Silly Multimeter Question?

Mod feel free to move if in the wrong room.

I have a cheapy "Electro-Tek" digital mm from Wallyworld.

I have used the 20mA/10A setting to test current draw on many of my lights. Just tonight I saw .97 on my Madmax+, and .27 on my Metal Gear 1W.

Does this mean 97xmA for the MM+ and 27xmA for the MG?

If not, how do I find the truth?

Whow wait a second! Now I see if I change the lead position I can use the other scales. On a 200mA scale I just saw 184 on the metal gear. Now I'm REALLY confused!

PlayboyJoeShmoe,

The .97 reading from the 20ma/10A setting is telling you that the current is 0.97 amp, or 970mA. The .27 = 270mA
( Note: 1A = 1000mA )
Most multimeters are not too accurate when using the amp meter feature, so at 200mA scale, 184mA sounds not too off.

I would believe the later readings also. Many meters are more accurate when the reading is a higher percentage of full scale. The first set of reasdings is a really tiny percentage of 10A so the accuracy will be much less.

The problem with trusting the 200mA scale is that it's inserting a 1 ohm resistor in series with everything. Thus, the meter may give a more accurate reading, but the additional resistance is modifying the behavior of the circuit significantly.

Using the 10A scale give a truer-to-life reading becuase there is less resistance being added to the circuit. You could run into precision issues because of the low relative reading.

You are also adding resistance to the circuit with your DMM probes. These can add 0.1 ohms or more just by themselves.

The ultimate best way is to use an external 0.1 or 0.01 ohm resistor, calibrate it (attach measurement points onto the resistor leads such that the measurement is accurate for a known current), and use the voltage scale of your DMM to measure the voltage drop across the resistor. Insert the shunt resistor in series like you would your DMM.

If you use a 0.1 ohm resistor, multiply the reading by 10 to get amps; with the 0.01 ohm resistor, multiply by 100. Use the 2V scale with an 0.1 ohm, and the 200mV scale with an 0.01 ohm resistor.

The advantage of using an extrernal current shunt like this is that you can do accurate voltage/current measurements at the same time using a single meter, without swapping probe sockets.

I ran into the same problem a while ago.
So... if I didn't want to fiddle with the extra resistor as evan9162 is suggesting, which one of the amp scales do I trust (more?) in my DMM?

GreenLED: evan9162 provided the answer in his 2nd paragraph.
[ QUOTE ]
evan9162 said:
Using the 10A scale give a truer-to-life reading becuase there is less resistance being added to the circuit. You could run into precision issues because of the low relative reading.


[/ QUOTE ]

BTW: @ 700mA a additional 0.1 Ohms in series would cause 50-100mA decrease in current (~0.1V drop in Vf).

Eg. If your meter is reading 700mA with the 20A setting, then when you remove the meter, the real amount current going thru the lux is ~ 800mA.

in addition ... the meter leads are typically 0.2ohm or more... even on the 10A scale.. if you are trying to do direct-drive luxeon measurements you'd better replace your leads with some seriously heavy duty wire (i use 12ga monster cable.. 1foot long)...

-awr