Today's Message Index:
----------------------
1. 05:57 AM - ELT Antenna (Owen Baker)
2. 12:30 PM - parelling two rp3 indicators (Bill S)
3. 03:12 PM - Re: Change to Shottky? (Eric M. Jones)
4. 04:56 PM - Re: Re: Change to Shottky? (Robert L. Nuckolls, III)
Message 1
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1/17/2014
Hello Bob Nuckolls, You wrote: =9CI've heard the rumor that some
ELT's were triggered by local radiation from a comm transmitter . . .
but
I've not seen any documentation or FMEA that supported the
assertion.=9D
Please let me give you one factual data point on that issue. I was
unable to launch on the first attempted test flight of my experimental
amateur built KIS TR-1 airplane because every time I keyed the VHF comm
transmitter to talk to either ground or tower the ACK ELT-01 began to
transmit.
The solution was to move the ELT antenna further away from the VHF comm
antenna and to reorient the ELT antenna from its original location and
orientation. Both antennas were originally, and remained after
relocation of the ELT antenna, inside the fiberglass fuselage.
Kelly McMullen wrote: =9CGood separation between ELT and com
antennas is probably best defense.=9D
That is what worked for me.
=98OC=99
'O C' Baker says "The best investment you can make is the effort to
gather and understand information."
=========
Time: 05:02:21 AM PST US
From: "Robert L. Nuckolls, III" <nuckolls.bob@aeroelectric.com>
Subject: Re: AeroElectric-List: Re: ELT Antenna
At 09:16 PM 1/15/2014, you wrote:
>
>Almost all ELTs, especially the older ones do in fact radiate
>anytime they are near high powered VHF transmitters in the FM, TV, etc
bands.
This phenomenon IS demonstrable . . . and rare. The
ELT isn't actually producing any output power unique
to its function. Very strong sources of local radiation
(typically FM stations and the old analog TV stations)
would find its way backwards into the output stage of
the ELT's transmitter (constantly hooked to the antenna -
receiver circuits are not so prone to exhibit this
behavior). If there are two or more strong sources,
then they can MIX or heterodyne against each other
and produce a variety of spurious signals on lots
of frequencies.
Analog TV is gone and about the only source of such
coherent energy would be an FM transmitter but you are unlikely
to spend much time flying around in the FM station's
radiation pattern and in close proximity . . . within
a mile or two.
One might hypothesize that the modern ELT's with
TWO transmitters attached to the antenna are even
more likely to exhibit the behavior . . . but you
still need to be poking around the vicinity of the
strong transmitter at altitudes generally 1000'
AGL or less.
I've heard the rumor that some ELT's were triggered
by local radiation from a comm transmitter . . . but
I've not seen any documentation or FMEA that supported
the assertion. Certainly, modern ELTs are subjected
to the standard DO-160 radiated susceptibility testing
and perhaps even HIRF (high intensity RF) testing.
None of this testing will concern itself with the
intermodulation phenomenon cited above but it would
watch for damage to the ELT and/or false triggering.
Bob . . .
=
Time: 07:11:05 AM PST US
From: Kelly McMullen <kellym@aviating.com>
Subject: Re: AeroElectric-List: Re: ELT Antenna
Hmm, I don't know why the switch to digital TV will make much
difference. They are still broadcasting in my area on VHF RF channels 7
and 8 through 13 in my local area. I used to have an antenna
configuration on my Mooney that consisted of VHF com in front of
windshield, top of fuselage behind baggage compartment, Loran about 18
in. behind that, followed by ELT antenna right in front of vertical
stab. When flying VFR transition over Phoenix Sky Harbor I would get bad
squelch break anywhere within 10 nm of the antenna farm located on South
Mountain, approx 8 mi south of Sky Harbor. Made hearing controllers very
difficult.
I isolated it to the ELT by doing a flight with external ELT antenna
disconnected, which completely eliminated problem even within a mile of
the transmitters. This was a 1st generation ELT. Moving 1st com antenna
from in front of windshield to the belly virtually eliminated the
interference. Removing Loran antenna (custom version of Comant CI121)
helped as well.
I understand later versions of ELTs have somewhat better isolation of
the transmit oscillator, but doesn't completely eliminate. Good
separation between ELT and com antennas is probably best defense.
