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u r right please proceed with utmost care

--- In Electronics_101@..., "Bene Tam" <bene-tam@d...>
wrote:

hi all

I am a firmware engineer, recently my power adapter for the laptop

is broken

Although I am going to buy a new one out there, i would like to

find out how to build a simple substitute.

There should be 3 stage, right?? AC-AC, then AC-DC, then DC-DC

stage 1, use transformer, say turn ratio 5:1 to lower the voltage

to 18VAC-48VAC, right?

stage 2, use bridge rectifier to convert the AC to DC, the outcome

will be 24-68VDC, right??

but is there any combined alternative for stage 1 and 2?? seems

some sort of switching technique out there?!

stage 3, use some sort of IC, just those from national semi. or

other MOS switching circuit to convert that to exactly 20VDC3A
output, right?? this stage is the most difficult to me, anyone can
tell me more detail??

Thanks much

Check your current requirements and your voltage requirements.

Most likly, the laptop will run on 12 and 5 VDC like home machines
and possibly, 5 and 3.5 v.

Since you also, PROBABLY do not need anything more than 12 VDC, don't
generate 24VDC on your second stage. Using a LM7812 and LM7805 will
drop the voltage to 12VDC and 5VDC regulated, but will shed a LOT of
heat.

A dual transformer to get 6 and 12 volts will offer a much lower
voltage to start with.

A simple power supply is just AC to AC via the transformers like you
mentioned.

Then a bridge rectifier or 4 diodes arranged in a bridge. This
yields DC voltage.

Add to that a capacitor that is rated for double the voltage you
expect. Search this list for "Mariss" or "80,000" and find the post
that referrs to how to size the capacitor.

Then after cap, run the DC voltge into a voltge regulator such as a
LM7812 to get 12 VDC regulated.

Check the specs for amperage for the voltage regulator(s).

None of this is rocket science and is a good E-101 project.

Dave

--- In Electronics_101@..., "Dave Mucha" <dave_mucha@y...>
wrote:
...

Check your current requirements and your voltage requirements.

Most likly, the laptop will run on 12 and 5 VDC like home machines
and possibly, 5 and 3.5 v.

Dell and KDS laptops require about 20V at 3A. He's not talking about
the internal voltages, he needs the one that charges the battery.

My KDS power supply was intermittent when I bought it. Computers4Sure
told me tough, warranty service by KDS, and KDS took 2 months to
respond. I had already gone to Boeing Surplus (Kent, WA) and found the
Dell laptop supplies were within 10% of the same rating, just had to
put a different connector on it.

Alien Steve

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That's called a buck-boost regulator.

Check and and


Alien Steve

--- In Electronics_101@..., "Bene Tam" <bene-tam@d...> wrote:

Thanks all man

Yes, I am saying that the old one has a rating of 20VDC 3A.

How can I generate the 20VDC 3A then?? I see 2 difficulties

If I use a turn ratio of 10:1, after the bridge rectifier, the DC

supply would range from 12VDC(9VAC) to 33VDC(24VAC), how can I get a
20VDC3A with promise??(since 12V is step-up, other is step down)

Can the switching regulating IC handle both step-up and down under a

fixed configuration?

Or I should use a turn ratio of 5:1, then we have 25V(18VAC) to

68V(48VAC) at stage 2, then use step-down on stage 3? but how to do
that at stage 3?? seems no single IC can do so?!

Do i have to design my own PWM-controlled circuit for the 20VDC3A

output?

Thanks for advice

i have the nice stuff for u .use the bridge rectifier?IC BR68 which has a current rating of around 3A.. and a regulator IC LM323 which also has a xcapability of handling around 4.5A so why are u waiting for

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No that's not quite right. The supply you had is not especially easy
to recreate. It is cheaper to buy than design in this case.

The original supply was a standard offline switching supply that
outputs 20V at 3A. It was probably designed for low THD (a.k.a. high
power factor or power factor corrected).

The stages are rectify the incoming AC then regulate the DC output
with a flyback regulator. The AC voltage is first rectified; there is
no input transformer. There is probably some circuitry that doubles
the input voltage if it is between 150VAC to 95VAC and does not double
it if it is higher. The high voltage DC powers a flyback regulator
which regulates the output to 20V. It also provides isolation from the
mains to 2500V or so.

The tricky part is getting the right magnetics for the flyback
regulator and making everything stable.

You probably should not attempt this if you are not already familiar
with offline switching supplies. Try reading the section in The Art of
Electronics about off line switching supplies.

You can cobble something together that will work but you will need to
switch voltage ranges depending on whether you are operating from
120VAC or 230VAC.

--- In Electronics_101@..., "Bene Tam" <bene-tam@d...> wrote:

hi all

I am a firmware engineer, recently my power adapter for the laptop

is broken

Although I am going to buy a new one out there, i would like to find

out how to build a simple substitute.

There should be 3 stage, right?? AC-AC, then AC-DC, then DC-DC

stage 1, use transformer, say turn ratio 5:1 to lower the voltage to

18VAC-48VAC, right?

stage 2, use bridge rectifier to convert the AC to DC, the outcome

will be 24-68VDC, right??

but is there any combined alternative for stage 1 and 2?? seems some

sort of switching technique out there?!

stage 3, use some sort of IC, just those from national semi. or

other MOS switching circuit to convert that to exactly 20VDC3A output,
right?? this stage is the most difficult to me, anyone can tell me
more detail??

Thanks much