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Author Topic: Linear regulated laptop supply  (Read 4073 times)
KK7UV
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Posts: 23




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« on: May 05, 2012, 07:02:10 AM »

I'd thought about building a linear regulated supply for my laptop to use at my radio desk, and save the switchmode supply for use away from the desk.   It seems simple to do using the LM338K TO-3 on a heat sink.  Input will be a 120:24v transformer.   Output for laptop is 19v at 3.4A.   The laptop power jack is a simple 2-wire coaxial affair.

Anyone tried something similar?   My only concern is whether the switchmode supply that came with the laptop has any sort of "smart" charging feature for the laptop battery using current or voltage sensing.  The labeling does not indicate so. 
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W8JX
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Posts: 6664




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« Reply #1 on: May 05, 2012, 11:58:16 AM »

I would tend to advise against this. Laptops are very sensitive to charger type and voltage and the supply also acts to limit peak charge rates as well. Just mail order another one on line. You can get them pretty cheap direct form Hong Kong or China. Last HP ones I bought were OEM and they were like 17 bucks shipped from China.
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You can embrace new computer/tablet technology and change with it or cling to old fall far behind....
KK7UV
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Posts: 23




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« Reply #2 on: May 05, 2012, 01:30:17 PM »

I can certainly buy a second switchmode supply for the desk, but the idea is to eliminate the horrible RFI it creates, which is why I was thinking of a linear regulated supply to replace it at the operating desk.

Are you saying that the coaxial (two-wire, plus and minus) type supplies are capable of "smart" charging like the three-wire systems?

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W8JX
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« Reply #3 on: May 05, 2012, 01:46:20 PM »

What I am saying is that I have found some laptop for be Very sensitive to voltage and current capacity of adapter. I do not understand what all the RFI fuss is about with switches. I am not saying it is not possible but I have got a LOT of them here powering various devices and i have yet to hear the first RFI from any of them. I live in country and is pretty RFI quiet here too so I think i would hear it.
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You can embrace new computer/tablet technology and change with it or cling to old fall far behind....
AA4PB
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Posts: 13032




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« Reply #4 on: May 05, 2012, 02:04:58 PM »

I would think that if your supply is a single voltage with only two wires then the battery charge controller is built into the laptop. In that case, a linear supply with the same voltage and at least as much current should work okay. I'd probably put in a crowbar circuit to protect the laptop from over-voltage in the event of a pass transistor failure.

The RFI issue depends a lot on your antenna. If its located well away from the house and you don't have common mode currents on the coax shield then you probably don't hear RFI from things in the house or shack. I don't hear any RFI, but my tower is located 100-feet from the house with feed lines running underground.
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N3QE
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Posts: 2422




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« Reply #5 on: May 06, 2012, 10:47:57 AM »

You might just try adding the filtering that the laptop and PS manufacturer should have put in to begin with.

At the very least, common mode ferrite chokes right at the power supply AC in and DC out, and maybe another one right at the laptop DC in.

Your laptop almost certainly has DC/DC converters inside it and keeping that noise inside the laptop could be more important than filtering out the noise coming from the external PS.

Tim.
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