Kelly
Message 2
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| Subject: | parelling two rp3 indicators |
Message for Bob, Ref. this email you sent to me Dec. 22 2011
Conserning the capasitor on the attached drawing. What should the
specs. be? I'm not up on electronic ckts. Thanks Bill S.
At 10:59 AM 12/22/2011, you wrote:
>I am wanting to parllel two Ray Allen RP3 led position indicators
>useing only one POS5 position sensor. RAC said that thay wont work
>just parelling them and that I need to add another position sensor
>or a switch which I would rather not do. Can anyone sudjest an
>electrical ckt. that would work for this? Thanks Bill S.
>
You need a 'buffer-amplifier' between the position
feedback potentiometer and ONE of the two indicators.
The problem with paralleling the two indicators arises
from the fact that they're not a 'high impedance' voltmeter.
The system is calibrated for one pot driving one indicator.
Adding a second indicator doubles the load on the position
signal from the potentiometer.
The 'fix' is to convert one of the indicators into a
high-impedance voltmeter. You need an operational
amplifier with rail-to-rail inputs and outputs. A device
like the LM7321 would probably work.
http://search.digikey.com/us/en/products/LM7321MF%2FNOPB/LM7321MFCT-ND/18
78646
Adding this device to the second indicator prevents
it from loading the potentiometer. You need to
fabricate something like this . . .
http://aeroelectric.com/Pictures/Schematics/Ray-Allen_Dual_Indicators.pdf
It could be fabricated on an etched circuit board
that would fit inside a d-sub connector back shell.
Bob . . .
22 2011
Message 3
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| Subject: | Re: Change to Shottky? |
When using diodes in parallel, the Vf drop will stay the same as the lowest diodes
Vf but reverse leakage Ir (and capacitance) will add. But it is often cited
as a bad idea because as one diode heats up, its Vf will decreaseand it will
draw more current, etc. This is called a thermal runaway failure. You can avoid
this by placing the diodes in tight thermal contact with each other, or using
a small resistor in series with each, or making certain the Vfs start and thermally
track each other as closely as possible (and the diodes were made at
the same time, same batch, etc.).
Schottky diodes especially trade off forward Vf with reverse leakage. So always
look for this when trying to get a very low forward Vf. I cant say reverse leakage
is always a bad thing; it depends on the use. The spec sheet for STPS60L30
states:
Dual center tap Schottky rectifier suited for Switch Mode Power Supply and high
frequency DC to DC converters. Packaged in TO247, this device is intended for
use in low voltage, high frequency inverters, free-wheeling and polarity protection
applications.
This statement by STM is a good indication that this part has a huge reverse leakage
Ir, since HF inverter transformers in power supplies can allow for that
reverse DC leakage easily while they benefit from the very low Vf. And it does.
Ir(max) Tj=125C, 500 mA! Can your application accept that?
The spec sheet for IXYS DSSX61-0045A Ir (max) is 20 mA.
I dont want to specify or analyze specific diode choices, but I think IXYS DSSX61-0045A
represents a good compromise for power blocking diodes in price/performance
/Vf/Ir mostly because it is packaged in an isolated SOT-227 package. There
are lower Vf diodes, and lower Ir diodes, and diodes that are far pricier.
But this is a great compromise and very versatile diode. Remember, Schottkys
dissipate less and often require no heatsink compared to a similarly-rated P/N
diode.
> >>From: Thomas Blejwas
> Subject: Re: AeroElectric-List: Re: Change to Schottky?
>
> -Eric, I think that I misinterpreted the use of the word "average."
> - (My-aero, mechanical, and civil engineering degrees were not big on diodes.) -Say, for the specific diode STPS60L30CW: http://www.st.com/en/resources/technical/document/cd00001857.pdf- , Fig. 1 provides "average forward power dissipation versus average forward current (per diode)."- I
> now understand that the term "average" is the average for-the cycle (not
> the average between the two diodes), i.e., the average amperage for a square-wave
cycle with a delta of 0.5 would be half the peak to peak amperage.
>
>
> >>- So, if I understand you correctly, the voltage drop with both diodes connected
to the same source-should be calculated using the total current, as if there
were only one diode connected.
Right, if I get what you're saying. Think of Vf standing for Vertical fall (waterfall).
Adding waterfalls in parallel, doesnt make each Vf waterfall taller.
You can get more current, but you cant reduce the Vf potential loss. It is what
it is. At the top of the waterfall you have a potential; at the bottom you have
lower potential.
