DC plugs and outlets [RE-wrenches]

[Kurt Nelson]

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Linear regulators are not very efficient because they use the same
amount of energy at the higher supply voltage as they provide at the
lower, load voltage.  I have used them mostly in answering machines and
small 9.5 volt radios and other tiny loads that typically draw 50mA or
so and usually in small cabin, 12 volt systems that need to be as
efficient as possible.

Greetings Kurt,

For full rated output of such a regulator (1-1.5 amp) they require a
heat sink.  If you are drawing 30-100 mA, heat is not much of a problem.
In the early uses for such small loads I bolted the tab to the ground
connector of the plug, which was unused (not used as negative).  This
allowed for a little additional heat sink.  If you do not heat sink and
over load the device by driving it at outputs approaching full rating,
it either shuts off temporarily (internal thermal overload), or over
time simply dies.  

They are clearly more efficient when there is less of a voltage drop.
For instance a 60 mA load at 9 volts consumes .54 watts, but when
operated from a 12 volt source with a linear regulator in line it
actually draws .72 watts.  At 24 volts it would draw 1.4 watts while
still only delivering .54 watts.  This is not very efficient unless the
load is very small like the 60 mA that many of these devices draw.

I have used a Grundig radio, model YB400PE for years with such an outlet
and plug and it wakes me every morning to NPR radio.  The clock (in
standby) or the radio operating at normal (even fairly loud) volume does
not produce noticeable warmth at the ground pin of the plug. I have used
these on 9 volt answering machines (Unidens work well) with the same
results, both in my own home and widely at other installations (mostly
smaller cabin systems).  In standby, there is perhaps 20-30 mA draw, and
current jumps around a bit when in play mode, but things do not heat up
significantly.

All this said, the group is correct.  This does not meet code and I
guess I will not recommend it, period.  I may still do it occasionally.

Kurt Nelson

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