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Speedy11(at)aol.com
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 Posted: Thu Apr 17, 2008 4:44 pm Post subject: A Bit of Help, Please |
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I am installing Whelen LED taxi/recognition lights.
Each one draws 0.9 amps.
I am using an DPDT on-off-on switch to operate them. One position for steady on, one for flashing. I want to use only one switch instead of two.
I'm using Eric Jones' wig wag flasher for the recognition mode.
For the flash switch selection, I've wired one side with leads from the flasher module that activate the flash.
For the steady on switch selection, I've wired power through the other side of the switch.
When in flash mode, to keep power from going back through the wires and causing the lights to stay on constantly, I've added zener diodes.
The diodes are Radio Shack 12v INT4742A. The package says the characteristics are current = 21mA and max power dissipation = 1.0W.
I've tested the lights with the diodes in the power wires for 45 minutes without a problem.
My question is - Are these diodes sufficient for long term use with lights drawing 0.9 Amps?
Thanks in advance,
Stan Sutterfield
Need a new ride? Check out the largest site for U.S. used car listings at AOL Autos.
[quote][b]
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Joemotis(at)aol.com
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nuckolls.bob(at)cox.net
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| Posted: Thu Apr 17, 2008 6:22 pm Post subject: A Bit of Help, Please |
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At 08:39 PM 4/17/2008 -0400, you wrote:
| Quote: | I am installing Whelen LED taxi/recognition lights.
Each one draws 0.9 amps.
I am using an DPDT on-off-on switch to operate them. One position for
steady on, one for flashing. I want to use only one switch instead of two.
I'm using Eric Jones' wig wag flasher for the recognition mode.
For the flash switch selection, I've wired one side with leads from the
flasher module that activate the flash.
For the steady on switch selection, I've wired power through the other
side of the switch.
When in flash mode, to keep power from going back through the wires and
causing the lights to stay on constantly, I've added zener diodes.
The diodes are Radio Shack 12v INT4742A. The package says the
characteristics are current = 21mA and max power dissipation = 1.0W.
I've tested the lights with the diodes in the power wires for 45 minutes
without a problem.
My question is - Are these diodes sufficient for long term use with lights
drawing 0.9 Amps?
Thanks in advance,
Stan Sutterfield
|
It's not clear as to why you selected Zener diodes for this
task. Zeners are for voltage regulation . . . and it seems
that the task you're describing uses the diodes to prevent
reverse current flow under some conditions. Now, if you've
used a 1N4742 zener in the forward biased mode to replace
a simple rectifier diode, then the current ratings stated
for zener operation do not apply. A 1N4742 forward biased
zener is probably good for about an amp . . . but in the
reverse direction, the condition that you want the current
to be zero, the 12 zener mode of operation will kick in and
it will cause the difference between your 14v system and the
12v rating of the zener to be impressed across parts of the
system that you want to be completely 'cold'.
See:
http://www.aeroelectric.com/Pictures/Misc/RS_Diodes.jpg
The smallest diode suited to the task would be the
1N4001 offered as RS catalog number 276-1101. However,
higher voltage ratings don't hurt a thing so any of the
1N400-series devices could be used.
My personal preference for mechanical robustness are
the 1N5400-series, 3 amp devices. Again, having a part
that is electrically "too big" isn't a factor for your
application. These diodes are about 3x the size
and have more robust bodies and leadwires. These are
the diodes shown in this picture:
http://www.aeroelectric.com/Pictures/Contactors/s701-2.jpg
When we were selling this assembly, the fatter diodes
were selected for ease of assembly and robustness.
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
----------------------------------------
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Speedy11(at)aol.com
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| Posted: Sat Apr 19, 2008 4:57 pm Post subject: A Bit of Help, Please |
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Bob,
Thanks for your robust reply. Very helpful.
My intention is to prevent reverse current flow in certain conditions.
I connected the zener diodes in line where needed and did a test of the system and the diodes did stop the reverse flow of current - as desired. My concern was that the selected diodes (RS INT4742A - RS part number 276-563) may not be robust enough to stop the 0.9 Amp current over the long term.
I may as well insert the correct diodes now rather than redo the work at a later time. So, it sounds like I should change the diodes to another type or another rating. Correct? Perhaps the 1N5400 series would be best.
Stan Sutterfield
| Quote: | >I am installing Whelen LED taxi/recognition lights.
| Quote: | Each one draws 0.9 amps.
I am using an DPDT on-off-on switch to operate them. One position for
steady on, one for flashing. I want to use only one switch instead of two.
I'm using Eric Jones' wig wag flasher for the recognition mode.
For the flash switch selection, I've wired one side with leads from the
flasher module that activate the flash.
For the steady on switch selection, I've wired power through the other
side of the switch.
When in flash mode, to keep power from going back through the wires and
causing the lights to stay on constantly, I've added zener diodes.
The diodes are Radio Shack 12v INT4742A. The package says the
characteristics are current = 21mA and max power dissipation = 1.0W.
I've tested the lights with the diodes in the power wires for 45 minutes
without a problem.
My question is - Are these diodes sufficient for long term use with lights
drawing 0.9 Amps?
Thanks in advance,
Stan Sutterfield
|
It's not clear as to why you selected Zener diodes for this
task. Zeners are for voltage regulation . . . and it seems
that the task you're describing uses the diodes to prevent
reverse current flow under some conditions. Now, if you've
used a 1N4742 zener in the forward biased mode to replace
a simple rectifier diode, then the current ratings stated
for zener operation do not apply. A 1N4742 forward biased
zener is probably good for about an amp . . . but in the
reverse direction, the condition that you want the current
to be zero, the 12 zener mode of operation will kick in and
it will cause the difference between your 14v system and the
12v rating of the zener to be impressed across parts of the
system that you want to be completely 'cold'.
See:
http://www.aeroelectric.com/Pictures/Misc/RS_Diodes.jpg
The smallest diode suited to the task would be the
1N4001 offered as RS catalog number 276-1101. However,
higher voltage ratings don't hurt a thing so any of the
1N400-series devices could be used.
My personal preference for mechanical robustness are
the 1N5400-series, 3 amp devices. Again, having a part
that is electrically "too big" isn't a factor for your
application. These diodes are about 3x the size
and have more robust bodies and leadwires. These are
the diodes shown in this picture:
http://www.aeroelectric.com/Pictures/Contactors/s701-2.jpg
When we were selling this assembly, the fatter diodes
were selected for ease of assembly and robustness.
Bob . . . |
Need a new ride? Check out the largest site for U.S. used car listings at AOL Autos.
[quote][b]
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
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nuckolls.bob(at)cox.net
Guest
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| Posted: Sun Apr 20, 2008 4:46 am Post subject: A Bit of Help, Please |
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| Quote: | Subject: Re: A Bit of Help, Please
To: aeroelectric-list(at)matronics.com
Bob,
Thanks for your robust reply. Very helpful.
My intention is to prevent reverse current flow in certain conditions.
I connected the zener diodes in line where needed and did a test of the
system and the diodes did stop the reverse flow of current - as
desired. My concern was that the selected diodes (RS INT4742A - RS part
number 276-563) may not be robust enough to stop the 0.9 Amp current over
the long term.
|
That's what I understood. I was mystified by the
use of a zener diode. These will "stop reverse flow"
only if the voltage remains below the rating
at which reverse flow is desired and expected . . .
in the case of the 1N4742, 12 volts.
The "ratings" numbers for these devices are
given in there reverse current flow mode of
operation where they dissipate higher power
for the same current flow. Since this package
is generally good for about 1W of dissipation,
their max reverse current should be limited to
about 1w/12v = 0.08 amps. The 0.02 amps you cited
was the "test current" at which the 12v regulation
accuracy was given . . . not the operating
current where the current is dissipation limited.
You're using these devices in their forward
conduction mode where voltage drop is not related
to the zener voltage rating of the device, i.e.
12v but the forward conduction rating which is on
the same order as the ordinary silicon diode
rectifier of about 0.8 volts. Here the same
dissipation limits apply . . . but to get 1w
of heat out of it in the forward mode, you can
now force 1.2a through the device. MUCH larger
than any of the reverse current ratings given
in the 1N4742 data sheet . . . because folks
are generally expect to use theses as reverse
current regulators, not forward current
power steering devices.
http://en.wikipedia.org/wiki/Zener_diode
This is why the 1N4742 didn't smoke during your
extended bench test . . . you were not using it
in the manner described in the data sheets hence
non-hazardous dissipated power at higher than "rated"
currents.
| Quote: | I may as well insert the correct diodes now rather than redo the work at a
later time. So, it sounds like I should change the diodes to another type
or another rating. Correct? Perhaps the 1N5400 series would be best.
|
The significance of my earlier posting was that
mis-application of the zener as a power steering
device was marginally satisfactory given that
voltages in your system are expected to exceed
the 12v rating of the device whereupon some amount
of reverse current may flow under conditions where
you don't want it to happen. If you picked a zener
that was larger, say 16v or even higher, then
it could be expected to function as you intended
even if it wasn't exactly the right part for the
job.
The silicon rectifier also has a voltage rating
stated as a limit. See:
http://www.diodes.com/datasheets/ds28002.pdf
Note that these little critters are the same
physical size as the 1N4742 but have voltage
limits ranging from 50 to 1000v. Any of these
devices could be used in your application. I
suggested the 5400 series devices be considered
also:
http://www.diodes.com/datasheets/ds28007.pdf
These to are offered in the same ranges of
voltage limits as the 4000 series devices and
while electrically much larger than you need in
this particular case, there's no great transgression
in using such devices at much lower levels
than their rated limits.
In this case, I favor the 5400 series devices
for their mechanical robustness. Larger package,
larger leadwires, much easier to work with in
these situations and less likely to be damaged in
service due to handling.
