Ericthelightweight Posted February 24, 2017 Report Share Posted February 24, 2017 Having picked up a copy of Bill Piggott's excellent 'Original Triumph TR' at the Stoneleigh spares day last weekend, I'm feeling all enthusiastic to don my anorak and bring my already quite original 3a even closer to its build spec. One thing I'm considering is to revert it to positive earth. Why would it have been changed to negative in the first place? I always though it was mostly to do with galvanic body corrosion but a quick internet search on the subject just brings even more confusion. Thoughts anyone? What would I need to change? Keith Quote Link to post Share on other sites
Ericthelightweight Posted February 24, 2017 Author Report Share Posted February 24, 2017 Hmmmm, two minutes after posting this I found another post proposing to go the other way, with most of the info I was looking for... Should have searched on 'negative' rather than 'positive' Quote Link to post Share on other sites
Lebro Posted February 24, 2017 Report Share Posted February 24, 2017 negative earth is universal these days, it is useful to convert to negative because it allows the use of alternators, sat navs etc. Bob. Quote Link to post Share on other sites
barkerwilliams Posted February 24, 2017 Report Share Posted February 24, 2017 Wasn't negative earth adopted because it has less corrosion of the body panels? Alan Quote Link to post Share on other sites
Lebro Posted February 25, 2017 Report Share Posted February 25, 2017 Yes I believe that was the original reason, but I also think that current thoughts are that it makes little difference. Bob. Quote Link to post Share on other sites
iain Posted February 25, 2017 Report Share Posted February 25, 2017 Yes I believe that was the original reason, but I also think that current thoughts are that it makes little difference. Bob. Nice pun Bob! Iain Quote Link to post Share on other sites
RobH Posted February 25, 2017 Report Share Posted February 25, 2017 Creeping off-topic I know, but I have often wondered why positive-earth was ever used - and why some manufacturers actually converted to it. My 1930 car is negative-earth but post-war cars from the same maker were positive-earth so there must have been a reason for making the change. I found this clipping which might explain why (about half way down the page): http://lajagclub.com/the-argument-for-positive-ground/ Quote Link to post Share on other sites
Ericthelightweight Posted February 25, 2017 Author Report Share Posted February 25, 2017 Wow... Well that's by far the best explanation I've seen so far. Before I posted the start of this thread I had typed something like: 'positive vs negative earth' into t'internet and found all sorts of conflicting statements, mostly from vintage tractor owners. In my case I was really just considering going down the originality route since it turns out (back to Bill Piggotts's book) that my car which I've had a couple of years now seems to be substantially as it left the factory, so there's not a great deal to do. So... going back to it, assuming no satnavs, electronic ignition etc., any pointers? It's still on a dynamo which seems to be able to cope with normal leisure use. Cheers Keith Quote Link to post Share on other sites
Lebro Posted February 25, 2017 Report Share Posted February 25, 2017 Creeping off-topic I know, but I have often wondered why positive-earth was ever used - and why some manufacturers actually converted to it. My 1930 car is negative-earth but post-war cars from the same maker were positive-earth so there must have been a reason for making the change. I found this clipping which might explain why (about half way down the page): http://lajagclub.com/the-argument-for-positive-ground/ Well it was going OK until he started talking about transistors only being "NPN", & only working with -ve earth cars - Nonsense. For a start early (Germaium) transistors tended to be PNP, & anyway either polarity of transistor can be used in circuits designed for -ve or +ve earth. Bob. Quote Link to post Share on other sites
RogerH Posted February 25, 2017 Report Share Posted February 25, 2017 Hi Bob, that's what I was thinking. In the early days getting hold of a decent NPN was difficult. The cars of the early 60's disintegrated into rust and I'm sure +ve polarity was blamed for this. Not the rubbish steel and no protection. Roger. Quote Link to post Share on other sites
Ericthelightweight Posted February 25, 2017 Author Report Share Posted February 25, 2017 Having worked in the motor industry from the early 70s and witnessed the introduction of modern e-coat systems, I quite agree with your little burst of irony Roger. Body protection is light years ahead of where it was when our TRs were built. The TR4a I had in about '74 was only about 8 years old but was already a basket case. Keith Quote Link to post Share on other sites
RobH Posted February 25, 2017 Report Share Posted February 25, 2017 Yes I ignored that little gem of duff gen as it didn't seem relevant. (Way off-topic and very geeky, but I still have a collection of very old transistors somewhere - and they are all pnp. Mullard OC16, Newmarket V30/10p that sort of thing; there is even one very early one still in its packet and labelled as a 'crystal triode'. I think the first npn transistor I encountered was an OC139 which you needed to make a complementary-pair amplifier output stage with an OC72.) If you don't believe what the chap says about positive-earth, what other explanation is there for a change from neg to pos in the '40s and '50s by at least one manufacturer? Quote Link to post Share on other sites
RogerH Posted February 25, 2017 Report Share Posted February 25, 2017 Hi Rob, I wonder if your old trannies still work !!! they had a habit of growing hairs on the inside of the can etc. How about this for polarity. Tyres have a fair carbon content and the car can earth through them (sort of). If the body was at 12V+ (or higher for HGV's) then there would be a leakage path from chassis to earth (that is planet Earth). Were early tyres less reliant on carbon. Roger Quote Link to post Share on other sites
