REFLECTOR:A (re)volting problem
Al Gietzen
reflector@tvbf.org
Fri, 3 Oct 2003 07:31:40 -0700
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The plot thickens. Yesterday evening, I went out to get my night
landings
in for currency and tested the pitot. I pushed in the circuit breaker
and
turned on the pitot switch. After 5-10 seconds, the "10" (amp, I
assume)
breaker popped. I cycled it again, same results. I checked and the
voltage
dropped from 12.8v to about 11.0v, then the breaker would pop after a
few
seconds.
Based on my passing understanding of electricity (I once passed by the
Electrical Engineering building), when you go up in volts, you go down
in
amps. I assume the converse holds true; when the voltage is cut in
half,
the amps double? So, an instrument rated for 10a at 24v is going to
need a
20a circuit at 12v (actual draw is approximately 80%, or 16a)?
Chuck:
Current = voltage/resistance. As others pointed out a couple of days
ago, for the same resistance cutting the voltage in half cuts the
current in half as well. Look for another problem, like a partial short
somewhere, or something faulty about the PT.
As I also mentioned, heated pitot tubes generally are self-regulating,
i.e.; as the temp goes up the resistance goes up in order to limit
current and overheating. Your case may vary, but the heated PT that I
have with 12v draws about 3.5 amps initially, dropping to 2 amps after a
couple of minutes. It is recommended to use a 7 amp breaker with 24V,
so a 5 amp ought to do it for 12v.
If you are making power, like in an electric motor, making the same
power will take twice as much current with half as much voltage. Power
= volts x amps.
Al
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<div class=3DSection1>
<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'> </span></font></p>
<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'>The
plot thickens. Yesterday evening, I went out to get my night =
landings</span></font></p>
<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'>in
for currency and tested the pitot. I pushed in the circuit breaker =
and</span></font></p>
<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'>turned
on the pitot switch. After 5-10 seconds, the "10" (amp, =
I
assume)</span></font></p>
<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'>breaker
popped. I cycled it again, same results. I checked and the =
voltage</span></font></p>
<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'>dropped
from 12.8v to about 11.0v, then the breaker would pop after a =
few</span></font></p>
<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'>seconds.</span></font></p>
<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'> </span></font></p>
<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'>Based
on my passing understanding of electricity (I once passed by =
the</span></font></p>
<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'>Electrical
Engineering building), when you go up in volts, you go down =
in</span></font></p>
<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'>amps.
I assume the converse holds true; when the voltage is cut in =
half,</span></font></p>
<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'>the
amps double? So, an instrument rated for 10a at 24v is going to =
need a</span></font></p>
<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'>20a
circuit at 12v (actual draw is approximately 80%, or =
16a)?</span></font></p>
<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'> </span></font></p>
<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>Chuck:</span></font></p>
<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'> </span><=
/font></p>
<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'>Current =3D
voltage/resistance. As others pointed out a couple of days ago, =
for the
same resistance cutting the voltage in half cuts the current in half as =
well.
Look for another problem, like a partial short somewhere, or something =
faulty
about the PT.</span></font></p>
<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'> </span><=
/font></p>
<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'>As I also =
mentioned,
heated pitot tubes generally are self-regulating, i.e.; as the temp goes =
up the
resistance goes up in order to limit current and overheating. Your =
case
may vary, but the heated PT that I have with 12v draws about 3.5 amps =
initially,
dropping to 2 amps after a couple of minutes. It is recommended to =
use a
7 amp breaker with 24V, so a 5 amp ought to do it for =
12v.</span></font></p>
<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'> </span><=
/font></p>
<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'>If you are =
making
power, like in an electric motor, making the same power will take twice =
as much
current with half as much voltage. Power =3D volts x =
amps.</span></font></p>
<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'> </span><=
/font></p>
<p class=3DMsoPlainText><font size=3D2 color=3Dblack =
face=3DVerdana><span
style=3D'font-size:11.0pt;font-family:Verdana;color:black'>Al</span></fon=
t></p>
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