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From: firstname.lastname@example.org (N. Thornton)
Subject: Re: Smallest component to step down 240VAC to 18VDC?
Date: 14 Oct 2002 05:57:04 -0700
NNTP-Posting-Date: 14 Oct 2002 12:57:05 GMT
LLA have you considered using a lower current supply, such as a 16v
0.5A supply? That would be perfectly practical to carry. The downside
is youd need to leave it plugged in 9x as long: ie pretty much all the
time. But I can think of scenarios where that wouldn't be a problem,
hence the thought. And it'd cost you 10 bucks.
email@example.com (Byron A Jeff) wrote in message news:...
> In article <firstname.lastname@example.org>,
> The little lost angel wrote:
> Since everyone is now finished telling LLA how bad an idea a transformerless
> PSU is, let's get back to the issue.
> - Are transformers with a AC->DC circuit the only way to step
> -down mains AC to low DC voltages?
> No. You can use a switcher to step the voltage down. BTW transformers only
> transform AC to AC. A bridge rectifier does the AC to DC conversion.
> - Are there no small IC that can
> -convert 240V to something like 16V to 24V range?
> Most switching ICs have a much lower input voltage. You'll probably have to
> do it with discretes.
> - Could I do something like
> -1) AC to DC rectifier(? or converter or whatever?) with four diodes
> -(IIRC from high school)
> Yes. And actually this can help you. I agree that you can't get away without
> a transformer. However you can get you magnetics much smaller by changing your
> switching frequency. This item is usually called a flyback transformer and
> it operates at a much high frequency than the mains. Because the frequency goes
> way up (50-100 Khz) the transformer size goes way down.
> -2) then split it (a la Kirchoff/Resistive divider) across say 6
> -LM117HV (60Vin, adj Vout, 1.5A) to put a nominal 40Vin to each,
> Won't work. The Vin of LM117 is only 40 volts. Linear power dissapation isn't
> the correct path anyway. There's just too much power to contend with and
> linear power supplies convert it all to heat. I thought you wanted a power
> supply, not a space heater!
> Plus you haven't even considered the power you want to get out.
> -3) then tie the six 16Vout outputs back to a single output connector?
> Poor approach.
> - Reason being... I lost my 16V 4.5A laptop charger and need to
> -make one small enough to pack in my bag. :(
> One of the reason's it's small enough is because it's a switcher like I've been
> describing. The other is because it probably is completely surface mounted
> which presents a challenge for a hobbyist.
> There's nothing simple about the project. Which is why the laptop manufacturer
> can charge $100-$150 USD for replacement power supplies.
> For more information about what I'm talking about, check out this article:
> You have to have a transformer. However you can control the size of frequency
> of it.
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