[USRP-users] 12V to 6V Converter for USRP Supply

Radio User radiogeek381 at gmail.com
Sun Feb 17 20:16:37 EST 2013


HHH wrote:

> Would just find an adjustable regulator from ebay cost only several USD
more cost effective?

Good question.    Answer: Not likely.

The regulators I've seen on eBay are all based either on the LM317 or the
LT1764.
These are linear regulators.

1. The LM317 is good to about 1.5A with careful attention to thermal
engineering.
(This includes fiddly stuff like the kind of thermal compound, careful
application,
the right surface finish on the heat sink, and the right fasteners and
torque.
You'll no doubt find folks who say "it works fine for me, and I use
discarded
bandaids and paperclips to hold it to the heat sink."  Good for them.)

2. The specified drain for the N200 and WBX together is about 2.6 amps.
That's
3 in round numbers.  3 > 1.5.

3. The LT1764 is a little beefier, but the thermal challenge remains. Max
spec'd
output current is 3A.  Too close to the requirement for my tastes.  The
low drop out is a nice feature (and the compelling one if your real problem
is
a battery voltage that is close to the output requirement). But it is still
a
linear regulator.

4. Linear regulators have three major problems in this application

     I) Given a 12V input and a 6V output at (let's say) 2.5A, we'll
dissipate
        6*2.5 = 15W in the series pass device (the regulator package).  In
still
        air (no fan) with a really good contact to the heat sink, that's
going to produce
        a temperature rise at the heat sink surface of T = 5000 deg C / S
where S is the
        exposed surface area of the heat sink in square centimeters.
(Source --
        Reference Data for Radio Engineers converted from the constant 55
deg C per watt per in^2.)
        The sample I looked at seems to have about 40 cm^2 of heat sink
(charitable)
        so the still air temp rise at 2.5A is going to be about 125C.  So
this solution
        requires a fan, where the temp rise is much much smaller than 5K
C/W/cm^2.
        (And if you think I'm being conservative, note that real models can
predict
        substantially higher temp rises for this load and heat sink. 5K deg
C/cm^2
        may be quite optimistic...)

        I don't want a fan.

      II) At 2.5A a linear will waste one watt for every watt delivered to
the radio.  That's additional
        drain on the batteries.  Compare this to the switcher efficiency at
90 to 95% where
        a switcher will waste 100mW or less for each watt to the radio.

      III) Linear regulators tend to produce lots of broadband noise,
especially when
        you push them. The decoupling on these supplies is not adequate for
a radio
        application.  Assuming all the other problems could be addressed,
careful users
        will want to add significant decoupling/bypass caps to the output
of any linear
        regulator.  A bazillion uF cap is not sufficient, it ceases to
behave as a capacitor
        well below frequencies of interest.  And most cheap power supply
        manufacturers aren't using the highest quality caps in any case.

So, yes there are inexpensive linear regulator solutions here.  They require
moving air past them to stay within thermal limits, they are running close
to (or over)
their capacity, they may produce broadband noise that needs to be knocked
down,
and they are only 50% efficient on a good day.

Given the cost of a USRP, saving $30 or even $50 by buying a cheap power
supply
seems like a false economy.
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