The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: firstname.lastname@example.org (Byron A Jeff)
Subject: Re: Smallest component to step down 240VAC to 18VDC?
Date: 10 Oct 2002 07:13:44 -0400
Organization: College of Computing, Georgia Tech
In article <email@example.com>,
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?
- 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
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup