Is Rubber An Insulator Or A Connector?

[DerekW]

Not a problem as such, I'd just like an explanation from our electrical/electronic wizards.

My "new" green estate is now fixed; all instruments, displays and systems are working as they should do and an 8 mile test drive along country roads filled me with much joy. But whilst repairing the displays I twice met an apparent anomoly.

When I was a toddler, my railwayman Grandfather used to take me to his allotment at Willesden Junction. To get to the allotment he carried me on his shoulders over the electrified lines that ran alongside the LMS main line, explaining to me to me that it was safe because he was wearing wellies and rubber doesn't conduct electricity. And yet....

Back to the present: I had to replace the bulb in the auto gear indicator, which is an LCD set in the fuel/oil level/ engine temp instrument. This is a pig of a job because the indicator has to be dimantled to get to the bulb and it's almost impossible to do that without snapping one of the fixing catches. Anyway, when the thing was finally apart I found that there was a set of contacts in the housing, a matching set of contacts on the display, and a block of rubber between them which seems to transmit the signal current between the two.

Next job was restoring the full readouts on the left hand dot matrix display. As is well known, this is invariably caused by faults in the flexible connector. I rejected the idea of restoring tracks with a pencil and trying to increase the contact pressure between tracks and display, opting to buy a repair kit from French ebay.(At £23 it looks to be expensive but is a whole lot cheaper than a new display). Guess what? It was a strip of rubber which replaces the original connector strip.

So was my Grandfather wrong? Can I no longer trust the rubber handles on my pliers to protect me? What's happening, Dudes? (to use the vernacular). Sufficient to say that both displays now work perfectly.

Derek

[techmanagain]

All depends on the mix of the rubber - or rather the ad-mixture. That's what you are paying for.

[XM v6 sadist]

Hi Derek

Having one of the £23 pieces of rubber that I haven't got around to fitting, it looks to me that the rubber strip has some sort of conductor in it (the faint lines) which I guess how it works.

Cheers

Tony

PS. It was a good job it was never raining when your grandfather was carrying you, one slip on a wet rail with a slippery welly and you would never have enjoyed the delights of owning an XM.

[DerekW]

Tony,

Yes, but nobody in their right minds goes gardening in the rain, or the dry for that matter. And from my recollections, my childhood was filled with sunshine!

I saw what could have been faint markings, what do you think they are, carbon impregnations? At least it works and hopefully won't deteriorate in the future - no loss of flexibility and no possibility of adhesive failure - highly recommended..

Derek

This post has been edited by DerekW on July 18, 2007 05:10 pm

[DerekW]

And if it is impregnated with carbon, how did they manage to admix it (techmanagain) in umpteen dozen closely spaced lines, with no cross-conducting? And how did an ebayer with a very limited market manage to pursuade some manufacturer to make it? Or has he just drawn on a rubber strip with pencil?

This is the penalty of having an enquiring mind - I have to know how everything works.

Derek

[XM v6 sadist]

Hi Derek

Yes I was somewhat surprised when the rubber strip came through in the post. I'm curious. I guess that a small experiement could be in order? Attach a small electrical charge across the strip and see if there is conductivity - which surely there must be???

Cheers

Tony

PS. I'm a scientist so I love a good little experiment.

[DerekW]

Hi Tony,

Go for it! I hope I never have to dismantle the instrument panel again!

Let us know your findings, I suppose a resistance check would be the easiest way and would be least liable to cause damage.

Derek

This post has been edited by DerekW on July 18, 2007 06:45 pm

[Peter.N.]

I can remember when I was a kid, seeing a railway worker thow a rubber mat on the conductor rail and lay across it! We had always been warned not to get anwhere near them, and at 600v DC you wouldn't get a second chance., so he obviously had great faith in his 'pure' rubber mat. Later when I started work in the TV trade, we had a large rubber mat covering the floor in the workshop, and you could do the trick of holding a screwdriver on the EHT connection of a CRT (about 7 or 8 KV in those days) because of the insulation from the floor your body would gradually reach EHT potential, the trick then was to walk across the floor and touch somebody on the ear, or just holding your finger near was enough, a spark would jump to the unsuspecting victim, causing him sometimes to react quite violently (depending on how sporting he was) The only problem was that it gave you a shock as well!.

But to more serious matters, as has previously been stated, although naturally an insulator, rubber can be made conductive by the addition of other compounds or coatings, TV remote control keypads are a prime example.

Peter.N.

[techmanagain]

My memory of high tension shock came from leaning on the front mudguard of an army truck while in "shirtsleeve order" (i.e. rolled up) and a brother mechanic also leaning across to put FOUR fingers on four spark plags of the open Ford V8 engine (while it was running)., with his elbows on the metal bodywork of the truck . He says he felt nothng, but I certainly did - and how!!

[Peter.N.]

I know this isn't really the place for it, and I will probably get told off by Noz, but I have just remembered something that happened to me in the north of Scotland last year.

