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Author Topic: Car Adapters  (Read 2561 times)

Colin Bishop

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Car Adapters
« on: November 02, 2006, 05:26:33 PM »

Can one of you electronic wizards out there set my mind at rest please?

I have just bought a Garmin Nuvi 350 GPS navigator for the car. It's an impressive piece of kit and now I can get to and from the shops safely and listen to the bongs every time I get near a camera.

Like all these gadgets it comes with an adapter to plug into the car's cigar lighter socket. This means running untidy wires across the dashboard if you don't want to rely on the internal battery. I therefore bought a car adapter extension lead and, with the cunning of the true modeller, have managed to tuck almost everything out of sight which looks much neater. The Garmin adapter plugs into the extension lead under the front seat and the other end is wired into the cigar lighter supply under the centre console.

The cigar lighter supply is 12v and is always on. Judging by the AC adapter supplied with the GPS, the Garmin car adapter probably has some means of stepping the voltage down to 5v. It also has a pretty green LED on it and a fuse inside. What I'd like to know is what method is used to step down the voltage (just a resistor?) and whether I can just leave it in a live condition when I unplug and remove the GPS itself. Alternatively, should I fit a switch in the 12v section to isolate the Garmin adapter? Hope this makes sense!

Thanks in anticipation.

Colin
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boatmadman

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Re: Car Adapters
« Reply #1 on: November 03, 2006, 04:24:18 AM »

I cant be certain, but I suspect the adaptor will have a small transformer to step down the voltage.

You can leave it on all the time, the current draw of a small transformer isnt enough to affect your battery (but what do I know!) ;)
Ian
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Doc

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Re: Car Adapters
« Reply #2 on: November 03, 2006, 05:09:17 AM »

Colin,
There are several methods for reducing voltage in these kind of adaptors, no idea which yours may use.  In general, leaving the thing 'on' after moving the GPS shouldn't make much (if any) difference to your car's battery.  A switch shouldn't be necessary, but if you just want to, then why not.  Just remember to turn it on, you know?
 - 'Doc
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OMK

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Re: Car Adapters
« Reply #3 on: November 03, 2006, 05:28:17 AM »

Colin, the transformer idea, suggested in the above post, is most likely is a no-go, since transformers only work on alternating currents.
If, as you suggested, a resistor is used, that wouldn't work neither, since all the resistor does is reduce the flow of current - not voltage. My guess would be that the actual stepping-down of the voltage is carried out inside the navigator itself... usually in the form of an on-board (+5v) regulator.
I could be out of my depth here, but my take is that your 12v adaptor is just a straight-through 12v-to-12v connector. If you have access to a multi-meter, try measuring the output voltage -- it's possible the output will be at the same potential as the input.
The only resistor inside the adaptor would be the one limiting the current to that green LED.
Leaving the adaptor 'live' should cause no harm at all.

If, on the other hand, the output of your adaptor *is* lower than the the input, then it would appear that the regulator is inside the adaptor -- not the navigator. If this is the case, when you remove the nav' from the car, the regulator gubbins inside the adaptor would only be pulling a few measly milliamps anyway.... hardly enough to threaten premature discharge of your car battery.

Either way, I would say that it's safe if you need to leave it permanently plugged in.
However, I'd take a couple voltage readings with the multi-meter first, just to be certain.
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Colin Bishop

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Re: Car Adapters
« Reply #4 on: November 03, 2006, 09:24:41 AM »

Thanks guys, that's generally reassuring. I have considered the multimeter but the output end is a mini USB connector and you can hardly see the connectors, let alone get a probe on them! The step down may be in the GPS itself but the other two charging methods, Computer USB and mains adapter, both output 5v. The car adapter can accept 12-24v which is what got me wondering a bit. The current draw is not likely to dent the car battery if the vehicle is in use every day and I can always unplug the connector if I'm going on holiday. (I usually leave a solar panel on the rear parcel shelf connected to the cigar lighter socket if I leave it at the airport anyway)
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Doc

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Re: Car Adapters
« Reply #5 on: November 04, 2006, 01:29:16 PM »

PMK,
" the transformer idea, suggested in the above post, is most likely is a no-go, since transformers only work on alternating currents ", sorry, certainly didn't mean to imply the use of a transformer, even though transformers can be used in DC circuits (pulsing DC circuits).  They certainly aren't used often in low cost devices since transformers are not exactly low cost devices themselves, and the circuits you find them in are sort of sophisticated thingys (most 'high-end' mobile radios for instance). 
Resistors are used in voltage dropping cicuits commonly called 'voltage dividers'.  Not something that's the 'best' answer in limited supply situations since a fair amount of power is lost (heat) with them.  If the voltage/current supply isn't limited (battery powered) that's usually what the voltage control knob is attached to (variable resistor, as in a mechanical speed controler for instance).
Your statement is generally correct, just be careful when you take it to the 'bank'!  It might be a withdrawal instead of a deposit...
 - 'Doc
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wombat

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Re: Car Adapters
« Reply #6 on: November 04, 2006, 01:40:26 PM »

I suspect that the item will be a small switching supply - you can build these for very little money and they give a few amps with a reasonably high efficiency - certainly more efficient than a linear regulator. Though I would suggest that it is unplugged when not in use.

Tim the Wombat
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cbr900

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Re: Car Adapters
« Reply #7 on: November 04, 2006, 02:12:26 PM »

Most of the cars I have owned over here when you turn the key off, everything except the clock is cut, seems the most logical idea to me, at least then left in the socket or not will not flatten anything, (keep the GPS lose the car) ;D ;D ;D




Roy
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wombat

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Re: Car Adapters
« Reply #8 on: November 04, 2006, 02:21:04 PM »

It depends on the car I think Roy

My VW leaves the cigarette lighter live with the ignition off, IIRC SWMBO's Peugeot deactivates it.

Tim the Wombat
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Colin Bishop

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Re: Car Adapters
« Reply #9 on: November 04, 2006, 03:49:44 PM »

Well, I've just fitted an illuminated switch in the centre console so that should fix it one way or another! I've had previous cars where the cigar lighter turned off with the ignition but it doesn't on my Mondeo. Thanks to all for your advice/comments.
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OMK

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Re: Car Adapters
« Reply #10 on: November 08, 2006, 12:22:45 AM »

Bish':
You mean to say you KNEW the solution to the problem all along?.. yet you made us spout all this drivel for nothing <wink!>?
Sadist!

Doc:
I *knew* that line about the trnsfrmr would throw a spanner in the works.
I hit the send button before realising that you got in there first. Meaning, when I mentioned "...suggested in the above post", I meant the OTHER above post - the one from Boatmadman.
It's your fault - we must have both been typing replies at the same, only you snuk in there first. I thought to myself: "Hmm... sequence schqweekance... I bet ol' Doc will think 'above post' means him.".
Anyhow, I'm hearing you. I know you didn't imply any transformer.
I'm quite clever like that.

The bank? Where I live, withdrawal is spelt withdrawl and pronounced: "this is a stick-up!".

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Doc

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Re: Car Adapters
« Reply #11 on: November 08, 2006, 11:55:52 AM »

PMK,
Oh well, like I never made a mistake before?  As for the banks here, you go in on your knees, unless you happen to be carrying a large gun.  Very difficult to rob a bank here!  People keep applauding and patting you on your back...
 - 'Doc
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Colin Bishop

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Re: Car Adapters
« Reply #12 on: November 08, 2006, 12:51:18 PM »

No, I didn't know the answer but you lot placed enough doubt in my mind that I thought I'd better put the switch in. Took a couple of hours.  ;)
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