Today's Message Index:
----------------------
1. 02:27 AM - Re: Computing a Resistor Value (BobsV35B@aol.com)
2. 06:13 AM - Re: Computing a Resistor Value (Robert L. Nuckolls, III)
3. 06:13 AM - Re: Computing a Resistor Value (Robert L. Nuckolls, III)
4. 06:13 AM - Re: Computing a Resistor Value (Mike Welch)
5. 08:03 AM - Re: Computing a Resistor Value (AlRice)
6. 08:25 AM - Re: Computing a Resistor Value (user9253)
7. 10:24 AM - "Regulated" 12 v supply (jonlaury)
8. 05:03 PM - Re: "Regulated" 12 v supply (Robert L. Nuckolls, III)
Message 1
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Subject: | Re: Computing a Resistor Value |
Good Morning Al,
I am sure you know more about Ohm's Law than I do, but why don't you
consider using a Zener Diode?
If you use a resistor, the voltage output will change as the load changes.
I have no idea whether or not the load does change during operation of the
AV80R but dropping resistors do not work well at all with a varying load.
Happy Skies,
Old Bob
In a message dated 12/12/2010 8:05:24 P.M. Central Standard Time,
Allen@allenrice.net writes:
--> AeroElectric-List message posted by: "Al Rice" <Allen@AllenRice.net>
I want to install my AV 80 R GPS in my panel and wire the power lead to
the bus without installing a clunky cigarette power socket on my panel. The
GPS input voltage is 5 V and it draws 1.5A (at least that's what the
cigarette power lead says on it). My buss voltage is 13.5V, so how do I compute
the resistor ohm's and watts needed for my power lead? I know enough about
Ohm's law to be dangerous.
Thanks.
--------
Al Rice
Sky bolt 260
RV-9 A
Helping with my grandson's Pi et
Message 2
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Subject: | Re: Computing a Resistor Value |
At 07:59 PM 12/12/2010, you wrote:
>
>I want to install my AV80R GPS in my panel and wire the power lead
>to the bus without installing a clunky cigarette power socket on my
>panel. The GPS input voltage is 5V and it draws 1.5A (at least
>that's what the cigarette power lead says on it). My buss voltage
>is 13.5V, so how do I compute the resistor ohm's and watts needed
>for my power lead? I know enough about Ohm's law to be dangerous.
And indeed, a total reliance on Ohm's law in this
instance would be very hard if not dangerous to
the GPS. You need to acquire the automotive power
adapter suited for using/charging this product
in the car . . . then adapt that same device to
the airplane. I see that the advertising literature
speaks of a USB/PC cable. Check the instruction
manual to see if connection of the GPS to a powered
up PC will also charge the battery. It may not . . .
some USB ports on computers won't deliver 1.5A.
If so, then there are dozens of "USB power adapters" designed
to accept 14 volts from a vehicular power system
and deliver the smooth, regulated 5v dc necessary
for safe operation of the GPS. But accommodating the King
automotive adapter cable in the airplane is THE
100% way to go.
Bob . . .
Message 3
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Subject: | Re: Computing a Resistor Value |
At 07:59 PM 12/12/2010, you wrote:
>
>I want to install my AV80R GPS in my panel and wire the power lead
>to the bus without installing a clunky cigarette power socket on my
>panel. The GPS input voltage is 5V and it draws 1.5A (at least
>that's what the cigarette power lead says on it). My buss voltage
>is 13.5V, so how do I compute the resistor ohm's and watts needed
>for my power lead? I know enough about Ohm's law to be dangerous.
And indeed, a total reliance on Ohm's law in this
instance would be very hard if not dangerous to
the GPS. You need to acquire the automotive power
adapter suited for using/charging this product
in the car . . . then adapt that same device to
the airplane. I see that the advertising literature
speaks of a USB/PC cable. Check the instruction
manual to see if connection of the GPS to a powered
up PC will also charge the battery. It may not . . .
some USB ports on computers won't deliver 1.5A.
If so, then there are dozens of "USB power adapters" designed
to accept 14 volts from a vehicular power system
and deliver the smooth, regulated 5v dc necessary
for safe operation of the GPS. But accommodating the King
automotive adapter cable in the airplane is THE
100% way to go.
Bob . . .
Message 4
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Subject: | Computing a Resistor Value |
> The GPS input voltage is 5V and it draws 1.5A (at least that's what the
>cigarette power lead says on it). My buss voltage is 13.5V=2C so how do I
compute
>the resistor ohm's and watts needed for my power lead?I know enough about
>Ohm's law to be dangerous.
> Thanks.
> --------
> Al Rice
Mornin' Al=2C
When I first read your post=2C I didn't think the resistor was the way to
go=2C either=2C
same as Dick Tasker described. My thinking was you're asking quite a bit f
rom just one
resistor.
My initial thought was "why not build a 5 volt circuit" (like this one)?
http://www.tkk.fi/Misc/Electronics/circuits/psu_5v.html
Recently=2C I asked this forum if they knew how I could turn my Ray Allen
servo resistance
output (0-5K ohms) into a visual LED indicator. Our buddy Joe Gores offere
d the perfect suggestion
of a circuit referenced from Mike Linse.
Included in his suggestion was that really handy 5 volt circuit above=2C
plus it is super easy to build.
The circuit had a final voltage output of 4.96 volts=2C and when combined w
ith the circuit Mike Linse
talked about=2C the whole project worked exactly as I was hoping for.
From the article's data points about this 5 volt circuit=2C it says it ma
y be limited to only
one amp=2C plus you should consider a heat sink. So I got to thinking.....
....
Here's a question for for you guys that rearrange electrons for a living.
....could a guy
build TWO of these 5 volt circuits=2C and combine the '5 volt output' in pa
rallel?
Mike Welch
Message 5
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Subject: | Re: Computing a Resistor Value |
Thanks for the advice everyone. I learned a lot with this thread. I guess I'll
stick with the cigarette adapter and mount the socket behind the panel where
I can't see the ugly thing.
--------
Al Rice
Skybolt 260
RV-9A
Helping with my grandson's Piet
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=323176#323176
Message 6
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Subject: | Re: Computing a Resistor Value |
Al,
The rating on the 5 volt adaptor is what it is capable of, not necessarily what
the GPS uses.
There are options:
1. Like everyone is saying, using a resistor by itself is a bad idea.
2. A resistor and a 5 volt zener diode. Without knowing the current requirements
of the GPS, it will be trial and error. 7 ohms, 10W would be a starting point.
3. You could build the 5v regulator circuit suggested by Mike Welch. A 7805 in
a TO-220 case bolted to the airframe and using thermal conductive paste.
4. Take apart the OEM adaptor and solder wires to it and connect to the aircraft
power.
5. Buy one of these: http://www.mouser.com/ProductDetail/Murata-Power-Solutions/78SRH-5-2-C/?qs=35WhQNrE6p2lJ6BniHrImQ%3d%3d Probably generates less heat than other options.
Joe
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=323181#323181
Message 7
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Subject: | "Regulated" 12 v supply |
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?
Thanks,
John
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=323192#323192
Message 8
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Subject: | Re: "Regulated" 12 v supply |
At 12:20 PM 12/13/2010, you wrote:
>
>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.
Emacs!
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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