How To Waste Three Hours

[Mike Allfrey]

To All Of You Electrical Experts Out There!

The front LHS parking lamp went out. Removed the bulb and found it to be fairly black. Bought some new Narva bulbs, now, like everything else imaginable, made in China. Installed a new bulb and not a glimmer. Thinking that the Chinese may not get them all properly made, I tried another three of the ten purchased. Not a glimmer!

Using my digital multimeter, there is 12.6-volts measured at the positive contact point and the body of the Lucas lamp assembly. Continuing with the multimeter, I checked for continuity between the bulb's bayonet body and earth, there is no resistance. Cleaned the contact point on the bulb with a piece of fine wet-and-dry paper. Can feel definite spring pressure as bulb is pushed in. Contact in bulb holder is right in centre, still not a glimmer. Checked for continuity from bulb holder to a good earth on chassis - no resistance.

Tried one of the new bulbs in the RHS lamp body, it works!

So, what on earth is going on here? Anyone know? The lamp was new in 1998 and there is no evidence of corrosion.

I am off on the Jowett Jaunt tomorrow and, all I can say if I get pulled over (and that will be my sort of luck) is, "It was working earlier Officer."

Some hints on a probable cause of this problem will be greatly appreciated.

All the best,

Mike A.

[Mike Allfrey]

The problem is resolved!

At the front underside of the bonnet there is a group of connectors. There is one double connector (of the Lucas snap type) with a red cable entering it and two red cables exiting it. This is the parking lamp circuit. After pulling out the cable for the LHS lamp, and pushing it back into place, the lamp came on.

This tells me that the original connection was sufficient to give a multimeter reading of 12.6-volts, but with the load of just a 6-Watt bulb, the continuity broke.

This has taught me to not always believe what is seen on a multimeter's liquid crystal display. I was deceived (and impressed!) by the 12.6-volts coming up instantly with no erratic numeral display, usually associated with a poor quality connection.

In my experience, the vast majority of such concerns have been 'Earth' related. At least with a more modern car with an electric temperature gauge, when there is a poor quality earth somewhere, the temperature gauge needle swings from hot to cold, and back, in time with a turn indicator flasher unit's pulsations. With the Jowett's accurate capillary tube type temperature gauge, such a useful diagnostic tool is not built in!

I wonder if there is an attachment that induces some load on a circuit that can be attached in the multimeter's live lead while taking a voltage reading. I suppose a small bulb in a suitable bulb holder could be inserted somehow.

Probably I will never be in this predicament again though!

All the best,

Mike A.

[Forumadmin]

I have had such indication frequently. Most notably across the ignition switch. What you have is a high resistance connection. Digital multimeters draw little current (bring back the Avo!), thus with little current the voltage drop is small (remember Ohm's law?); but, when the bulb is inserted, the voltage drop is high. Simple solution is to check the voltage with a load, as you say a test lamp and the meter across the lamp. The test lamp will probably not light anyway so you do not need the meter. If you use a low wattage bulb then the test lamp may just glimmer.

Similar indications can occur with a broken (or high resistance) earth circuit. What may happen then is the device may just work (for instance both stop and tail lights glimmering together) where the return current that normally should go via the earth connection is going through more than one bulb in series to return to the battery. I test circuits with the meter set to resistance (ohms) and (with battery disconnected) a thick, low resistance wire as a return. Check the circuit resistance between the battery terminal (either + or - depending on whether I am testing the feed or return connections). The resistance should be less than 1 ohm.

[Mike Allfrey]

Thanks Keith,

I was swayed by the solid-looking, instant 12.6-volt reading. Moral - never assume that what you see is actually there!

How is Jenny coming along? Give her my good wishes.

Mike A.

[Keith Andrews]

Removed the bulb and found it to be fairly black

There could be your answer there
Black a sliver or whitish coating on the inside of the buld is due to a bad ground.
The bad ground has resistyance, this increases the draw of current thru the filiment which causes excessive heat
So much heat the tungstin flimiment slowly vapourises and consenses on the the inside of the bulb glass.

