Measuring frequency with a multimeter is fun :)

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Haggai
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Measuring frequency with a multimeter is fun :)

Vichy VC99 (which is a great multimeter, btw) has both frequency and duty cycle measurements, so I decided to test my P60 dropins.

I hooked them directly to a 3xAAA holder (because it's easier to attach crocodiles to it than to a 18650 battery), and attached the multimeter leads in parallel.

Results (in the format of MODE PWMfreq/dutyCycle):

tMart Q5 #1: H no PWM / M 6.2kHz/40% / L 9kHz/9% / strobe 14.8Hz/50%

tMart Q5 #2: H no PWM / M 6.2kHz/40% / L 7kHz/9% / strobe 14.9Hz/50%

Manafont XML: H no PWM / M 123.5Hz/30% / L 126.6Hz/5%

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.

 

Did I say I love this multimeter? Embarassed

 

Two pictures of the setup (try to find which wire goes where... apparently the polarity affects duty cycle measurements [if I'd reverse the green/yellow leads it would read 70% instead of 30%] but not the frequency).

Manafont XML on medium:

Edited by: Haggai on 04/11/2011 - 16:00
Don
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Niiice!

 

The numbers from my light tests are always to be found here.

https://spreadsheets.google.com/ccc?key=0ApkFM37n_QnRdDU5MDNzOURjYllmZHI...

arenat
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From this manufacturer I have an LCR meter .The only instrument in that cheap category I own. While the case is awfully cheap the board and especially the LCD display are top quality, the best I have.

Payed for it a fraction of the brand name cost...

Haggai
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I wish they made some cheap battery internal resistance meter.

This could be really useful.

Foy
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Haggai - What did you pay for the Vichy, if you don't mind me asking.

needsabetterdmmFoy

No referral links and nothing embedded . . . ever.

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Haggai
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Flashlight Foy wrote:

Haggai - What did you pay for the Vichy, if you don't mind me asking.

needsabetterdmmFoy

Not at all.

I bought it from DX for 33$: http://www.dealextreme.com/p/3-0-lcd-handheld-digital-multimeter-voltage-current-resistance-temperature-2-aaa-37082

photon1k
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Thought provoking... Made me go and do a little reading. Thanks for posting this Haggai.

kragmutt
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I got one too, but still waiting for it to come. Glad to hear that you like it.

Kokopelli
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I had the same one a couple of weeks ago. It seems to measure a bit lower over 2Amp with the stock leads I'm afraid. I'm happy I now can use my crocodiles I got with my Turnigy charger. They seem way thicker than any leads I have. Haven't made the same measurements yet though.

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

It shouldn't have worked. What I was trying to measure was DC current change, but the multimeter measures frequency through voltage changes (it's parallel to the circuit), and since the flashlight is on and the driver just switches the LED on and off but keeps drawing a small current for itself, the voltage in the circuit is constant! (or so I thought)

Well, the DC voltage is indeed constant. (it decreases steadily, but does not fluctuate)

So I measured the AC voltage for the XML at the battery, and discovered: H 0.000v, M 0.120v, L 0.035v... Which means that the current ripples of the PWM process are measured as alternating current (with load - negative polarity, no load - positive polarity?, maybe that's why it's important for duty cycle readings), and that's what my multimeter actually measures. Neat!

The voltage itself is probably proportional to the duty cycle (linearly? non-linearly? I need another PWM mode to know that...) - maybe when reading AC it's trying to fit the square wave onto a theoretical sinusoid?

I guess the 0.120v at medium is strong enough for the meter to measure, but it probably has a noise filter at about 0.040v. When I used my finger as a resistor in the baterry-multimeter circuit, I probably created some extra load which translated to higher voltage. I tested this as well, and with my finger-resistor I measured about 0.050-0.060v for an instant, which was probably just barely enough for the meter to read the frequency.

All this may be total bollocks, my knowledge of frequency counters, AC and PWM is very lacking; please correct me if I'm wrong...

Haggai
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Kokopelli wrote:

I had the same one a couple of weeks ago. It seems to measure a bit lower over 2Amp with the stock leads I'm afraid. I'm happy I now can use my crocodiles I got with my Turnigy charger. They seem way thicker than any leads I have. Haven't made the same measurements yet though.

The stock leads I got can measure at least 3.2A. They have 0.2Ω-0.3Ω resistance when measured closed circuit with this meter.

Do you see differences when you measure the same setup with just the leads replaced by the crocodiles?

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

Budgeteer
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I was considering to buy a ddm capable of frequency measurement but i'm not sure i would really like to fiddle with all those cables. PWM actually never bothered me.

The VC99 DMM always appealed to me but i never found it under 22€. The reviews says it has junk internals but for the price difference of a genuine fluke i can live with that easily. I always wanted the capabilities VC99 provides.

Looking to get this instead:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250800691624&ssPageNa...

kragmutt wrote:

They're gonna send you a green redcat with a black LED.

Haggai
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The UT33A is very nice indeed, but I strongly recommend against buying an unfused A port if you ever intend to measure anything main/car battery/high current related.

And there's the VC97 which is almost the same as the VC99 (minus the live bar which is really not very useful, the min/max function, and the range of voltage is less appropriate for li-ions) and can be found at DX for 26$: http://www.dealextreme.com/p/3-0-lcd-handheld-auto-range-multimeter-voltage-current-resistance-temperature-2-aaa-37383

angusinalberta
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Haggai wrote:

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.

Haggai wrote:

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.

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

 

arenat
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angusinalberta]</p><p>[quote=Haggai wrote:

 

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

 

 

Those counters aren't sensible enough to this experiment. Usually need some 'real' signal , in my Meterman 35XP is 2.5Vrms ...thought the solar panel could work but I suspect that his response is very slow to measure frequency...I'm back with the answer in a while.

Neither..

 

 

angusinalberta
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arenat wrote:
Those counters aren't sensible enough to this experiment. Usually need some 'real' signal , in my Meterman 35XP is 2.5Vrms ...thought the solar panel could work but I suspect that his response is very slow to measure frequency...I'm back with the answer in a while.

Haggai wrote:
So I measured the AC voltage for the XML at the battery, and discovered: H 0.000v, M 0.120v, L 0.035v... 

In other words the Vichy VC99 that Haggai uses works at a very useful 35 mV for frequency measurements, if not even lower.

If I didn't already have a couple of DVMs as well as an old oscilloscope I'd might consider getting a Vichy. OTOH I still prefer to see what I'm measuring with the scope.

Cheers,
Angus 

 

arenat
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Never mind, I was wrong ...Laughing  The measure is possible because the the high resistance of aaa cells which make a great ripple , probably more than 2Vpp, I doubt could measure something with a big cell ...

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Budgeteer wrote:

Looking to get this instead:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250800691624&ssPageNa...

Searching for a pocketable DMM too. UNI-T better than Xiole/Best brands? You can find them cheeper (10-13 on DX or ebay) but don't mind pay 5 bucks more for UNI-T if it's much better (still want a pocket one, I know that V97 is much better)

Haggai
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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 Smile ).

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. 

Haggai
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angusinalberta wrote:

Haggai wrote:

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 wrote:
Haggai wrote:

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! Smile

angusinalberta wrote:
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 LOL

SashiX
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Haggai wrote:

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 Smile ).

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 Big Smile 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
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I think the UT33A is a very decent multimeter. I almost bought one myself. Smile

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
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Kokopelli wrote:

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
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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.

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Nice !   

 

Looks like I need to budget for a new MM 

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