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In Car Laptop PSU - May 2004

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Heloman
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Registered: 03-2006

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Posted on Monday, 08 May, 2006 - 05:10 pm:   Edit Post Delete Post Print Post

I am building this item now since the need just came up, but I was wondering why the author of the article repeatedly reminds us to NOT USE the unit with the car running? Is it a design issue that he did not explain or is an obscure safety related issue
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John_becker
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Posted on Tuesday, 09 May, 2006 - 12:31 am:   Edit Post Delete Post Print Post

I imagine it's those dirty power line spikes on car electrics that've been discussed in another thread, suggesting a not-too-good PSU etc on the design.

J
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Obiwan
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Posted on Tuesday, 09 May, 2006 - 06:44 pm:   Edit Post Delete Post Print Post

John is right. He doesn't want to be responsible for a fried laptop! It may very well work, but by saying that, he's covering um, his end of it. If you use it with the car running and a spike comes through, it's your fault, not his.

So if you build it, I would suggest beefing up the protection part of it.

I have a small inverter that plugs into the lighter (now the power port) and works quite well. I've also got a much larger inverter sitting under the passanger seat for other devices, and works out quite well also!
Do Not Hit The Fly That Lands On The Tigers Head.
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Heloman
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Posted on Wednesday, 10 May, 2006 - 01:57 pm:   Edit Post Delete Post Print Post

Thanks for the replies. I have a small inverter and I could plug the existing "power brick" into that , but it seemed to me worthwhile eliminating one of the conversion processes from an efficiency perspective

Can annyone suggest an alternative input protection that will react fast enough if there is a spike coming through? For example sort of "high speed" filter for anything any inputs higher that 14V. Thanks again.
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Obiwan
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Posted on Wednesday, 10 May, 2006 - 06:45 pm:   Edit Post Delete Post Print Post

You should start with filters, caps and inductors. After that, there's a lot of choices out there.

If it were me, I'd employ several devices, just as I would several types of capacitors.

With the caps, you're looking for different qualities in several of them. By putting them in parallel, you get the best of all of them.

Same with the protection. I doubt you'll find one device that works just as you want it to, so use several.

Probably not more than three. You're looking for speed and power.

If you have an electronics catalog, just browse through it and see protection circuits they offer, and look at their qualities and see what you can see. (for one, you learn a lot this way)

I don't have anything right off the top of my head, I'd have to look through the catalog also, new devices come out all the time.

But filtering is primary. You should be able to add these devices to your project without taking up much room at all.
Do Not Hit The Fly That Lands On The Tigers Head.
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Petelobus
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Posted on Thursday, 11 May, 2006 - 08:47 am:   Edit Post Delete Post Print Post

I'm along with Obi on what he says, but, at the risk of repeating stuff in other threads, remember that, with the engine running the battery voltage can rise to nearly 15v continuously, and, at starting, fall to 8 or 9v. This can really screw up regulator dissipation calcs if you don't take it into account. Can't remember if the project was linear or switch mode. Its much more of a problem with linear supplies.

Ian
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Mark
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Posted on Thursday, 11 May, 2006 - 09:09 am:   Edit Post Delete Post Print Post

I suggest 4 things to protect the circuit in an automotive environment:

A fuse! close to the converter before the protection circuitry. That will limit the duration of any high fault current. I think a standard 5Amp 5x20mm would be OK.

A Transorb - 33V bi-directional straight across the incoming power after the fuse. That takes out high speed high voltage spikes that might come from inductive load switching.

A capacitor - low ESR 2200uF 25V should be OK.

A Zener diode - 18V with a decent power rating
5 Watts or more should be tough enough. It should survive long enough for the fuse to blow if somebody tries a two battery jump start! or disconnects the battery earth strap while the engine is running (it happens!)
It also protects the whole circuit from reverse polarity (again in conjunction with the fuse!)

Old fashioned Zener diodes and fuses are really effective.

Mark
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Obiwan
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Posted on Thursday, 11 May, 2006 - 07:26 pm:   Edit Post Delete Post Print Post

And remember that fuses have time ratings.

So even though have you have say a 1A fuse, you can apply 1A to it, and it will stay up for some amout of time.

So you need to make sure that your protection circuits will be able to absorb the transit as long as the fuse hangs in there.
Do Not Hit The Fly That Lands On The Tigers Head.

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