[Forumadmin]

Increased resistance reduces current!

A jamming spring in the connector or some other cause of an intermittent connection might cause the circuit to break and then reconnect. This would cause arcing at the break and thus cause localised heat. This might transmit up the rod to the filament of the bulb; but I doubt it would balance the loss of heat caused by the loss of current supply to the filament. Although the filament is coiled I doubt the inductive effect of the rapidly breaking connection would have much effect and increase the temperature of the filament. The heat from arcing might also effect the seal of the bulb.

The blowing of the bulb filament usually draws a high current which would probably break any bad connection in the circuit or melt a narrow copper wire, so that when you put a new bulb in it does not work. (How often do the trips go in your consumer unit when a bulb blows? They actually should do to protect the wiring!)

The whitening is caused by the evaporation of the tungsten when the filament breaks. The blackening is explained below.

In a conventional lamp, the evaporated tungsten eventually condenses on the inner surface of the glass envelope, darkening it. For bulbs that contain a vacuum, the darkening is uniform across the entire surface of the envelope. When a filling of inert gas is used, the evaporated tungsten is carried in the thermal convection currents of the gas, depositing preferentially on the uppermost part of the envelope and blackening just that portion of the envelope. An incandescent lamp that gives 93% or less of its initial light output at 75% of its rated life is regarded as unsatisfactory, when tested according to IEC Publication 60064. Light loss is due to filament evaporation and bulb blackening.[64] Study of the problem of bulb blackening led to the discovery of the Edison effect, thermionic emission and invention of the vacuum tube.

A very small amount of water vapor inside a light bulb can significantly affect lamp darkening. Water vapor dissociates into hydrogen and oxygen at the hot filament. The oxygen attacks the tungsten metal, and the resulting tungsten oxide particles travel to cooler parts of the lamp. Hydrogen from water vapor reduces the oxide, reforming water vapor and continuing this water cycle.[35] The equivalent of a drop of water distributed over 500,000 lamps will significantly increase darkening.[34] Small amounts of substances such as zirconium are placed within the lamp as a getter to react with any oxygen that may bake out of the lamp components during operation.

Some old, high-powered lamps used in theater, projection, searchlight, and lighthouse service with heavy, sturdy filaments contained loose tungsten powder within the envelope. From time to time, the operator would remove the bulb and shake it, allowing the tungsten powder to scrub off most of the tungsten that had condensed on the interior of the envelope, removing the blackening and brightening the lamp again

[Keith Andrews]

Increased resistance reduces current!

Correction in terminlogy then inceasre in watts withing the circuit...so if the bulb is rated at say 2 watts and the bad ground draws anothe 1/2w then the re is too much energy being drawn thetu the buln, the filliment runs too hot and slowly evaporates aand vcondeses onto the glass...and will eventaully blow the bulb
Rule of thumb without getting way over board complex...bulb that needs replacing and has a darkened glass check connections to stop repeat blowing of bulb.
I think most people will understand that.
Discover of the edison effect and invention of the vac tube is technology 150 yrs old as is the rest...manufactureing , filitment alloys etc have progresed hugely since then..dont even come close these days...So as to what relivence 100 yr old cinimea bulbs have to todays bulb technology????? and water issues.. ????Mr.Philips changed that after edisons patients ran out.

[Forumadmin]

Still wrong! If you have a bad ground there will be less volts across the bulb, hence less watts used by the bulb as the resistance in series with the bulb will reduce the current through it. There is a possibililty that the bad earth would not let the tungsten operate at a high enough temperature thus causing the blackening.

Moisture around the bulb might also gradually seap through into the bulb (it would only need a few molecules). The water vapour then is split by the high temps into H and O and this oxidises the tungsten.

Yes, you are correct, the tungsten is now doped with various elements to help prolong life and different inert gases are used; but the basic technology of a filament lamp has not changed. Neither has chemistry or physics!

And yes again, if you have a blackened bulb make sure the enclosure is water tight and connections are good. I seal my sidelights with silicone to make absolutely sure. I now also use halogen bulbs as side lights and have never had one go in 10 years, whereas before there were at least 2 per year.

