Measuring frequency with a multimeter is fun :)

[Haggai]

@angusinalberta

@Haggai

I thought about the experiment a little bit, and was totally baffled.

I beg to disagree. Your analysis is well reasoned.

Whether it was a stroke of genius or simple serendipity to connect the leads across the battery to measure frequency/duty cycle it doesn't matter, you got a reasonable result. I wouldn't have anticipated enough signal to even attempt it this way.

Thanks.

It was actually from lack of want to uncover the LED protection at the pill top and connect the probes directly to the LED's leads.

@angusinalberta

@Haggai

Funny thing is, I couldn't measure the XML in low mode until I used my finger as an extra resistor in the battery/multimeter circuit. I've no idea why, but it was consistent... and I measured everything again with my finger in the circuit and got the same results.

This suggests that your finger wasn't acting as a resistor at all. Your body became an antenna, picking up some of the rf radiating from the battery wires.

This is interesting.

And you are probably right! I disconnected the Hz input of the multimeter from direct contact with the battery, touched one pole of the battery with one hand and held Hz probe by its plastic shielding in my other hand - I still got a freq/duty cycle reading! Amazing! :)

@angusinalberta

There are two further experiments you could try.

Experiment 1: Check for inductive coupling between the circuit and the meter.
Clip the two leads of your meter together. Yes, as in short circuit. Bring those leads close to the battery wires. Do you get a frequency reading? If not, then coil the leads around a pencil and try again.

Experiment 2: Measure the pwm frequency/duty cycle directly using a light sensor.
Use a photo diode or silicon solar cell connected directly to your meter.
If you don't have either a photo diode or solar cell then any diode at all that has a transparent cover will work, such as an led. For more details refer to http://budgetlightforum.cz.cc/node/1853

Cheers,
Angus

I tried both experiments with the Manafont XML.

With the first I couldn't get any signal at all.

With the second with a 5mm LED I could only read AC/DC voltage (DC in the range of 0.1v-3v, AC in the range of 0.05v-0.5v, depending on the distance from the emitter and its operation mode H/M/L), but again could get no signal. Maybe it's just the LED I used isn't fast enough in converting light to electricity.

But when I tried it with an XP-G on a board - I could measure frequency and duty cycle! Very nice. The frequency was spot on, about 123Hz for the medium mode and 127Hz for the low mode. But duty cycle readings where way off - medium measured 60% and low measured 40%. This just can't be right - there's too much difference in the amount of light output for 100%/60%/40% modes to be correct.

I also tried my original method for fun with an 18650 battery, to see if internal resistance has anything to do with the values measured as arenat said, and it doesn't:

H 0Hz / 100% VDC 3.690 VAC 0.002
M 123.6Hz / 30% VDC 3.940 VAC 0.230
L 126.7Hz / 5.2% VDC 4.080 VAC 0.040

(oh, and I got my first spark from an 18650 today... I attached the crocodiles to the battery with magnets, and when I removed one crocodile head in order to change mode there was a very tiny but visible spark between the crocodile head and the battery along with an audible bzzzzt... a bit frightening, but everything's okay now :))

[SashiX]

@Haggai

I'm not sure the Vichy brand is better than the UNI-T, it's just much more senisbly priced.

UNI-T have a very large selection of multimeters, scopes and the like, which all seem very good to my eyes.

There's a big fan of UNI-T in th eevblog forums (kiriakos, I think) and one model was even tested by Dave from eevblog and found to be not so horrible even though it's not a Fluke. So I guess this brand is considered okay even though it's Chinese.

They have a very nice pocket multimeter called UT10A, if you don't need current measurements - it has AC/DC voltage, resistance, diode test and frequency amongst other stuff. You can get it for about 14$ here: http://www.goodluckbuy.com/index.php?target=products&bid=42&sl=EN&aff_id=9128&product_id=30210 (but you can register and use their free GL points system to get some discount, it's really fun :) ).

And if you want the cheapest meter with fused current ports, there's this: http://www.dealextreme.com/p/2-2-lcd-handheld-digital-multimeter-voltage-current-resistance-1-6f22-48195, but it doesn't have frequency.

Thanks for the help, Haggai :D Yep, I've seen it before (UT10A), but what you think about UT33A (suggested by budgeteer before)? Looks like it's an auto-range meter for only 4 bucks more :~ Dunno I will measure more then 12V. It's basically for li-ion and ni-mh, and for flashlight currents (up to 10A). Don't know if I need fused ports for that :~ For sure I can buy one here where I live, for 5-6 bucks (like this one), but I think it will be very innacurate in measuring almost everything. Don'y mind to pay 10 bucks more and have a decent DMM.

[Haggai]

I think the UT33A is a very decent multimeter. I almost bought one myself. :)

It might pose some problem jumping between ranges when measuring li-ion batteries near 4v, but otherwise I don't see any problem with it if you don't intend to measure high current and take reasonable precautions. If you want 10A DC you shouldn't have any problem, I think, but you'll just have to be careful (warning from the manual: "When the testing leads are connected to the current terminals, do not parallel them across any circuit". You don't have a fuse there).

The DT830D DMM you linked to at DX is a PoS. I have one just like that (DT830B 3$ from eBay) and it can't measure any current over 1A, I wouldn't trust its accuracy even in the 500mA range, it can't test a high power diode, and it has serious safety issues (mine doesn't even have a fuse on the mA input, which together with having the mA and V inputs in the same port is unsafe; and I don't think it has any voltage input protection either). Buy it only to compare low voltage readings if you really want to.

[Kokopelli]

@Kokopelli

I'll try this as soon as I charge my black Trustfire's again.

Ok, time to update.

My IMR cells arrived today and even though they weren't fully charged to 4.2V I wanted to try them on a 2.8A light and use my Turnigy wire this time, for the measurement. The cells were 4.0V and tailcap readings were 1.8Amps with the stock leads those came with my Vichy 99, and 2.6Amps with the crocodile leads came with my Turnigy Accucel 6 charger.

So my leads seem to have a higher resistance (or dirty/weak contacts) than yours, or your Manafont drop-in pulls even more. And a thicker wire solves the problem easily.

[SashiX]

So I bought that UT33A for about $18. I really like UT10A small, pocket size, but it doesn't measure AC nor DC current :~ Wish UT33A measure temp and has backlight like UT33C, should be UT33C with auto ranging, lol :bigsmile: Will see if it's good enough.

[old4570]

Nice !

Looks like I need to budget for a new MM