I think you got sucked into the 12v zener
selection due to a mis-understanding of its
rating as being "appropriate" to your 14v
airplane. The problem was that zeners are
not intended for or generally used as reverse
power blocking devices. The silicon rectifiers
come with lots larger voltage LIMIT ratings
so while much larger than the system voltage
of your aircraft, they are not being mis-applied
by using say a 600v device in your 14v
airplane.
So yes, my recommendation is that you avail yourself
of Radio Shack's standard inventory and substitute
any one of the 5400 series devices for the 1N4742
in the application you've described.
Bob . . .
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
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Speedy11(at)aol.com
Guest
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| Posted: Mon Apr 21, 2008 8:33 pm Post subject: A Bit of Help, Please |
|
|
Bob,
Your analysis of my situation was dead on accurate. And your info was invaluable.
I sincerely appreciate your response!
Stan Sutterfield
| Quote: | >Bob,
| Quote: | Thanks for your robust reply. Very helpful.
My intention is to prevent reverse current flow in certain conditions.
I connected the zener diodes in line where needed and did a test of the
system and the diodes did stop the reverse flow of current - as
desired. My concern was that the selected diodes (RS INT4742A - RS part
number 276-563) may not be robust enough to stop the 0.9 Amp current over
the long term.
|
That's what I understood. I was mystified by the
use of a zener diode. These will "stop reverse flow"
only if the voltage remains below the rating
at which reverse flow is desired and expected . . .
in the case of the 1N4742, 12 volts.
The "ratings" numbers for these devices are
given in there reverse current flow mode of
operation where they dissipate higher power
for the same current flow. Since this package
is generally good for about 1W of dissipation,
their max reverse current should be limited to
about 1w/12v = 0.08 amps. The 0.02 amps you cited
was the "test current" at which the 12v regulation
accuracy was given . . . not the operating
current where the current is dissipation limited.
You're using these devices in their forward
conduction mode where voltage drop is not related
to the zener voltage rating of the device, i.e.
12v but the forward conduction rating which is on
the same order as the ordinary silicon diode
rectifier of about 0.8 volts. Here the same
dissipation limits apply . . . but to get 1w
of heat out of it in the forward mode, you can
now force 1.2a through the device. MUCH larger
than any of the reverse current ratings given
in the 1N4742 data sheet . . . because folks
are generally expect to use theses as reverse
current regulators, not forward current
power steering devices.
http://en.wikipedia.org/wiki/Zener_diode
This is why the 1N4742 didn't smoke during your
extended bench test . . . you were not using it
in the manner described in the data sheets hence
non-hazardous dissipated power at higher than "rated"
currents.
| Quote: | I may as well insert the correct diodes now rather than redo the work at a
later time. So, it sounds like I should change the diodes to another type
or another rating. Correct? Perhaps the 1N5400 series would be best.
|
The significance of my earlier posting was that
mis-application of the zener as a power steering
device was marginally satisfactory given that
voltages in your system are expected to exceed
the 12v rating of the device whereupon some amount
of reverse current may flow under conditions where
you don't want it to happen. If you picked a zener
that was larger, say 16v or even higher, then
it could be expected to function as you intended
even if it wasn't exactly the right part for the
job.
The silicon rectifier also has a voltage rating
stated as a limit. See:
http://www.diodes.com/datasheets/ds28002.pdf
Note that these little critters are the same
physical size as the 1N4742 but have voltage
limits ranging from 50 to 1000v. Any of these
devices could be used in your application. I
suggested the 5400 series devices be considered
also:
http://www.diodes.com/datasheets/ds28007.pdf
These to are offered in the same ranges of
voltage limits as the 4000 series devices and
while electrically much larger than you need in
this particular case, there's no great transgression
in using such devices at much lower levels
than their rated limits.
In this case, I favor the 5400 series devices
for their mechanical robustness. Larger package,
larger leadwires, much easier to work with in
these situations and less likely to be damaged in
service due to handling.
I think you got sucked into the 12v zener
selection due to a mis-understanding of its
rating as being "appropriate" to your 14v
airplane. The problem was that zeners are
not intended for or generally used as reverse
power blocking devices. The silicon rectifiers
come with lots larger voltage LIMIT ratings
so while much larger than the system voltage
of your aircraft, they are not being mis-applied
by using say a 600v device in your 14v
airplane.
So yes, my recommendation is that you avail yourself
of Radio Shack's standard inventory and substitute
any one of the 5400 series devices for the 1N4742
in the application you've described.
Bob . . .
|
Need a new ride? Check out the largest site for U.S. used car listings at AOL Autos.
[quote][b]
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
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nuckolls.bob(at)cox.net
Guest
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| Posted: Tue Apr 22, 2008 4:28 am Post subject: A Bit of Help, Please |
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|
At 12:27 AM 4/22/2008 -0400, you wrote:
| Quote: | Bob,
Your analysis of my situation was dead on accurate. And your info was
invaluable.
I sincerely appreciate your response!
Stan Sutterfield
|
My pleasure sir . . . it's what we do!
Bob . . .
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
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