Measuring current draw. What Am I doing wrong?

Raoul_Duke · Jun 28, 2006

I have a Vital gear F2.

I have two lamps; the one I'm using is a Digilight DRB 9V hp lamp (I think) the other is on loan to a friend but was labled G&P G90 9V.

They were both bought from Dan at blackrifles.com. I emailed after I had recived them enquireing if they were the 175 lumen lamp. He said they are the 175 lamp, one was a direct pull from the digilite
They have a good output, both equal, and to my eye, roughly the same output as the surefire P91. Although they are alot whiter than the P91. Maybee the P91 is brighter but my eyes cant see it. It think the Digilite or G&P lamps beat the P91, especially in the light output verses runtime dept.

Anyway, I can't power the two lamps from the protected 123's that I bought, they will double tap ok, but not without two taps.

So I am using Unprotected 123's. But Idealy I'd like protected.

I thought prehaps I had the 1.12A or 1.22A lamps, so I tried to measure the current it was pulling last night.

I unscrewed the switch, took my multimeter & and put one lead on the end of the cells, and one to the body of the light.

The lamp slowly lit up, but It was very under driven. under 1/2 the output at least. it was reading aroung 0.8 amps.
I tried another multimeter ( both fluke's ) and blew the fuse in the leads. Doh the fused were only good for 500mah.

so using the same leads on both multi meters I only got 0.7 amps.

I changed the settings and checked the leads on the ohms setting, and It read 3 ohms.

so do the leads and meters resistance stop this lamp from running at full brightness?

I'm guessing so. but what can I do to check the real current draw under full load?

nzgunnie · Jun 28, 2006

3 ohms is very high for just the leads, our flukes read about 0.12 ohms for the leads. But then our leads don't have fuses.

Apart from making sure you're using the correct range, I can't see that your doing anything wrong.

yellow · Jun 28, 2006

my very cheap meters go down to 0.0 when measuring the resistance of the leads, must be something wrong with Yours.

when measuring any current of a lamp, there is always an unprotected Amp-position with any meter I have seen.
showing 10 or 20 Amps, the positive lead then has to be put into another contact possibility.

Everything else sounds ok (lead 1 to batt, lead 2 to body)

scott.cr · Jun 29, 2006

Agree with Yellow; there are probably three (+) plugs for your meter probes. One of them will be for higher-amperage readings.

evan9162 · Jun 29, 2006

Sounds like you're using the 1A scale. Use the 10A scale and appropriate probe socket. This will introduce a minimum amount of resistance into the circuit. The ~1A scale introduces about 1 ohm of resistance into the circuit which would account for the low current and performance when measuring.

Raoul_Duke · Jun 30, 2006

I was using the 10 A setting, and all the leads were in the correct sockets.

It was my leads that were the problem.

I used some different leads ( with no fuses,) Just straight leads &...

The Lamps (Digilight DRB HP 9 Volt) was 1.2Amps settling to 1.19 A, and the P91 was 2.5 settling to 2.49 amps. Which sounds about right to me.

The Light output was the same as if the switch was installed.

Looks like I need to get the newer more efficient Digilight lamp to have a chance at running on protected cells.

Cheers for all the replies

andrewwynn · Jul 1, 2006

or soft-start.. to hold current back during the start-up.

i use 1' lengths of monster cable speaker wire with banana plugs for my ammeter leads.. it's amazing how much the leads will throw of any readings over 1A.

-awr

jayflash · Jul 2, 2006

I've used the stock Fluke leads or those from good quality accessory lead kits for dead short cell testing. How much more amperage might be shown on the meter by switching to a 12ga wire? Just using bare, stranded, wire to increase contact area on the tested cells may be better than the tiny point on the test probe, too.

andrewwynn · Jul 2, 2006

i usually measure about 40mohm per lead when i've tested my fluke leads.. so 80mohm.. if you pull 10A through 80mohm you drop 0.8V on the leads.

