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Old 6th Oct 2018, 2:52 pm   #1
ORAWA01
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Default Checking bulbs with multi-meter

Are multi meters connectivity or ohm setting able to check the light bulbs - whether it is shot or still workable?

This morning, I tried with my mmeter, but nothing was reading. And the bulb still worked.
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Old 6th Oct 2018, 3:02 pm   #2
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Default Re: Checking bulbs with multi-meter

You can test incandescent bulbs, but not CFLs or LEDs.
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Old 6th Oct 2018, 4:12 pm   #3
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Default Re: Checking bulbs with multi-meter

Further to the above, on the rare occasions when a mains voltage incandescent filament lamp tests open circuit on an ohmmeter but still illuminates, it should be taken out of service as it will fail altogether in the near future, generally blowing a fuse or tripping a breaker when it does so.

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Old 6th Oct 2018, 7:30 pm   #4
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Default Re: Checking bulbs with multi-meter

Could be that there is a break in the filament, but the ends are close enough that mains voltage is sufficiently high to strike an arc and weld the ends together. I recall getting a clear bulb whose filament had fractured to light by carefully waggling the bulb until the ends met. It was a low wattage bulb for my darkroom safelighbt and I didn't have a spare.
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Old 6th Oct 2018, 8:25 pm   #5
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Default Re: Checking bulbs with multi-meter

They are energy saving bulbs from Diall 9W.

Are they kind of CFLs?
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Old 6th Oct 2018, 8:30 pm   #6
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Default Re: Checking bulbs with multi-meter

If they are recent they are likely to be LED

I usaully come accross DIALL in B&Q

A picture wold most likely resolve their type

You can't really check them with a meter.

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Old 7th Oct 2018, 8:20 pm   #7
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Default Re: Checking bulbs with multi-meter

Compact Fluorescent lamps are rarely marked as such, they are marketed as "Low energy", which doesn't mean very much.

LEDs I suppose are deemed to be sexy enough to be called what they are!
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Old 8th Oct 2018, 11:16 pm   #8
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Default Re: Checking bulbs with multi-meter

Compact Fluorescent Lamps usually have a bridge rectifier connected directly across the mains input, and LED bulbs probably have a similar arrangement. There will be no DC path between the two ~ terminals of the bridge. The output of the bridge rectifier goes straight to an electrolytic reservoir capacitor. It's doubtful whether the ohmmeter's internal battery will be sufficient to turn on the two forward-biased diodes in the bridge rectifier, so it will probably register an open-circuit.

Interesting question, though; I must try it myself with a selection of multimeters.
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Old 10th Oct 2018, 2:54 pm   #9
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Default Re: Checking bulbs with multi-meter

Only incandescent lamps can be checked with a meter, and even then not terribly reliably.
Incandescent lamps normally fail in service by breakage of the filament, this can be detected by a test meter.
New lamps that have suffered transit damage or are of defective manufacture, may contain air and therefore fail instantly when lit. No simple test will detect the air ingress.

Also, no simple test will detect lamps that are in fact sound, but are wrongly labelled.
As a teenager I purchased very cheaply a bulk supply of lamps that were marked "115 volt 60 watt" but were in fact 240 volt.

Also a supply of large GLS lamps simply marked "250 volt dim" with no wattage stated. Measurement showed them to be about 15/20 watt, but the same size as a high wattage lamp. Presumed to be war surplus.

No simple test would detect these oddities, though a cold resistance test and comparison with a known lamp would.
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Old 10th Oct 2018, 6:09 pm   #10
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Default Re: Checking bulbs with multi-meter

Back in the 1960s the local hardware shopkeeper used to have a live lamp holder on the counter to test all bulbs before sale.
Those days are long gone now though.
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Old 10th Oct 2018, 7:27 pm   #11
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Default Re: Checking bulbs with multi-meter

Bulbs being called “sexy” am I missing something?
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Old 10th Oct 2018, 7:35 pm   #12
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Default Re: Checking bulbs with multi-meter

And Woolworths had a rather safer tester that consisted of several lamp holders for different based lamps, mounted atop a large box. Two momentary action switches were located on opposite sides of the box and had to be depressed at the same time to light the lamp.

One hand for each switch precluded inserting a finger into the lamp holder whilst live.
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Old 10th Oct 2018, 7:36 pm   #13
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Default Re: Checking bulbs with multi-meter

I remember that Woolworths used to test bulbs when you bought them. They had a box with a couple of different lampholders on the top and a pushbutton switch on each side. The assistant had to first put the bulb in the appropriate holder and then push both buttons to make the bulb light.
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Old 10th Oct 2018, 7:37 pm   #14
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Default Re: Checking bulbs with multi-meter

Quote:
Originally Posted by broadgage View Post
Incandescent lamps normally fail in service by breakage of the filament, this can be detected by a test meter.
The same applies to most CFLs, except that they have one at each end of the tube and they're only accessible by dismantling.
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Old 16th Oct 2018, 2:58 pm   #15
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Default Re: Checking bulbs with multi-meter

