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spook51
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Joined: Mar 26, 2004
Posts: 548
Location: East Midlands
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 Posted: Mon May 31, 2004 12:21 pm Post subject: Polarity Converter |
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| One of my cars is a 1954 Riley with positive earth. I'm heading off to Le Mans in it next week and would like to use my GPS (Haicom 204S + Axim X3i and TTN3). Any idea where I can obtain a polarity converter to provide a negative earth supply to a cigarette socket? |
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DavidW
Pocket GPS Moderator
Joined: 17/05/2003 02:26:21
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Location: Bedfordshire, UK
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| Posted: Mon May 31, 2004 6:40 pm Post subject: |
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Whilst I'm no expert on positive earth vehicles, so long as your Riley is 12V, can anyone see any problems if you wire an insulated cigarette lighter socket (available from Maplin for around a pound) with the centre positive and the outside negative?
The only thing that comes to mind is that in a positive earth vehicle, you probably want to fuse the negative line rather than the positive (3A is likely more than enough).
Only one thing - don't hold me responsible if you blow your system up.
David |
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spook51
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Joined: Mar 26, 2004
Posts: 548
Location: East Midlands
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| Posted: Mon May 31, 2004 10:36 pm Post subject: |
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I wondered about that but hoped to hear from someone who has actually done it. I don't really want to risk frying a few hundred quids worth of PDA and GPS kit, not to mention the car which still has its original wiring.
Thanks for the suggestion however I think for this trip I might rely on my wife's navigation - probably end up in Poland..... |
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Scott
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Joined: 17/11/2002 23:42:58
Posts: 66
Location: United Kingdom
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| Posted: Mon May 31, 2004 11:51 pm Post subject: |
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| DavidW wrote: | Whilst I'm no expert on positive earth vehicles, so long as your Riley is 12V, can anyone see any problems if you wire an insulated cigarette lighter socket (available from Maplin for around a pound) with the centre positive and the outside negative?
David |
You'll get a nasty short condition if any exposed 'earth' connections on the PDA/GPS come into contact with the car chassis. |
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DavidW
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Joined: 17/05/2003 02:26:21
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Location: Bedfordshire, UK
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| Posted: Tue Jun 01, 2004 12:10 am Post subject: |
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Whilst I'm not familiar with the hardware the original poster has, most Pocket PCs, even if they look metallic, are in a plastic case.
If so, I don't think there'll be a problem - but I can't guarantee it.
David |
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icsys
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Location: South Lancashire, UK
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| Posted: Thu Jun 03, 2004 11:10 pm Post subject: |
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I'm no auto-electrician but wiring a cig lighter socket into a positive earth vehicle should not be a problem and work fine. Simply put the positive wire to any metal point and take a negative from the nearest available point (as you would a positive on normal vehicles)
You should always fuse the live feed and never the earth.
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iPAQ 2210 | Navman 4100 BT Receiver
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DavidW
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Location: Bedfordshire, UK
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| Posted: Fri Jun 04, 2004 10:53 am Post subject: |
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I think we have some agreement on this - that so long as you remember to fuse the negative (not the positive as you would on a negative earth vehicle), you should be fine - assuming that the Riley's electrics are 12V and not excessively noisy.
David |
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icsys
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| Posted: Fri Jun 04, 2004 8:01 pm Post subject: |
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I think we know what we are saying David but the wording makes it look confusing. 
Perhaps change the word 'earth' at the end of my last post to 'negative' then delete this sentence. (it will give you some more editing practice at least  )
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Ian.
iPAQ 2210 | Navman 4100 BT Receiver
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spook51
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Joined: Mar 26, 2004
Posts: 548
Location: East Midlands
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| Posted: Sat Jun 05, 2004 7:11 am Post subject: |
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Thanks for your deliberations however I think a PDA and GPS are a bit too expensive too risk so I've decided to opt for a less reliable method of navigation - wife + road atlas...  |
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alix776
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Joined: 03/05/2003 14:45:49
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Location: leyland lancs ENGLAND
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| Posted: Sat Jun 05, 2004 9:28 am Post subject: |
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why not wire the lighter straight off the battery pos to pos neg to neg the would give a negative earth
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currently using aponia truck navigation on windows phone. Good bye IOS don't let the door hit you on the way out .