[Keith Andrews]

Now u get a 2w bulb connect across a battery..it works
Now conect a 24W bulb (the bad ground) into the circuit, the 2 W bulb will over heat and blow
R=V/A
W=AxV

[Forumadmin]

With a supply voltage of 12v, the 2W bulb takes (I=w/v) =1/6 Amp= 0.17A
Resistance of 2W bulb is (R=V/A) 72 ohms.
The resistance of the bulb does not change (assuming no temperature effects).
Similarly the 24W bulb takes 2 amps and has a resistance of 6 ohms.

Now put these two resistances in series and you have a total resistance of 78 ohms.
The current through the circuit (A=V/R) =0.15A

Since the current is now less through the 2W bulb than when directly connected across the battery, it is unlikely to blow.
The voltage across the 2W bulb when in series with a 24W bulb is (V=A*R) = 11 v

If what you say does happen, then I am surprised.

Anyway I am not friends with Alain Roland (being half Welsh)!!

[Keith Andrews]

If what you say does happen, then I am surprised.

Well I suggest that u try it..nothing like reality over therory.

[Forumadmin]

There is a posssibility that a poor earth or feed in the dynamo circuit would create higher than the rated voltage of the bulbs, thus blowing them. There is also a possibility that unless the bulbs run at there rated voltage blackening of the bulb occurs. This certainly happens with the heaters on flourescent tubes.

The test would need to be to connect a 2w and a 24w in seies and then a 2w in parallel to these two. Then put a 12v battery across the two circuits. Even better would be to do this with 10 sets of three bulbs at the same time , thus getting a statistically believable result.

[Keith Andrews]

Which is what I said above
1/Cicuit battery to switch to say left rear tail light bulb to ground good groud long lasting clear bulb
2. Battery to switch (as above) to righ rear tail light to ground and not having a common ground for both sides, and this ground is corroded , or no star washer and has poor groun .. this bulb with have short life and discoloured glass

Therefore althu the bulbs are on the same circuit and switch one will have a tency to have to be replaced more often than the other.

And A note: ALWAYS use star washers under ground lugs to the body...this is what those funny little spikey washes are for, breaking thru new paint, and or thru dirty metal and dig into good steel to make good grounds....stainless star washes are best as they are stronger with far better bite.
Do not just scrape back a bit of paint or just clean the body surface the bolt /screw the ground to the body without a star washer...such lazy practice will come back and bite u in the bum...this basic principles apllies to all electical connections.

[Forumadmin]

Well if it was what you said, I misinterpreted it. The bad earth would have to be on the dynamo circuit, not the lamp circuit, to create a higher voltage than normal. A poor earth on the battery might cause the dynamo to run at say 16v and this might go through the lighting switch to the lamps and return to the dynamo earth (i.e. engine). Faults in the regulator might also cause this; but I have not analysed all fault possibilities. A poor earth on the lamp circuit (except from battery to chassis) can only ever result in a lower than normal voltage. A poor earth from lamp to chassis would not result in a high voltage on the circuit with the poor earth. Indeed the opposite is true as other bulbs would see a slight increase due to a lower load on the supply voltage.

[Keith Andrews]

Think of it like this a poor ground has resistance correct?
A heater element, jug element, a bulb filiment has resistance...hence why they heat up
A bad ground or even +Ve on a battery which, is easy to pick up because the cable with the bad ground will be warm or hot.
They are hot because the little electons have trouble getting thru the resistance , get all excited and rub together, like rubbing your hands..and like your hands they heat up.
they they draw current...
On a battery system the voltage drops with resistance, just like water in a pipe over distance or a blockage...but the system still wants to maintain and equilibuim, so the current draw increases....the narrow part of the water pipe has higher flow
On a water pipe the pressure is the voltage and the flow is the current..flow current see the relastationship.?

A poor ground/connection no matter where itis in a cicuit will screw how other compents in that circuit are ment to operate...