Local electrical shop had a tester, used to charge 2d for testing your bulb and also tested bulbs that they sold before handing them over. new bulbs were 1/-3d or 1/-6d as I remember,
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Old 16th Oct 2018, 3:22 pm   #16
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Default Re: Checking bulbs with multi-meter

I can see the merit in testing lamps when sold, but who would pay to test a lamp that they already owned ?
It would be cheaper and less trouble to try unknown lamps at home on a known working circuit.
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Old 16th Oct 2018, 7:54 pm   #17
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Default Re: Checking bulbs with multi-meter

Quote:
Originally Posted by Refugee View Post
Back in the 1960s the local hardware shopkeeper used to have a live lamp holder on the counter to test all bulbs before sale.
Those days are long gone now though.
The local electrical/DIY shop near me had a board rigged up at the counter with every type of socket and lampholder which he used to test bulbs etc before sale but the thing I remember was a piece of flex sticking out with bare ends which was line and neutral At least it was switched.... This would have been early-mid 80's.

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Old 16th Oct 2018, 9:04 pm   #18
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Arrow Re: Checking bulbs with multi-meter

Quote:
Originally Posted by budkor22 View Post
Are multi -meters connectivity or Ohm setting able to check the light bulbs - whether it is shot or still workable?

This morning, I tried with my Ohm-meter, but nothing was reading. And the bulb still worked.
If that bulb was of the S.E.S. type - outer thread connecting to one end of the filament - and a central pip, connecting to other end of the filament - that central pip, which is usually solder, can develop a layer of oxide that gives the problem you report, especially if the bulb has been in storage for some considerable time. Of course, when you then screw it into the threaded bulb holder, that very action scrapes off a layer of that oxide - and voila! The bulb illuminates - sometimes. The fix? Look at that pip: if in doubt, gently run the pip over fine emery cloth. Then use the Ohm-meter.

All above based on personal experience.

Al.
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Old 16th Oct 2018, 9:27 pm   #19
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Default Re: Checking bulbs with multi-meter

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Originally Posted by Phil G4SPZ View Post
Interesting question; I must try it myself with a selection of multimeters.
Coincidentally, two nights ago a 'Megaman Compact 2000' 11 Watt CFL bulb failed while I was in the room. The failure started with brief flickers, which got quickly worse and after about two minutes the lamp extinguished for good. No noise, bangs or smells of burning. So I had a faulty CFL to compare with its brand new replacement, which is a Philips 7 Watt model. Having done a few tests this evening, I have concluded - to my surprise - that, using a conventional multimeter to measure the resistance between the lamp pins, it IS indeed possible to tell the difference between a good CFL and a faulty CFL. Or, more correctly, a non-working CFL that has the same fault as mine...

I tried five different analogue multi-meters (is there any other kind?) set to their highest ohms ranges, each having different terminal voltages: a 40kOPV Taylor with a 1.5 volt internal cell driving the ohms ranges; a 20kOPV Avo 8 Mk I with a 15 volt battery; a vintage 20kOPV Espey 100 with a 24 volt battery; a 1,000kOPV Avo EA113 electronic multimeter; and a Heathkit Daystrom V-7AU valve voltmeter.

The Taylor cam measure resistance up to about 2 Megohms. The good CFL produced a small brief 'kick' from the pointer with the prods applied in either direction, before falling back to a high resistance near infinity. The faulty CFL produced an initial 'kick' but the meter then returned to infinity.

The Avo 8 and the Espey 100 can measure up to about 20 Megohms and 60 Megohms respectively. With the good CFL, both meters registered an initial kick to around 200k, settling quickly to around 500k in both directions. However the faulty CFL, as before, registered an initial kick to around 500k but then fell back to settle at infinity.

The Avo electronic multimeter and the Heathkit VVM showed readings of 4 Megohms and 8 Megohms respectively in both directions with the good CFL. The faulty CFL registered an open-circuit on both meters.

Finally, for the sake of completeness, I repeated the experiment using a modern cheap DMM and a 1980s Tandy DMM. Both digital meters displayed an open circuit on both the good CFL and the faulty CFL.

The general conclusion is that a good working CFL should measure around 500k when tested on the 'ohms x 100' range of an Avo 8 or similar decent conventional analogue multimeter having an open-circuit terminal voltage of at least 15 volts. A faulty CFL may show an initial reading but it will fall back to open-circuit. Other fault modes are undoubtedly possible which could give rise to different resistance readings, but hopefully these will differ from the 'good' readings listed above.

I would welcome any comments, particularly if anyone else can replicate my experiments using different types of meter or lamp.
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Old 16th Oct 2018, 10:56 pm   #20
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Originally Posted by Skywave View Post
If that bulb was of the S.E.S. type...
I have come across a number of small Edison screw-capped bulbs where the soldered connection to the screw cap has been left with a protruding blob of solder, large enough to prevent the cap screwing right down to the centre terminal at the bottom of the lampholder. Bulbs like E12 and E14 and even M.E.S. and L.E.S. are prone to this problem. The bulb tests perfect on an ohmmeter but won't light when screwed in. A moment's work with a fine file or piece of sandpaper will usually effect a cure.
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Last edited by Phil G4SPZ; 16th Oct 2018 at 10:57 pm. Reason: Clarity
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