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nuckolls.bob(at)aeroelect
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 Posted: Mon Dec 13, 2010 5:03 pm Post subject: "Regulated" 12 v supply |
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At 12:20 PM 12/13/2010, you wrote:
| Quote: | --> AeroElectric-List message posted by: "jonlaury" <jonlaury(at)impulse.net>
I have a video camera on my tailwheel so that I can see under the nose of my plane when taxiing. The camera calls for a "regulated" 12v, (+ or - 10%) power supply.
Will a 12v inline zener diode do the job, or do I not understand how zeners work? |
A zener works like a "smart weight" on the end of a
spring. Suppose you hung a kid's swing from the ceiling
on a bungee cord and with nobody in the swing, it
was 3' off the floor. For the kid's safety, you
want them to enjoy the swing at an altitude of
12 +/- 1 inches off the floor.
Suppose the heaviest 50# kid caused the swing to
assume an altitude of 16" . . . you would need
to ADD weight to achieve the desired altitude.
Suppose further that the lightest 30# kid pulls
the swing down to 24" off the floor. Obviously,
still more weight needs to be added to achieve
the 12" target.
24-16
The spring rate for this example is ------ = .40 inches/lb
50-30
So added weight need to lower the altitude with the
50# kid by 4 inches is 4/.4 or 10 pounds. Added
weight for the 30# kid needs to lower the swing
an additional 12 inches is 12/.4 or 30 pounds.
Finally, you want the swing to hang at 12" for
everyone to get into our out of the swing. This
means that while the swing is completely unloaded,
the greatest compensating weight would have to be
applied and then adjusted downward quickly and
smoothly as the passenger gets into or out of
the swing. The study above says a total weight
of 60 pounds will hold the empty swing at the
right "altitude".
Okay, our zener is a "smart weight". The bungee
is replaced with a resistor (R). The range of
swing customers have a "weight" of 30 to 50
millamperes. The target "swinging height"
or Vout of 5 volts.
[img]cid:.0[/img]
Obviously, we must hang from a ceiling greater
than 5 volts . . . for the purposes of this
study, let's call it a Vin of 12 volts.
So we need a constant differential or "drop"
of 7 volts in our resistor. Let's set up the
resistor so that the swing is capable of carrying
passengers just above the largest anticipated
demand . . . 60 mA.
8V drop at 60 mA calls for a resistor of
R = 8v/.06a = 133 ohms.
Our "smart weight" is a zener diode operating
at 5.1 volts above the floor. Further, with no
passengers in the swing, the zener needs to
dissipate a total power value of
5.1v x .06 amps = 300 milliwatts.
So a 500 mW or larger zener will suffice.
The resistor has a constant 8v drop across it
at the .06A so it dissipates worst case:
8v x .06A = .48W
I'd go for a 1W resistor to run cooler.
Now, with the "smart weight" maintaining
5.1 volts off the floor, any load of 0 up to
and including 0.06 amps can be connected
across the zener with an expectation that the
5.1 volt value will be maintained.
This example is probably in the neighborhood
of a best case for using a zener supply. Total
power being dumped in the components is maxed
at 0.78W. If one were to design a similar architecture
for a 1.5A supply, the wasted power goes up to
18 watts . . . really gets things hot unless
provided with HEAVY heatsinks.
The architecture of choice is to utilize one
of the modern DC/DC converters that yield 90 plus
percent efficiencies at all load levels.
HOWEVER, it's not a free lunch. There can be
terrible noise generators for adf and perhaps
even VHF comm radios. So as a matter of normal
practice, I ALWAYS close these critters up in
metallic enclosures with input and output
filters.
You asked about a 12 volt source. Your input
can be anything from 10 to 15 volts. So you need
a buck-boost DC/DC converter that provides an
output from a source that may be either above
or below the output. How much current are we
talking about? You might want to consider a
set of alkaline cells and a toggle switch to
turn the system on only when actively in use.
I suspect a set of dedicated batteries would
last a very long time.
Bob . . .
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jonlaury
Joined: 06 Nov 2006
Posts: 336
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