The people that owned the cottage we were staying in, had a fairly newborn foal which my wife wanted me to take a photograph of. It had moved away from the side of the road so I had to go up the owners drive to get near it. I leaned on the fence and tried to attract the foal to me, as it got near me I leaned harder on the fence to stretch out to reach it, at the instant it was about to touch my hand, I felt a sharp pain in the arm and a spark jumped from the tip of my finger to the poor creatures nose. Needless to say, it took off, to well out of photographic range. As I had been leaning on the fence I had pushed it down and come into contact with the electric fence behind it, just at the critical moment!

Peter.N.

[kenhall1202]

Hello readers

I work in the chemical industry and have visited a factory that made carbon black for the tyre industry by burning heavy oil with a shortage of air. The carbon black produced is extremely fine and is compounded with raw rubber by the tyre manufacturer to give it tensile stengh and wear resistance. So I thought if I stick my multimeter probes into a tyre tread block I ought to get a resistance reading of some sort - but no! not even on the highest Meg Ohm range so obviously the carbon particles must be well insulated from each other by the rubber. So that might explain why you don't get a shock through your black wellies. Then again the lack of electrical conductivity might be because tyre manufacturers are replacing carbon with (non conductive) silica to reduce rolling resistance (eg Michelin Energy).

However at my present employer's factory site (which handles many highly flammable solvents) it is mandatory to wear anti-static safety footwear on the plants. The idea is that a static charge is prevented from building up on your person because there is a constant leakage route to the ground or metal floor. (Unlike getting out of an XM in dry winter weather where I have had many a belt from the spark that jumps from your finger to the handle as you close the door!!) As far as I know the footwear soles are made from a polyurethane compound with some unknown additive to make it slightly electrically conductive (which I have confirmed by getting a reading of several hundred meg ohms across the sole of an old pair of boots). That doesn't answer Derek's question but I imagine the rubber conductive strip used in the LCD display is a similar sort of compound possibly produced as a multilayer sandwich of alternating conductive / non conductive layers.

Derk, I would be interested to have the contact details of your French ebay supplier of the rubber strip. Thanks.

Regards KenHall

[DerekW]

Thank you all for your help and comments.

My early working experience was aircraft based and I just remembered that aircraft then had conducting tailwheels to dissipate static that built up in flight.

KenHall, your suggestion of layered construction certainly makes sense for the LCD gear indicator but the dot matrix strip makes face contact. if you want to purchase the magic rubber strip the contact details on French ebay came from a member of our forum. I had it written down but can't find it at the moment, so after I've finished this post I'll go searching and post again. If you want to contact him the seller did send me an email, his address is eclipse212<keyboard sign for at>cegetel.net. Sorry about the rigamarole about @ but I understand it's to frustrate spammers.

Derek

[DerekW]

Hi KenHall,

Do a search on ebay.fr for "nappe afficheur gauche", that will take you directly to eclipse212's sales page. Information came courtesy of franpa2.

Derek

[xmexclusive]

Hi All

I spent most of my working life along side the Juice Rail. 600 volts was only the nominal voltage and most substations actually kick out nearer 850 volts with the breakers set for anything up to 10k amps. DC of course so unlike AC it holds you on if you were to touch it. Short between the rails accidentally with a shovel blade and you get a 20 foot high column of flame, smoke and molten steel and the circuit breaker perhaps a mile away will flicker at most and stay in. All normal commercial rubber in general use was treated as a conductor due to the presence of carbon black to prevent it degrading in sunlight. Hydraulic hoses were particularly conductive for some reason. Pure neoprene rubber is an insulator so before the modern emphasis on Health & Safety the maintenance specialist would unroll his mat on the ballast, position himself on the mat and then drill and tap holes in the web of the rail with it live. The man himself decided if the weather conditions made it safe to work.

So Derek the answer to your question is that rubber can be both an insulator and a conductor depending on the material composition. It is easy to manufacture a wide insulating strip with thin conductors bonded to it. The contact spacing on modern electronics is virtually standardised so our friend in France has just identified a source of bulk strip or a standard connector that he can buy in bulk that fits the XM display. My guess is that his charge is 1% material cost but good luck to him because none of us have a clue how to source the stuff direct and cut out the middleman.

Regards

XMexc

This post has been edited by xmexclusive on July 19, 2007 08:33 pm

[bigjohnh]

Chaps

This type of strip is used in all lcd displays in all clocks, watches etc.

As a kid I dismantled my LCD watch and between the display lcd and the circuit board was a stripey rubbbery strip but the giveaway was the alternating white/ black layers. These were much finer than the contact spacing so the rubber was effectively an insulator in one direction, along it's length, and a conductor in the other. Thus the contact was made between circuit board tracks and the conductive bits of the LCD without any alignment problems. The resistance of the conductive rubber can still be quite high as the currents involved in a LCD are absoutely tiny, I suspect so small as to be immeasurable.

The implication is that this magic rubber is more common than you might initially think and is probably being knocked out by the mile in some far eastern factory.

Hope this helps

John