Re: Measuring mA (debate solution needed)
- From: Benj <bjacoby@xxxxxxxxxxx>
- Date: Sat, 04 Aug 2007 23:38:41 -0700
Phil Allison wrote:
** Time you measured the internal resistance of a DMM when set to the 200
/ 300 mA range.
A typical 3.75 digit meter I have here exhibits a whooping 5.5 ohms !!!
The voltage drop at full reading = 1.8 volts !!
Most of it is due to the 500mA glass fuse employed on that range.
Your friend is right & you are 100% wrong.
Phil is 100% RIGHT ON!
Since you trying to measure LED currents in a given circuit, you
really DON'T want to have the measurement changing the circuit values.
(Especially in this case where LED values are probably not well known
and perhaps flaky from unit to unit as well.)
The big mistake is assuming that meters don't change the circuit. As
you see above that isn't true. Of course they ALSO change a voltage
measurement as well with a parallel loading due to meter input
impedance. But here 10 meg vs 20 Ohms won't be seen.
Plus, the fact that the 20 Ohms is already in the circuit makes it an
easy way to go. In this case it'd be wise to measure a whole bunch of
these dual LEDs in the "on" state and then set the value of the 20
Ohms based on the maximum current you observed. You want to keep the
max current somewhere below 30ma. This is why you don't want to insert
extra resistance from the meter that will change the observed current.
Even more than you might think given that LEDs are non-linear
devices! My feeling would be that resistor accuracy could be an issue
(you could temporarily put 1% Rs in the circuit) but it probably isn't
important in that the LEDs will be rather variable and the idea is to
just keep the current below max values. If you set R to give the
highest current 5% lower than 30 ma, then that also takes care of the
resistor accuracy problem. Hell, make it 10% lower just to be on the
safe side. As someone already noted the current regulation in this
circuit is less than optimum already. An open collector from a higher
voltage would be better.
Next time listen to your friend! :-)
.
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