I received a 3xT6 (the 3800 lumen one) the other day. It was faulty on arrival, putting out very low light levels. I took off the tailclip and measured the current with a DMM. I got 85mA on high (and less for medium and low). I am guessing that these units draw around 3A on normal high, and the low reading I got explains the low output.
At 85mA, the XM-L T6 would put out around 30 lumens. With 3 emitters, this is around 90 lumens - and this is about what I thought the torch was putting out. So the readings kind of confirmed what I was looking at.
I saw on another website that sometimes the current is not DC, it is AC. If this is the case, then the current reading I made is useless. Is this the case? Do some/all of these torches use AC, and do I have to make current readings in a different way?
You always have to measure DC ampere, but if the light is drawing the current in pulses, not all meters can show the correct current.
Another problem is voltage drop over the meter and probes, it can easily change the result by 30% when measuring a few amperes from a single LiIon battery.
Yes, I found your website - excellent stuff on the hobby chargers! I am seriously thinking of getting one of those - but am still looking into it.
I surmise that most DMM would give you the RMS value, which would be your 30% drop. The problem is getting from RMS to peak unless you know the waveform. But RMS is really all you want anyway - it is the 'effective' current. I think the problem is in knowing what you are getting. is it AC or DC? Even AC would not be correct as it would (I presume) be assuming that you are looking at a sine wave.
The 30% drop has nothing with RMS or not to do, but with voltage drop in the DMM and probes. In my DMM guide I have just added a high current example, where the high error is obvious.
RMS is fine for AC or if you want to calculate the heat loss in a wire, but not for measuring the tailcap current, there you need to measure DC.
One problem is that many flashlights uses a pulsing current draw and this confuses some DMM's, the other problem is the loss in the DMM and probes. Measuring current from a 3.7 volt battery, while loosing 1 volt in the meter and probes will never give the correct current draw.
Get the 10A test probes, unless if you got a > $20 meter then most likely they'd come with one. They usually are 18awg, enough for LED commercial flashlight use. The TK70 is starting to push it already.
Neither a >$20 (or >$200) meter or these probes will solve it.
I just read the comments about the probes, somebody measured them to 0.14 ohm (I do not hope this is correct), this would give a drop of 2.8 volt when measuring 10 ampere. This is completely useless for measuring current on TK70.
Also, this is without the voltage drop in the DMM (Somewhere between 0.4 and 1 volt for typical meters).
Those lead are ok on some DMM, but surprisingly not a good fit for cheapo DMM. The shields on them are too high and basically you have to cut them off and generally futz with it. Worked pretty good once I worked them into a fluke though.
Most cheapo DMM aren't fast enough that PWM bothers them anyway.
Yeah, they're funky out of the packaging. My operating hypothesis is that there're some kind of coating (oxidation???) on them , because they will work find even initially if you press hard. After futzing with them in several meters, they now work fine in a Fluke. Just weird.
They might not be fast, but PWM can anyway lead to wrong readings.
Why is that? I thought these DMM work by averaging over time not unlike analog galvanometer. A slow one shouldn't have any problem catching PWM correctly, but faster reacting one might get confused. I don't really know what's going on inside these, though.
They do, but that does not mean they average correctly. Anyway it was not only the pwm I was thinking about, but also the buck/boost converter , it is usual not very well filtered, i.e. there is some high frequency components in the current draw.
A typical flashlight has the following components to its current draw:
PWM for lights with pwm regulation, can be from 100 Hz up to 50 kHz.
Regulation, this is usual in the 10-100 kHz range, only lights with buck/boost converters has this..
Buck/boost converter, this is above 100 khz and might be into MHz.
Are you saying that the PWM and "noise" would throw the cheap meters off? I'd agree on this.
But how about for full power readings? Ok lets not talk about the TK70, or the 100W HID ballast I have been measuring. How about just a 1 x 18650 XM-L which we have been measuring at full power. Would the Fluke 189 and cheap meters be nearly the same then?
You said voltage drop in the meter, what causes that....too thin traces in the meter's PCB? What would be a good meter then? I don't think any of us would wanna spend on a Fluke 189. (I have a Fluke 67 IR thermometer which I have somewhere though, its for my HiFi hobby LOL!). Mastech?
I'm sorry if you are in the electrical field and have describe such stuff to newbies, for me my income comes from the retail, photography and IT fields which are totally unrelated. LOL!
The cheap meters has some inaccuracies that you cannet get around, but when measuring high current at low voltage, you will have problems with any DMM, even the most expensive Fluke meters.
Some of it you can fix by making your own measuring leds 10 cm long with thick wire, but you can not get around the voltage drop in the meter.
For my Fluke meters (189 & 289) it is 0.4 volt at 10 ampere, for my Gossen meter (Energy) it is 0.6 volt at 10 ampere and these meters are supposed to be some of the best DMMs on the market. Note: Fluke has also made some meters that has much higher voltage drop.
The only way to get around it is to use a clamp meter or a small resistor (See my guide).
This is only a problem at low voltages and high current, at 12 volt it is a lot less significant or when using a 10 ampere range to measure 1 ampere.
2100, if you have a second meter, you can measure the internal resistance of the first one. On the cheapo HarborFreight meter I have, it's 0.2ohm on 10A, and 1.5ohm on 200ma, but the 0.2 reading may not be accurate since it's difficult to measure low resistance (it's in last significant digit of a Metex).
Assuming it's true (and it seem not to be since reading w/ the device are not off by that much) and reasonably linear device, ohm's law dictates that @3A, it's 0.6v across just the DMM.
Personally I do not trust DMM to at these low ohm values. If you use wires with banana plugs in both ends and use the zero function it might be somewhere close to the real value for the 1.5 ohm measurement. For the 0.2 ohm it is much better to use a known current (measure it with the meter) and then measure the voltage accross the meter. Now calculate the resistance using ohms law: ohm=voltage/current
This may be an easy question for you guys, And i didn't see it answered. But im lost in how this works.
How can you tell how much amperage the emitter is getting in a two cell flashlight by doing tailcap amp readings?
I have a Beamtech T6 2x 18650 and it gives a 1.6A reading on an XML single emitter. But the output at the emitter suggests its seeing much more than 1.6A
What am i missing and if there a formula or can it not be done without desoldering a lead on the emitter?
