poster

Okay--lots of nerds on this site. I am looking for cheap amp meters that can read amps in the 5 to 50 mA range. They need to be cheap, as they are for a public school science class and I need a bunch of them. The multimeter that I bought and tried seems to be pretty much hopeless at anything below 200 mA. And, using ohm's law seems not to work either, as the ohm readings we want are low as well, and the meter does not do well. The meter works pretty well reading volts on 1.5 V batteries (1.56), but that's about it.

The project the kids are doing is with solar racers, so we are looking at small project motors that draw about 18 mA using 0.5 to 2V solar panels.

Any ideas?

Thanks,
Eric

scottman · Since 08 Jun 2007 150 Posts hillsboro Stoked

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=90899

CHEAP CHINA SHIT, WITH EMPHASIS ON THE SHIT

eric · Since 13 Jan 2006 1871 Posts XTreme Poster

Yeah--similar to what I have ( only I paid more). Hopeless for smaller electronics.

scottman · Since 08 Jun 2007 150 Posts hillsboro Stoked

the meter will read 200uA. That is 200 millionths of an amp! You may need to describe in detail "exactly" what you want to do. I bet the meter you have will work....

eric · Since 13 Jan 2006 1871 Posts XTreme Poster

my meter goes down to 20mA; not uA like the one you posted.

I will check it out.

thanks

stringy · Since 23 Jun 2006 1738 Posts vancouver XTreme Poster

eric,
have you considered approaching a company like tektronix in beaverton? They would probably loan you a handful if you told them you wanted them for school. I used to be employed at the fluke corporation up here in everett and I could see them helping out the community with meters.

Might be worth a call if you feel up to it. www.tek.com

good luck on your search. I prefer the high quality fluke meters. nothing compares to them.

stringy

eric · Since 13 Jan 2006 1871 Posts XTreme Poster

Nice--I grovel well, and will follow up on your link.

Thanks,
Eric

Hein · Since 08 Mar 2005 1314 Posts Possessed

Some panel mount analog DC amp meters might be
good. Analog meters are better for what you are
doing.

http://www.action-electronics.com/panelmeters.htm

Hein · Since 08 Mar 2005 1314 Posts Possessed

It might be interesting to evaluate the environmental impact of your designs from 'cradle to grave' This process starts with an accounting of all the resources that go into the creating the product, those consumed during it's usable life and finally those left when it is recycled.

Engineers are very interested in designing products that have an efficient life cycle. They are designing products so they can be more easily disassembled and recycled.

Old CDs make great wheels BTW.

Pitsco has a lot of solar car stuff. But resist the temptation to buy a kit. Half the fun of building stuff is scrounging for parts.

pdxmonkeyboy · Sat Sep 06, 08 7:07 am

Fluke makes some high quality stuff. i have one of their multi meters and have been very happy with it. Even though I only use it like once a year.

Since we are off topic, if you are doing a fair amount of wiring either in the home or on a project, spend the extra $10 and buy a pair of klein wire strippers. I re-wired my entire home and my hands were killing me using these cheap ass strippers. I bought a pair of kleins and I was amazed at the difference!! They have a curved handle that fits perfectly to the curve of your palm and they are sharp as hell. Every time I use them I am reminded how nice it is to use quality tools..

http://www.amazon.com/Klein-Tools-Stranded-Stripper-Cutter/dp/B00080DPNQ/ref=sr_1_2/103-1278535-6776664?ie=UTF8&s=hi&qid=1220713455&sr=1-2

mamann · Since 16 Jul 2007 80 Posts Goldendale

I second Scottman.

If you are looking for a cheap multimeter that will give you good results, the Harbor Freight 90889 is a great deal.

I picked up a bunch for $2.99 when they're on sale. They measure a ton of stuff and not just mA.

They are actually pretty accurate as can be seen in the photo below. I used a current source that is pretty much right on, that I checked with my fluke 187. The measurements are all in mA except the one for 1mA which I used the 2000uA scale.
I purchased them to use as panel meters, as they are cheaper and much more versitle than single application panel meters.

So in the end of the day, they are the cheapest DMM you can buy, with tons of functionality for basic measurements.

$2.99 just can be beat.

Mike

jcohenx · Since 28 Aug 2007 250 Posts Portland Obsessed

eric · Since 13 Jan 2006 1871 Posts XTreme Poster

Really great advice coming in. The Fluke meters are out of the picture, because we want to get 10-20 meters for use in a classroom of 30 with a budget of about $100.00 max. Kids will use them to explore the very basics of electricity using inexpensive, low amp/volt devices sources.

Example, what happens to amps/volts/ohms when you change the angle at which light strikes a solar panel? "Let's find out kids, take your panels outside, and record amps/volts/ohms as you raise your panel by 10 degrees. Did you find an optimal angle? did volts/ ohms and amps change? What happens to volts/amps/ohms if you hook up another motor in series--in parallel, etc"

The labs will culminate with kids putting together solar racers and hopefully using some of the things they have learned-one motor, or two, optimum panel angle...

So, simple experiments with simple devices with the idea of kids getting hands-on familiarity with these mysterious units of electricity. We currently have 4 multimeters that dance all over the place when we try and measure amps below about .5A. Even the volts are not so great. Our project motors are in the 5-25 mA range, and we use .5 -2V solar panels to power them.

jcohenx · Since 28 Aug 2007 250 Posts Portland Obsessed

eric · Since 13 Jan 2006 1871 Posts XTreme Poster

"No," to all of those questions...

jcohenx · Since 28 Aug 2007 250 Posts Portland Obsessed

I guess then I would pick up one of the Harbor Freight meters recommended by mamman and try it out. If you can look up the specs beforehand great. Milliamp current is pretty small so if your contacts from the meter to the test points to the assembly are flaky don't be surprised if you get flaky results. Also, remember that your meter readings are only as good as your probes so make sure you don't have any shorts.

scottman · Since 08 Jun 2007 150 Posts hillsboro Stoked

To determine the effect of light angle on a solar panel I would recommend getting some 1% resistors, say like 100ohm, 200, 500, 1K (whatever the closest values are) and then measure the voltage across the resistor when hooked to a solar panel. The resistor is a very stable load and will demonstrate the V=IR principle. An electric motor is a variable load (i.e. R is dynamic). So you may not get a stable current reading with a DMM. The way a DMM measures current is to use a fixed precision resistor and then measures the voltage drop across it. It does an internal calculation for I=V/R and report I. So your DMM will actually drop some voltage across it while measuring current. This voltage drop across the DMM may be the source of the error you see in your V=IR calculation. Just take a second DMM and measure the voltage drop across the current reading DMM and your math will all work out. Also with a solar panel you should look at maximizing the power transfer by matching the load impedance to the source impedance of the solar cell.

http://en.wikipedia.org/wiki/Impedance_mismatch