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Old 26th November 2012, 09:28 AM   #1
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Default Linear PSU for laptop

I need advice to make a linear PSU for laptop 19.5V 4.7A, suggestion will welcome.
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Old 26th November 2012, 10:10 AM   #2
dangus is offline dangus  Canada
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Why? The laptop has switching regulators and stuff inside, so it's unlikely to make any audible difference.
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Old 26th November 2012, 10:15 AM   #3
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My charger do noise so it's the reason to make a dedicate linear PSU.
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Old 26th November 2012, 10:29 AM   #4
Salas is online now Salas  Greece
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Google for LM338K circuits.
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Old 26th November 2012, 10:29 AM   #5
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Thanks Salas
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Old 26th November 2012, 01:45 PM   #6
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My Samsung laptop is noisy with SMPS and quiet when run on battery. And: it is also quiet when I run the SMPS charger through an isolation transformer -just for testing. I have a ground loop. Do you have a bench supply? You could try with that and make an earth connection to test for a ground loop.
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Old 26th November 2012, 02:27 PM   #7
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My Vaio is the same like your Samsung: quiet when run on battery and noisy with SMPS charger. I didn't tried with isolation transformer. My bench PSU can't supply 5A.
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Old 26th November 2012, 04:03 PM   #8
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The first thing i would do is determine if it is a regulated or unregulated power supply. Measure the output voltage of the power supply when it is unplugged from the laptop.
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Old 26th November 2012, 04:05 PM   #9
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Is DC.
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Old 26th November 2012, 04:36 PM   #10
gmarsh is offline gmarsh  Canada
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Your problem here is a ground loop, I suspect.

Laptops are especially terrible at creating ground loops when they're plugged in. Usually the negative output of the adapter is connected to AC earth ground, and at the laptop itself the negative terminal is connected to the laptop's ground - which in turn is connected to USB grounds, serial port grounds, cable shields, audio ground, etc.

So when the laptop pulls current from the adapter, the resistance of the DC cable and connector end up making the laptop's ground voltage float a few hundred mV above AC earth. Plug another earth-grounded device into the laptop and you create a different return path for the DC power... and a hell of a mess.

There's a few ways to fix this:

- Isolation transformer. Lots of isolation transformers will isolate hot/netural but keep ground connected from input to output - make sure that it's disconnected also.
- Find another 19V laptop adapter with a similar current rating, but with a 2 prong plug. Lots of Dells use these power adapters, but have a custom plug that you'll probably have to cut off and replace with one that matches your laptop.
- Horrible idea I'll probably get told off for suggesting: install a GFI outlet to plug the laptop into, make a big "DANGEROUS DEVICE WHICH VIOLATES LOTS OF CODES, ONLY PLUG THIS INTO A GFI OR YOU MIGHT DIE" sticker for the laptop adapter, and disconnect the AC ground on the adapter by some means (modify the power cord, etc).
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