How To Waste Three Hours

[Forumadmin]

Correct! A poor ground has resistance, and yes that will generate heat if a current passes through it. What it will not do is increase the voltage across ( or increase the current through or heat in or light from) a bulb connected to that poor ground.

This does assume that the source of electrical energy is essentially constant voltage (rather than constant current). An automotive battery delivers constant voltage within certain limits. The dynamo is a different story (without a regulator) as it tries to deliver its output with the load attached to it. Hence its voltage will rise if there is little load and without a change in the field current. Thus faults in the battery (e.g. a plate going high resistance or a battery feed or ground lead) or a fault in the voltage regulator might cause more than 12v (13.8v) to be across loads (a bulb) in the car.

This electricery gets more complicated when we start looking at alternating current. inductive loads and high frequencies where some peculiar things start to happen. An intermittent bad joint might heat up even more due to inductive loads (like the points in the distributor) where very high back electro-motive force creates an arc. There is very little inductive load in a bulb circuit so such an effect should not happen.

Other peculiar things start to happen with electricity with superconductors, where massive currents can flow with very little generation of heat, and semi-condutors where changes in resistance can be large with small changes in voltage e.g. diodes; but I do not think we have them in a Jowett.

[Keith Andrews]

This is a basic thread on bulbs blowing on a dc cicuit....and the effect of a bad ground (or any connection for that matter)has on that bulb
I again suggest connect a say 5w bulb acroos a battery, then introcuce a head lamp bulb in that circiut in series to act as a resistance as a bad ground/connection wil....and watch what happens to the 5w bulb as the curent /wattage that the head lamp bulb draws goes thru the 5w bulb W=AxV
As another excerise put a 12v headlamp across a 12v battery with a ammeter and volt meter in the circuit
Then do the same on a 6v battery.
And repeat on a 36 v battery.
Basic school kid experiments.
And lets not go int a/c currnts, alternators as a gainst generators rectifiers etc etc that is a new ball game.

[jowettgeoff]

Hi Mike,

Two possible causes spring to mind (both of which I've had recently!). Firstly there can be a bad connection through the spring between the bulb and the live terminal when the bulb is in its normal position. If you push the bulb in and it then lights up, this is the problem. It can be temporarily cured by just partially twisting the bulb into position on the bayonet (ie, not quite twisted the whole way), thus increasing the pressure on the connector. Another problem can be with the little rivets that hold the earthing socket to the lampholder. These can corrode and become 'high resistance'. It's possible to apply a blob of solder between the socket and the lamp holder, or more crudely, slip a short piece of thin flex between the bulb body and the holder, then connect the other end to one of the lampholder securing screws, or else trap it between the bullet connector and the earthing socket. Digital voltmeters can be a bit misleading as they have what's called a 'high internal impedance'. This results in them sometimes displaying a full 12 volts despite the fact that there may be a quite high resistance between the supply and the bulb. I tend to use a simple test lamp which can be far more reliable. Anyway, by the time this is posted you may have found a cure!
Good luck
Geoff McA

[Forumadmin]

then introcuce a head lamp bulb in that circiut in series to act as a resistance as a bad ground/connection wil....and watch what happens to the 5w bulb as the curent /wattage that the head lamp bulb draws goes thru the 5w bulb

What does happen to the 5w bulb? Does it blow? If so why?

The current a bulb draws depends on the volts applied across it and its resistance. The wattage of a bulb is only a computed nominal figure based on a fixed normal working voltage. If the high resistance low wattage bulb is in series with a high wattage bulb, the current the high wattage bulb draws will reduce from its normal amount and hence the energy it uses (watts ) will also reduce. Both bulbs are working below their normal working voltage and not using the same amount of watts.

Anyway I do not know how many more different ways I can put this.

If you put a bulb rated for 12v on a 36v battery it will blow, or at least burn very brightly. If you put on a 6v battery it will glow dimly. If you put three 12v bulbs in series on a 36v battery they will work normally and not blow. It does not matter what wattage the bulbs are, only their rated voltage. If you put dissimilar resistance (wattage) bulbs in series then the lower wattage bulb will blow because the voltage drop across the low resistance high wattage bulbs is small and thus overdrives the smaller bulb. Using the example bulbs we previously used 2x24W bubs and a 2w bulb in series driven by a 36v battery would put .43 amps through the circuit of total 84 ohms. This current would blow the 2w bulb that is rated only at 0.17 amps. Which I guess is what you might have experienced. But there should not be more than the rated voltage anywhere in the car!

[Keith Andrews]

What does happen to the 5w bulb? Does it blow? If so why?

How many times have sugested to jusrt do it..u will not take my word as to what happenes which is the same as a bad ground/connection resistance

"How To Waste Three Hours"
Pretty well sums it up m8

[David Morris]

Hi All,

Just a thought. I have ditched using a liquid crystal meter on car electrics. They draw so little current that problems like poor earthing don't show up. I would always recommend using a good old moving coil test meter. They will not give you anything like the number of false readings!

All the best,

David

[jowettgeoff]

I agree with David, but even moving coil multimeters can have an internal impedance of perhaps 20,000 ohms per volt, so a high-resistance fault can be difficult to spot for the uninitiated (not you David - obviously!).. The little screwdriver cum test lamps you can buy at car boot sales for about a couple of quid are remarkably handy. Or if you're a Yorkshireman/Scotsman, you can make your own from a bit of wire and a sidelight bulb .

G McA

[Keith Andrews]

When looking for a bad ground or conection look for voltage drop aross the connection and confirm with with resistance test.
But id suspect something, just clean up the contacts, re solder in case of dry joints and trow in fresh stainless star washers.
The old anologe type meters do have a resistance across them even thu very small in the armiture coil where as the digital do not draw any residual current and are far more accurate over a far wider range...Anologe are only acruate in the middle of the dial range.
Also digitals are far safer on electonics becuae of not drawing current...as in checking things like O2 sensors where any current load across as with a anologe causes deteriation....and the faster reading of digital gives far more accurate results.

One thing to watch with digital...the battery is in good conition and full charge...most take 9V batteries and get below 8.5V and your readings can be come very unreliable...espec voltage and resistance...This is usually the reason of digitals getting a bad rap.
A quick check is if your digital has a opitional background light see if there is any difference in low voltage less than 2v or resistance reading with light on and off.