[Elecraft] OT: Quiet 12V-to-5V converter

[Fred Townsend]

Hi John:
OK let's say your requirement is for a low noise, high efficiency converter
delivering 4.5 to 5V at up to 1 amp. There are hundreds of series linear
regulators that will do that job but the will dissipate more than 3 watts as
heat and that will require a small heatsink. For greater efficiency you will
want a switching regulator. 
Fortunately there are also hundreds of switching regulators available too.
Many of the manufactures have online software for designing the circuit.
What you want is a low RFI design. There are various categories of RFI.
Your receiver should already be protected against conducted RFI so you
should be protecting your receiver against radiated RFI. 
Radiated RFI comes from either magnet or electric fields. Of the two types,
electric fields are much easier to shield or reduce. Again, this translates
to a switching regulator that stores energy in capacitors rather than
inductors. For maximum efficiency the circuit should use two FETs (one for
switching and one for commutating). Searching the manufacture's web pages
for Low RFI switching regulators will usually find what you need. Then when
you have a working circuit, enclose it in a metal box to provide an
effective Faraday shield. The same manufacturer pages will also provide you
with app notes that will also provide a typical design. 

Finally keep your regulator as far away as possible from your receiver. DO
NOT put them in the same box.
Hope this helps.
73
Fred, AE6QL
-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of John Gibson
Sent: Friday, January 06, 2012 7:11 AM
To: elecraft at mailman.qth.net
Subject: [Elecraft] OT: Quiet 12V-to-5V converter

Dear Elecraft list members,

The recent thread on noisy switching power supplies leads me to a related
question. My son n8fyk, who teaches radio astronomy, has set up a project
with his students. They are using a 15m direct-conversion Radio Jove
receiver in the field to monitor radio emissions from Jupiter, the sun, and
the galaxy. The receiver operates from 12V, which is readily supplied by a
portable utility 12V battery pack. Their problem is powering their data
recorder, which needs about 5V at 0.7A.

The data recorder is designed to operate from three internal AA alkaline
cells, but these give a limited run time. My son would like to be able to
leave the receiver and recorder for unattended operation for several days at
a time. 

One solution would be to use a 12V-to-5V power converter between the 12V
battery and the data recorder. For long battery life, it would be nice to
have the efficiency of a switching converter. Such a converter would have to
be RF-quiet, so as not to interfere with their weak-signal reception. 

I will be interested to hear what list members recommend.

Thank you and 73,

John, no8v
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