stuart Posted February 25, 2017 Report Share Posted February 25, 2017 (edited) Hi Rob, I wonder if your old trannies still work !!! they had a habit of growing hairs on the inside of the can etc. How about this for polarity. Tyres have a fair carbon content and the car can earth through them (sort of). If the body was at 12V+ (or higher for HGV's) then there would be a leakage path from chassis to earth (that is planet Earth). Were early tyres less reliant on carbon. Roger Rubber in those days was a completely different mix to todays so probably. Stuart. Edited February 25, 2017 by stuart Quote Link to post Share on other sites
Lebro Posted February 25, 2017 Report Share Posted February 25, 2017 (edited) Sorry, can't accept that - there can only be leakage if there is a complete circuit. For current to leak from chassis to the road, the non chassis side of the 12V battery would have to be directly connected to the road. Clearly this is not usually the case. Bob. Edited February 25, 2017 by Lebro Quote Link to post Share on other sites
Paul Down Posted February 25, 2017 Report Share Posted February 25, 2017 Yes I ignored that little gem of duff gen as it didn't seem relevant. (Way off-topic and very geeky, but I still have a collection of very old transistors somewhere - and they are all pnp. Mullard OC16, Newmarket V30/10p that sort of thing; there is even one very early one still in its packet and labelled as a 'crystal triode'. I think the first npn transistor I encountered was an OC139 which you needed to make a complementary-pair amplifier output stage with an OC72.) If you don't believe what the chap says about positive-earth, what other explanation is there for a change from neg to pos in the '40s and '50s by at least one manufacturer? This takes me back to school boy days making transistor radios. The only reasonably priced audio transistors available then were "red spot PNP" I believe they were actually rejects. Costing 10 shillings ( 50p now) which in those days was a weeks pocket money. On one occasion after connecting one back to front the red spot changed to purple much to my horror and the transistor was no more. As radio/TV dealers had in the main been brought up with thermionic valves they were a bit wary of transistors. It was quite a profitable sideline mending them for them while still at school. Quote Link to post Share on other sites
RobH Posted February 25, 2017 Report Share Posted February 25, 2017 You have a point Roger but its difficult to see how an actual connection to the mass of earth will influence this - after all we only use the term 'earth' as shorthand for the return connection in what is really a self-contained circuit. A car could be suspended in mid-air and there would be no effect on galvanic erosion process which does not depend on an external voltage but forms because of the connection of dissimilar metals in the presence of a conducting fluid. Its that process which destroys unprotected areas of bodywork and chassis of course. I think the reason for going positive-earth may be related to electrolysis which requires an external power source as as in the electro-plating process. This is not galvanic erosion. Considering the 'leaky and damp' cotton-covered wiring described in the link above, having the wires positive and the chassis negative would have resulted in erosion of the copper wires by plating material to the steel. Changing to positive earth would prevent that happening and protect the wires, perhaps at the expense of the bodywork. Does that sound plausible? Quote Link to post Share on other sites
RobH Posted February 25, 2017 Report Share Posted February 25, 2017 Paul - yes the old red-spot, white-spot things were production line rejects for sure, as were the MAT gold coloured transistors and other bits that Clive Sinclair use to sell. Remember the 'power' amplifier Super IC 10 with the finned heatsink? The real full-spec deal was a Plessey part. Ah the days when Henry's Radio was Shangri-la. Nostalgia. Quote Link to post Share on other sites
RogerH Posted February 25, 2017 Report Share Posted February 25, 2017 Blimey Henry's radio - did they have a shop in Tottenham court road. Roger Quote Link to post Share on other sites
RobH Posted February 25, 2017 Report Share Posted February 25, 2017 Yep thats the place Roger. Spent many hours in there back in the day.... As apprentices we occasionally used to get bussed up to the big trade exhibitions at Earls Court and the like - but you didn't stay in there long before sloping off to Henry's or the other similar outfits in the Edgware Road. Quote Link to post Share on other sites
barkerwilliams Posted February 25, 2017 Report Share Posted February 25, 2017 OK, while you are on the subject does anyone know why are car bodies still connected to either terminal anyway? Most car electrical bits these days have both a negative and positive wire, so why use the body anyway? Why not just keep it insulated? Years ago cheapskate manufacturers missed out the return circuit and just hung off the nearest bit of bodywork (bare metal anyway). But now electronics needs a better supply so twin wire is the name of the game - and the bodywork is usually painted these days. Alan Quote Link to post Share on other sites
ed_h Posted February 25, 2017 Report Share Posted February 25, 2017 (edited) The wet fabric insulation theory is the best one I've heard on this question. At least it has a plausible scientific basis. As to why use the body as a conductor at all, I think cost would be enough motivation. And later, weight. Ed Edited February 25, 2017 by ed_h Quote Link to post Share on other sites
stillp Posted February 25, 2017 Report Share Posted February 25, 2017 The leaky insulation theory falls down when you consider that whatever the polarity, the voltage difference across the insulation is the same. Pete Quote Link to post Share on other sites
ed_h Posted February 25, 2017 Report Share Posted February 25, 2017 Yes, but there is a big electrochemical difference. It's analogous to electroplating. Reverse the polarity, and you get way different results. Ed Quote Link to post Share on other sites
Lebro Posted February 25, 2017 Report Share Posted February 25, 2017 I used to go to a shop by the Kingston Apple market. Can't remember it's name. Could have been Watts Radio ? Bob. Quote Link to post Share on other sites
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