DMM - I'm confused

LED Flashlights – General Info
1

I do own a cheap (10€) DMM and I also had the chance to compare it to more expensive ones. Of course I used thicker leads..

It reads the same V and A. And thats pretty much all they can or what I need..

But: There have been a lot of threads about several DMM. And always people state "yeah, cheap ones read V very accurate, but they are crap on current because its more complicated, leads are crap and so on".. then there was another thread recently (cant find it right now) where someone brought up the theory that expensive DMM are calibrated on their leads. Someone else countered that measuring current is very simple and doesnt have to be calibrated, the leads only influence how much current can flow at all.

Now, whos right? Is measuring current really that simple and are cheap DMM as good as more expensive ones?

2

The leads are never part of the calibration, but they can influence the current. The DMM does show the current flowing, but this current can be to low (or high) due to resistance in the leads or in the DMM.

Cheap DMM's has many problems, but often they work fine with DC voltage and current.

3

dont be a confused nightcrawl ;)

hopefully HKJ can chime in and tell you more about it.

in short, 99.9% exact current (amperage) measurements at low operating voltages by the help of a single (cheap or expensive) DMM are still a great challenge in technology and engineering. clever circuit aids have been devised by HKJ, David Jones, et al. to circumvent the inherent difficulty of the task. if you do not buy such clever circuit aids, then you get the most truthful amperage reading by setting your DMM to the high amperage range (1A...10A) in place of the low amperage range (1mA...1000mA) --- this tip applies to our purposes: tailcap readings on flashlights at low operating voltages.

EDIT: HKJ beat me to it. hehe.

4

@kreisler: :P ;)

Yeah I always use the 10A unfused setting for readings. And thinner leads -> less amps. Thats logic and the reason I use 2.5mm² (14 AWG) speaker cables. Should result in something similar to the tailcap.

So by how much could the current vary on a cheap DMM? If I read it correctly, the power goes through a tiny resistor inside the DMM and something will be measured there..

5

A unfused DMM might be better than a fused DMM, because it might have less resistance, due to the missing fuse. Less resistance means less voltage drop.

How much a reading varies depends on both the meter calibration and waveform. The resistor in the meter might also change value when it gets hot and then skew the reading.

A flashlight does not always draw current in a steady manner, sometimes it is in pulses (i.e. pwm) and not all meters can handle that type of waveform correctly.

6

Great, thank you. My DMM flattens the PWM pretty good, amps usually match the data from the manufacturer.

The part about the resistor getting hot also explains the "10A unfused, 10 sec max. each 15min" printing. I always thought thats more of a security measure to prevent a meltdown than an advise for more accurate readings. Might be both then.

Now I'm a less confused nightcrawl. :)

7

Actually probably short form of nightcrawler, or Cacing.

8

i should be kreisl then ;)

9

Been my nickname for ages now. Nightcrawler was taken in one board and I didnt want to add numbers.. so I removed the "er" and kept the name. Also, there are thousands of Nightcrawlers, but only few NightCrawls. :)

10

The best way to measure current without affecting the circuit being monitored is with an AC/DC current probe system. These use a probe that clamps around the wire that you want to measure the current in, much like those clamp on amp meters that electricians use. But a current probe also has a hall-effect sensor in it that can measure DC current.

Good current probe systems (and those in the know, know that only Tektronix makes good ones) are hellaciously expensive. New they can run over $10,000. They consist of the probe, an amplifier, and a power supply for the amp. They are almost always used with an oscilloscope. If you know what you are doing, you can put together a system for under $1000. If you are stupidly lucky maybe $400.

My recommendation would be a Tek A6302 probe, AM503A or AM503B amp, and TM501,502,503,504,515, 506, 5003, or 5006 power supply mainframe. That combo goes to 50 MHz, 20 amps. You can get probes that go to 20,000 amps. The A6302 needs about one inch of wire to clamp around. Those different mainframes have 1 to 6 slots for adding additional test equipment modules (see http://www.barrytech.com/tektronix/tektm500/tektm500.html)

I'd avoid the plain AM503 amp. It is an earlier analog model that can be fussy (but can be had for cheap).

11

There is a lot of factors, battery, battery tubes, springs and so on, also how angry a person you are will affect your DMM reading as the harder you press down the higher the reading you will get, some under read some over read some are better then others, some cost more then others but most of this means shit once the tail cap and switch is screwed on as the current will be lower again and even lower at the led then what people get with 1 inch thick DMM leads at the back.

I'm crazy i use stock leads cheap DMM, my fluke went walk about when we moved, i have brought another cheap one to keep my old faithful in check it should be here soon, but you will get different measurements from different spots on the thread and battery tube and different readings depending on how warm or cold the battery is and since there is resistance in the switch, tail cap, threads, threads with grease, battery, battery tube, springs, solder, driver, pill, led wires and so on what you get at the tail with your newly modded DMM leads is not what the led is getting any way.

I have seen some crazy thine wires on some lights, my KD C8 that people claim 3 + amps, mine had wires from the driver to the led thiner then a bit of my hair and i have thine hair, i find it hard to believe that what people got on there tail cap readings is what the led got a skinny bit of wire poorly soldered like that would struggle at 3.6 volt at 3 amps IMO.

So maybe stock DMM leads reads closer to what the led is getting, but then again some of my lights seam to pack more of a punch then my cheap DMM says with standard leads shows, maybe Ive never seen a xm-l driven at 3 amps, but i know i have. but my eyes can not tell the difference between a xml driven at 2 to 2.5 amps and i can not tell the difference between 2.5 and 3 amps, this is another story about tint and what people see, but any how.

Some times i feel its more of a pissing contest, my car has 600 BHP and does 4 mpg and yours is 200 BHP and does 35 mpg and because we are not using a sanitized system e.g same meter, same thickness same length leads and are not all pressing the same hardness and using different batteries and people do tail cap readings with less then 4.2 volt we will always get different readings and then on regulated lights it does not really matter any way.

But in saying all this a claimed 3 amp light from forum members will sell more lights then some one claiming 1.5 amps, i rather under estimate mine then over estimate mine, so did this post help with the confusion or did it just show Ive had a ruff night and add more lolls.

12

You can put a lot of current through a very thin wire (just look at the bond wires to the led chip)... as long as the wire is very short.

One inch of 30 gauge wire-wrap wire is around 0.0085 ohms. Put 10 amps through it and it drops less than 0.1V (but eats 0.85 watts... gonna get hot)

13

Yeah turn into a light bulb filament after 45 Min's.