Measuring mA (debate solution needed)

The 20ohm resistor is In circuit,

5vdc -> led -> resistor -> led -> pin on shift register -> gnd

Leds drop ~2v each, spec sheet below, these are LY T776 S2 leds

Resistor is 20ohm

Shift register drain on-resistance is rated at 6.5 ohms @ 50ma and

25c.

Asides from suggesting he measures with DVM on mA scale, also suggest he goes to 33ohm resistor as he's right on the edge of max current for the Osram Leds. The is for a christmas decoration display panel with approx 4k leds on it :) Would suck for him if he ran it one night and the next night half the leds were dead or substantially dimmer than before..

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Check the specs on the multimeter. SOme of the cheapies from China drop half a volt right off the crack of the bat, no matter the current scale. Some drop voltage as a function of current.

Jim

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The 20 ohm resistor is MUCH more than a current meter will put in the circuit. Is the 20- ohm already there, or is he inserting it?

Measuring the voltage across an existing resistor of known value is generally more accurate, provided your voltmeter does not shunt the resistor too much (figure 10M for a modern cheap multimeter on a 2V range and virtually infinity on a 200mV range, neither of which will affect a 20 ohm resistor much. You can also measure the resistor and get closer if the value is not known. Chances are the mA meter will insert a resistance of 1 ohm on the 200mA scale, which could affect the current a bit. With 2 LEDs in series, depending on type, you probably don't have enough voltage to get a well controlled current from 5V.

Best regards, Spehro Pefhany

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Understanding measurement errors can get quite complicated, and the fact that you understand that there are errors rather than just writing down the number on the meter is a good start.

If it is a digital meter it will probably measure current by measuring the voltage across a small, known resistance, so the accuracy of the meter when measuring a small voltage is probably not an issue. On the other hand, if it is a simple analog moving coil meter the meter actually responds to current and the voltage has to be converted to current internally (e.g. by a series resistor).

As you rightly say, both methods will have errors, and if you really want to know which is best you will probably have to do some math.

For the voltage across the resistor you have:

1) Error in the resistor value - probably quite significant if you just go be the nominal value. Lower if you measure it, but there will be some error in this measurement too.

2) Current flowing through the meter - when you put the meter across the resistor you will have the resistance of the meter in parallel with the resistor which will reduce the total resistance. In this case I would expect this to be negligible unless you have a very poor meter.

3) Voltage measurement accuracy of the meter.

For the direct current measurement you have:

1) Change in resistance due to series resistance of the meter

2) Current measurement accuracy of the meter.

If you have the specifications for the meter you should be able to find its accuracy and series resistance and work out which sources of error are the most significant.

Personally, I would probably measure the current across the resistor because it is easier than breaking the circuit and inserting the current meter.

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voltage, of course intended

across the resistor

With one DMM doing a series current measurement, you can use a second DMM to measure the voltage dropped across the first one and calculate the actual series resistance the first one introduces. Might be interesting.

Chuck

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leds

coating site)

15KW generator project)

Given that the resistor is already in the circuit, so long as you know its resistance accurately, that will be the better way. I'm assuming a volt meter that has something like a 10 megohm input resistance, common in DVMs. If you insert a meter, you'll disturb things by whatever the meter resistance is. Say it's 0.1 ohms. That's 0.5% of the total of 20 ohms plus the meter. On the other hand, 10 megohms in parallel with 20 ohms disturbs it by about .0002%.

For current in an LED, neither matters a whit. But measuring the voltage across a resistor is almost always trivial compared with inserting a meter in series with an existing circuit.

How accurately you know the value of the 20 ohm resistor would be an issue, along with how stable it is with self-heating and stuff like that, but again, measuring current in an LED really doesn't require extreme accuracy. The max current rating of the LED is for rated life; it's not going to die in an hour if you go 1% over. You can measure the value of the resistor with the power turned off in that particular circuit; if the LED is reverse biased by less than a volt by the meter, the reading should be quite accurate.

I've never met a DC current meter that drops half a volt at (practically) no current, like Jim mentioned, and hope I never do. Even my cheapies don't do that. On AC, yeah, quite possibly, but on DC???

Cheers, Tom

"James Lerch"

** 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

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! :-)

I hate being wrong, but I'd rather be wrong and learn something that wrong and think I'm right! :)

Thanks Gents!

(off to develop a circuit to see just how much my mA meter changes things) :)

--
Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction,testing, and coating site)
http://lerch.no-ip.com/ChangFa_Gen (My 15KW generator project)

"Anything that can happen, will happen" -Stephen Pollock from:
"Particle Physics for Non-Physicists: A Tour of the Microcosmos"

" Press on: nothing in the world can take the place of perseverance.
Talent will not; nothing is more common than unsuccessful men with talent.
Genius will not; unrewarded genius is almost a proverb.
Education will not; the world is full of educated derelicts.
Persistence and determination alone are omnipotent. "

On Sat, 04 Aug 2007 15:17:31 -0400, James Lerch wrote: ...

You could definitively solve this once and for all by using two meters - interpose the ammeter, then use a voltmeter to measure both the drop across the meter and the drop across the load.

Good Luck! Rich

Phil Allison snipped-for-privacy@tpg.com.au posted to sci.electronics.design:

So what make and model of piece of shit is that so i can avoid buying one?

"JosephKK = Utter Fuckhead"

** Fluke 73 or Fluke 77 and many, many others.

Burden voltage is speced as " 6 mV per mA " on the 32mA and 320 mA ranges.

Equates to a 6 ohm resistance.

Fuckwit.

........ Phil

Phil Allison snipped-for-privacy@tpg.com.au posted to sci.electronics.design:

I threw away an old fluke 77 years ago. It did ok on voltage on voltage and soso on resistance. Another overpriced brand name mediocre meter.