> >>For this particular diode assembly-and my system (14A max.-at 13V), the voltage
drop from Fig. 9 at a Tj of 125 degC is 0.31 to 0.32, for a power dissipation
of-slightly more than 4W.- I would only use this diode if I were confident
that I could keep the Tj low, because it has high leakage at high temperature
and because this diode is only rated to 150 degC.- But the leakage versus voltage
drop is the tradeoff I see in the selection process.-
>
True. But even at a Tj of 125 degC, used as an isolation diode, Id be careful because
it doesnt isolate much. This might matter.
> >>Not clear to me that we need extremely low leakage. I don't understand-why
the voltage drop should be calculated as if all the current goes through only
one diode.- This implies that the current doesn't substantively split, because
if it did, the voltage drop in each diode would be lower.- If there is a good
authoritative-reference that you can suggest, I would appreciate it.- I've Googled
and found different views and opinions.
There is a substantial body of work on using Schottkys. Try: http://www.onsemi.com/pub_link/Collateral/AND9038-D.PDF or other application notes from major manufacturers.
Builders new to electronics often wonder if there is a particular best component.
There usually isnt Furthermore sometimes (amazingly!) the manufacturer hides
certain factslike price, which is a really important characteristic unless you
work for the military, or the 56-week lead-time, (happened to me!) or the fact
that the parts are counterfeit. (I will send you some fake MC33030P parts
if interestedstraight from 4-Star Electronicswho wouldnt return my money because
it took me 32 days to come to the hard-to-believe conclusion that their parts
were fakes.)
--------
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=417097#417097
Message 4
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| Subject: | Re: Change to Shottky? |
At 09:14 AM 1/16/2014, you wrote:
>
>The calculation here on the dissipation and voltage drops of
>Schottkys and P/N diodes are simply being pulled out of the air.
>What diodes are you referring to? You show me yours and I'll show you mine.
Okay, our MBR4060 mounted for convenient installation
heat-sinked to the airframe and provided with terminals
for attachment of wires.
With the pair carrying 10A I measure a drop of 0.45
volts, 20A drops 0.68 volts.
While one cannot expect two diodes to accurately
parallel and share a load, the fact that the voltage
drops at all when the second diode is added demonstrates that
SOME load sharing takes place. This offers a benefit,
(perhaps tiny but quantifiable) for spreading the thermal
stresses over the device's heat-sink surface combined
with a small drop in voltage. In no way is the practice of
paralleling expected to offer a critical performance
jump . . . the second diode was there in the package
. . . might as well do something useful with it . . .
>
>A "standard" 15A 50V P/N diode is 1.5 Volts Vf like 1N3208.
>So at 13V you get 11.5V out.
I just measured one of the RS 25A bridge diodes
at 10A and got 0.84 volts, 20A gave me 0.93 volts
>I use and sell IXYS DSSX61-0045A. Power_Deuce_Schottkys. At 15A
>they are 0.45Volts Vf. So at 13VDC you get 12.55VDC out.
Don't know where the 13v comes from. The normal feedpath
diode takes a feed from the main bus which also drives
the voltage sense pin of the regulator. Hence, this bus
is expected to be at 14.2 +/0 0.2 volts any time the
alternator is operating normally. If the alternator is
running, the bus is >14v . . . if the alternator is
not running the system voltage is <12.5
>I supply these on heatsink. They are isolated and paired so they can
>be used in Y-configurations or separate or paired.
>
>I've looked and don't see any better diodes for general aircraft
>purposes. If the difference was as small as some here have
>erroneously stated, we wouldn't be having this conversation. Most
>modern battery operated equipment have no p/n diodes. They waste too
>much power.
Kumquats and watermelons. Diodes used inside
a piece of electronics have little in common
with design goals for steering diodes in
power distribution systems. There are some really
fat P/N diodes used throughout the Lear, Cessna
and Beech fleets (and no doubt others) that cannot
be Shottky due to requirements for lightning and abnormal
surge voltage requirements. Requirements that
are not levied onto the internal workings of an
appliance.
I think the Hawker 4000 had some Shottky power
steering in their DC system (Low current stuff
driven by TR-sets off wild frequency 208, 3-
phase) and they had to add a lot of monkey-
motion in the form of Transorbs, etc. to make
it through lightning testing.
