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Calling Electronic Design Experts

Community Veteran
Posts: 16,815
Thanks: 1,112
Fixes: 13
Registered: 06-11-2007

Calling Electronic Design Experts

Instead of buying a 12volt dc to 240v ac inverter for the car. to run my laptop, does anyone know if it is possible to construct a small circuit, that works like this.....
voltage doubler circuit (12v dc X 2 = 24v dc) then use dropping resistor circuit to reduce 24v dc to 19.5v dc....at 3.35 amps ? ? . thus powering the laptop without the inverter/conversion dc
If so, I would appreciate it if they could show me the circuit with the necessary values of components, so that I can make it .....
Unless of course there is already a "commercial product" on the market that already does this?
8 REPLIES
Community Veteran
Posts: 6,735
Thanks: 12
Registered: 02-02-2008

Re: Calling Electronic Design Experts

Why not just buy one?
e.g. http://www.maplin.co.uk/120w-laptop-car-adaptor-44736 for £20
itsme
Grafter
Posts: 5,924
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Registered: 07-04-2007

Re: Calling Electronic Design Experts

If you want to do it to save money you won't. If it is an exercise in electronics then I can point you if the right direction but because of the currents and power involve it's not a simple solution as it will have to be a switching power supply. What you have described is a linear concept and to drop 4.5 volts at 3..35amps will require a 15 watt resistor. These are very large, expensive and will require a heat sink to dissipate the 15w. http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=6150303
Community Veteran
Posts: 16,815
Thanks: 1,112
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Registered: 06-11-2007

Re: Calling Electronic Design Experts

As usual, it was "just a thought" that crossed my mind in an idle moment !  Roll eyes
HP sauce..... thanks for the link.... seems reasonable
itsme.... Hmm yes, I thought there might be something more to it, than a "simple circuit".... that is why I asked !  Undecided
Thanks guys
alanb
Grafter
Posts: 459
Registered: 24-05-2007

Re: Calling Electronic Design Experts

I'm not an expert by any means, and I think you'd be better off buying a suitable adaptor, so I'm posting this just in case it might interest you. The type of circuit that is used to increase voltages is known as a charge pump. You can read more here ... http://en.wikipedia.org/wiki/Charge_pump
Moderator
Moderator
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Registered: 14-04-2007

Re: Calling Electronic Design Experts

...and a zener would take care of the voltage drop although there are chips to do that these days.
It's a few years since I was building power supplies.
Customer and Forum Moderator.
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itsme
Grafter
Posts: 5,924
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Registered: 07-04-2007

Re: Calling Electronic Design Experts

You will struggle to find a zener that can handle the current and as for using a charge pump this also have problems as the size of the capacitors, both physically and electrically will be large.
I'm now intrigued to find out what the commercially brought ones use as I believe the only option because of the currents is to have a transformer based design.
pierre_pierre
Grafter
Posts: 19,757
Registered: 30-07-2007

Re: Calling Electronic Design Experts

Had a look at mine, still no wiser but it has a B big heat soak
input 11-14V 8 amp, output 15/16/18/19/20/22/24 @ 3.5a
22-24V DV 2.9A 70 W max
alanb
Grafter
Posts: 459
Registered: 24-05-2007

Re: Calling Electronic Design Experts

Quote from: itsme
I'm now intrigued to find out what the commercially brought ones use as I believe the only option because of the currents is to have a transformer based design.

You're probably right. If they used a high-frequency transformer it could be a lot smaller than a typical mains transformer too. It would need a chopper circuit, or an oscillator, and some meaty power transistors to make a rising and falling 12v signal to feed the transformer primary. I'm guessing here, but it might work better if the transformer had two primary windings, with opposed phases, to generate a true alternating magnetic field in the transformer.