12ga wire is only 1.6mohm/foot.. 10A on a length of 6' of 12ga wire will drop 16x6= 96mV or 0.096V

for doing a 'dead short' test you want to have some resistance or you'll likely break things.. the meter leads might be just the right amount in that case.. but for current measurements.. lower the resistance the better..i use very short leads.. barely over a foot.. then i can get away with 14ga wire.

-awr

Handlobraesing · Jul 2, 2006

If you're measuring >1A stuff at low voltage, use a 0.01 ohm shunt. You can either buy one or research the ohm/foot for various wires and build one that's approximately 0.01 ohms (lets say the resistance is 0.01 ohm for 5.00ft. what you do is build it around 6 feet and take a measurement with a voltmeter leads placed on the wire so that the distance between the probes are exactly 5.00ft)

Put the voltmeter in mV DC settings. For each amp, you will get 10mV. With the typical digital voltmeter, you will get a resolution down to 100mA (0.1mV)

andrewwynn · Jul 2, 2006

i build one with wire and close measurements.. important: solder on the test point connections!

http://www.powerstream.com/Wire_Size.htm

that's a great reference for length per resistance.

I have to make very precise very low very resistance shunt resistors for hotdriver.. on the order of 0.001 to 0.002 ohm.... so i measure exact lengths of 20 to 22ga wire and it works great.

(i need to have 0.035V drop on the sense resistor at maximum current (up to 25A)).

that's definitely the best way (described above) to get the most accurate measurement.. but for most things... having very short very thick meter leads will do the trick.

-awr

Nubo · Jul 3, 2006

Raoul_Duke said:
I have a Vital gear F2.

I have two lamps; the one I'm using is a Digilight DRB 9V hp lamp (I think) the other is on loan to a friend but was labled G&P G90 9V.

They were both bought from Dan at blackrifles.com. I emailed after I had recived them enquireing if they were the 175 lumen lamp. He said they are the 175 lamp, one was a direct pull from the digilite
They have a good output, both equal, and to my eye, roughly the same output as the surefire P91. Although they are alot whiter than the P91. Maybee the P91 is brighter but my eyes cant see it. It think the Digilite or G&P lamps beat the P91, especially in the light output verses runtime dept.

Anyway, I can't power the two lamps from the protected 123's that I bought, they will double tap ok, but not without two taps.

So I am using Unprotected 123's. But Idealy I'd like protected.

I thought prehaps I had the 1.12A or 1.22A lamps, so I tried to measure the current it was pulling last night.

I unscrewed the switch, took my multimeter & and put one lead on the end of the cells, and one to the body of the light.

The lamp slowly lit up, but It was very under driven. under 1/2 the output at least. it was reading aroung 0.8 amps.
I tried another multimeter ( both fluke's ) and blew the fuse in the leads. Doh the fused were only good for 500mah.

so using the same leads on both multi meters I only got 0.7 amps.

I changed the settings and checked the leads on the ohms setting, and It read 3 ohms.

so do the leads and meters resistance stop this lamp from running at full brightness?

I'm guessing so. but what can I do to check the real current draw under full load?

Just put the meter in the circuit, which sounds like what you did. But 3 ohm is crazy, on amps setting, the meter should introduce minimal resistance. But you need a meter that is capable of measuring amps, not milliamps. Do you have an input socket for something like 10A, or 20A? This would probably be "un-fused". If you're trying to measure a >1 amp load on a setting designed for 500mA, you either blow the fuse, or perhaps have high resistance because the fuse is heating up. Or maybe the leads aren't making good contact to the meter?

Measuring current draw. What Am I doing wrong?

Raoul_Duke

Enlightened

nzgunnie

Enlightened

yellow

Flashlight Enthusiast

scott.cr

Flashlight Enthusiast

evan9162

Flashlight Enthusiast

Raoul_Duke

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andrewwynn

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jayflash

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andrewwynn

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Handlobraesing

Banned

andrewwynn

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Nubo

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