We're discussing the return on investment in
light airplanes for saving a watt or two in
a system powered with a 500+ watt engine driven
power source is not quantifiable. Energy 'squandered'
is a fraction of a percent of the whole energy
bucket no matter what kind of diode you use.
At the same time, the 'rule of thumb' for selection
of wires says 5% voltage drop is an acceptable
compromise between loss of electrical performance
and unnecessary addition to aircraft empty weight.
That figures out to 0.7 volts per power feeder;
but that too is a 'rule of thumb' . . . were it
twice that amount, no airplane is going to fall
out of the sky and no pilots are going to be
squinting out into the dark because 'violation'
of rule by a factor of 2 or even 3 has dimmed
the lights. No dollars will be saved. So the choice
of components is driven by FMEA and human
factors for reducing risk.
For example, we could replace the normal feedpath
diode with a relay that is controlled by an
aux battery management module . . . relay closed
only when the bus voltage exceeds 13.0 volts.
Hard contacts, lower voltage drop, more complexity.
Or put a switch in. But then, the whole purposed
for the diode was to prevent inadvertent back-feeding
the main bus from the e-bus due to improper operation
of switches.
I've looked and don't see any better diodes for general aircraft
purposes. If the difference was as small as some here have
erroneously stated, we wouldn't be having this conversation. Most
modern battery operated equipment have no p/n diodes. They waste too
much power.
It's easy to avoid getting wrapped around
the weight/energy-savings axles in this application.
Energy savings is trivial . . . weight savings can
be as much as 1/2 pound if the builder discovers
that THEIR implementation of the e-bus normal feed
diode doesn't really need that big heat sink.
If some diode is dropping enough BTU's to require
a hefty heat-sink, then it's not driving an e-bus.
The Endurance Bus was conceived to offer a
narrowly defined benefit . . . electrical endurance
that exceeded fuel endurance. With the electrically
dependent engine needing something on the order
of 100W power for operation, the concept of an
E=Bus is no longer useful. It's unlikely that
anyone will opt to carry enough battery to run
an engine for 3 hours. At the same time,
diodes used to steer power can STILL be sized
in the 8-12 amp range without extra-ordinary
heat-sinking in an all metal airplane.
But in any case, there's no substitute for
confirmation of one's decisions by direct
measurement of effects and benefits. Toward that
end, I supported an RS bridge in air with no heat-sink
and loaded it to 10A. After 20 minutes, the mounting
surface stabilized out at just over 100C. Given
the very limited ability of this surface to reject
heat, the stress across the internal thermal resistance
would be low the diode junction was probably not much
hotter . . . perhaps 110-120C.
But the thing didn't self-destruct. Then I bolted
it to an airplane . . . well . . . a part of
an airplane .040 x 5 x 5 inches. Didn't use
heat-sink grease under the part but I did
put a fender-washer on the back side to stiffen
the aluminum around the mounting hole with
a notion of bringing the rectifier into better
contact with the aluminum sheet.
If one intends to run loads in the 10A class
through a device like this, some heat-sink grease
or Sil-Pad underneath plus a backside 'stiffener'
is probably a good thing to do.
The second experiment was rather profound.
After 20 minutes, the metal around the
rectifier was running 40C. 15C over ambient
for a rise of about 2C/watt. Assuming I had
a lousy thermal interface of . . . say
1.5C/W. These devices will typically
offer a to case resistance 1.5C/W. So the
8w dissipated in the diode was taking
the junction to 8 x 3 or 24C hotter
than the 'airplane' at 40C. This minimalist
approach to heat-sink gave us a junction
temperature about 40C over ambient or 65C
. . . . . .WAaaay less than the rated
150C maximum.
If your airplane isn't metal, then you can
use the same part-of-an-airplane that I just
demonstrated. 25 square inches of aluminum
goes a long way toward cooling things off.
If that were a Schottky running 5 watts
of loss, then the temperature of the junction
can be expected to drop into the neighborhood
of 25C over ambient . . . and a 'savings'
of 3 watts.
The point of this discussion is to put the
worries to bed for application of any diode
technology for steering power to either an
E-bus or M-P bus of the variety we're
discussing today.
Further, note that 'losses' associated with
these diodes can be eliminated from the
concerns for battery-only operations. That's
what the Z-07 discussion is all about.
Bob . . .
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