Oh the joys of being a courier.
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DavidW
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Location: Bedfordshire, UK
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| Posted: Sun Jun 06, 2004 3:23 pm Post subject: |
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| icsys wrote: | I think we know what we are saying David but the wording makes it look confusing.
Perhaps change the word 'earth' at the end of my last post to 'negative' then delete this sentence. (it will give you some more editing practice at least ) |
I could do it that way - then someone will come along and read that instruction as being valid in a negative earth vehicle. I think it's clear enough as it stands, now.
The point is - you fuse the feed, not the earth. In a negative earth vehicle, the fuse goes in the positive line. In a positive earth vehicle, the fuse goes in the negative line. If you're paranoid, you could fuse both (some Nokia phone wiring looms are shown as being wired this way - possibly because they may be installed on a 12V supply from an isolated convertor on a 24V vehicle, where neither negative nor positive is earth).
David |
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icsys
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Location: South Lancashire, UK
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| Posted: Sun Jun 06, 2004 10:13 pm Post subject: |
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| DavidW wrote: | | The point is - you fuse the feed, not the earth. In a negative earth vehicle, the fuse goes in the positive line. In a positive earth vehicle, the fuse goes in the negative line. |
This is exactly my point about the word 'earth' David,
You fuse the feed (+ve), not the earth. In a negative earth vehicle, the fuse goes in the positive line. In a positive earth vehicle, the fuse goes in the earth line because it will be positive and is providing the feed.
Pwhew I think we should call it a day on this one now.
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MrT
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| Posted: Mon Jun 07, 2004 5:36 pm Post subject: |
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| icsys wrote: |
You fuse the feed (+ve), not the earth. In a negative earth vehicle, the fuse goes in the positive line. In a positive earth vehicle, the fuse goes in the earth line because it will be positive and is providing the feed. |
I disagree with the above, the fuse should always go in the line that is not earthed to the body of the car.
The logic behind this is that if the non earthed wire touches the earthed body, the fuse blows.
If you fuse the earthed supply to a device and the non earthed feed touches any part of the earthed body there is no protection, the current will just flow back through the earthed body and not through the earthed fuse.
If a supply is floating you may need to fuse both supply leads.
A wire fuse does not care which direction a current flows through it. |
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dastardly
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Joined: Apr 11, 2004
Posts: 26
Location: Kent - UK
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| Posted: Mon Jun 07, 2004 9:39 pm Post subject: |
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| MrT wrote: | | I disagree with the above, the fuse should always go in the line that is not earthed to the body of the car. |
Absolutely, that's the heart of the issue  .... and so on a positive earth car, you still fuse the 'live' feed (It just happens to be negative because the battery terminals are reversed compared to modern cars).
So far no worries, just remember that when you connect up your PPC - Positive goes to chassis and negative goes to live.
In all honesty though if it was me, I'd get hold of a 12v 6Ah rechageable jelly cell - they're pretty cheap, and then I wouldn't have to rely on 1954 car electrics: ie, dynamo & voltage regulator (no disrespect meant to the lovely car) 
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spook51
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Joined: Mar 26, 2004
Posts: 548
Location: East Midlands
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| Posted: Tue Jun 08, 2004 6:27 am Post subject: |
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Phew! That opened a can of worms - ever wish you'd never asked the question?
Thanks for all your responses. The car has a new battery and I fitted new brushes in the dynamo yesterday. Other than a recent coil, distributor and HT leads all of the rest of the electrical system is fifty years old - hence my desire not to take any chances.
I've shelved the idea for now but may try it later. Fortunately we are travelling to Le Mans in a group so I don't have to rely entirely on my wife's navigating.
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