On 3/16/2019 12:23 PM, Gary Hinson wrote:
You could check laptops and their power supplies in a friendly laptop shop
(when they are not too busy!), using a portable AM radio.
Naw -- virtually place that sells or repairs stuff, or where humans
live, is almost certain to be so full of noise that noise from one
device is unlikely stand out.
Alternatively, you could simply banish switchmode supplies of all types ...
so either get/build a conventional PSU or find a laptop that will work from
the shack 13.8V supply. Many seem to be rated for 18V or more
Yes, that's been true for nearly a decade.
but I suspect
they might work at 13.8V although their batteries may not charge (don't
quote me on that: you'd need to check for yourself).
The last ones I owned that fall into this category were from the
Thinkpad T41-T42-T43-T44 series. And they do work that way. Beginning
with the T60 models, introduced at least 10 years ago, they went to 20V
power supplies and don't run on 13.8V.
Hinson tip: look out for cardboard boxes of PSUs in charity shops. Either
pick out the bulky/heavy ones, or make an offer for the whole box. Check
them for safety before use. Supplies rated for wide-range inputs (e.g.
110-250V without a selector switch) are generally switchmodes. The flying
leads and special connectors might be worth keeping, even if the PSUs
themselves are junk. Unregulated supplies typically produce several volts
above their rating off-load and only vaguely approximate the rated voltage
at the rated current ... so avoid powering valuable electronics directly
from them. Any PSUs that get hot are clearly suspect.
This is good advice, which I've freely given for years. :) Another
detail about voltage regulation -- the vast majority of linear wall
warts and line lumps are transformer-rectifier-capacitor filter
supplies, so open circuit voltage will rise to nearly the peak value of
the transformer secondary with no load, and drop close to the label
voltage at the label current.
I break all the DC cables for these things and add PowerPole connectors,
and use an inline meter with Power Poles to monitor voltage and current
when matching a wart to a load. In my experience, you're pretty safe
matching voltage and current ratings of the supply to the voltage and
current spec of the device (usually marked on the SMPS that comes with it).
73, Jim K9YC
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