On Mar 30, 1:30*am, Peter Purdie wrote:
A standard height encoder alone would need more power draw than a modern
transponder with integrated height encoder.

And fitting an ARINC case TPDR into an average glider panel would
compromise the layout of thestuff you actually need on the panel to go
soaring.

At 03:00 30 March 2009, Tony Condon wrote:

King made a version of their KT-76 Transponder with only 150 watts of
power
and called it the -78. *Im curious if anyone has ever tried to use one
of
these in a glider. *If so, what sort of power draw did you experience?

Thanks!
-Tony Condon
Cherokee II N373Y


I would also discourage people from wasting time with older
transponders for reasons including power consumption, reliability,
vendor support and repairability, space, lack of encoder altitude
display, etc. Modern transponders are all solid state, the KT-76/78
uses a cavity tube, you know--glowing metal filament, all that stuff,
this will use a good fraction of an amp just to run the filament. The
power consumption is going to be ugly.

However the statement on encoder power consumption is just not
accurate. A "standard height encoder" most definitely will *NOT* draw
more power than a modern transponder with intergrated height encoder.

The "standard" aka most common encoder used in sailplanes is the
ACK-30, certainly in the USA it is almost exclusively used. Chosen for
it's low power consumption and low cost. Nominal consumption quoted by
the manufacturers is is 60mA @ 14VDC. Consumption with the heater off
is measured at less than 10mA @ 12 V and from my measurements even on
typical soaring days the average consumption is well below the
nominal. If the encoder is not exposed to cold air leaks and you don't
fly in cold wave etc. then a consumption of a few tens of mA is
probably correct for power budget purposes. And what's more the ACK-30
is pretty low cost around $150-$200.

A typical modern low power consumption transponder with internal
encoder is the Trigg TT-250. The vendor specs for that transponder are
a impressive 280mA @14V nominal consumption. That is still over an
order of magnitude higher than the ACK-30 encoder alone (the Trigg
will be closer to it's claimed nominal power consumption than the
ACK-30 encoder which has it's specs padded to account for heater duty
cycle).

The reason for being pedantic is I don't want people thinking that an
external encoder necessarily implies large power consumption. Some
may, but most gliders will be using an ACK-30 if the installation has
an external encoder.

Always be careful of comparing specs. There is no standard way of
measuring/quoting typical power consumptions and different vendors may
quote different specs. Here are some --

Internal Encoder
Trig TT-250 130W (nominal at connector) Mode-S 0.28A typical @ 14V

External encoder
Mode-S Becker BXP 6401-2-(01) 140W (nominal at connector) 370mA
typical @ 50 replies/sec @ 14V
Mode-C Becker ATC 4401-1-175 175W (nominal at connector) 700mA max @
14V, less than 400mA @ 12V typical measured in actual use (i.e pretty
much the same as the BXP 6401-2-(01) claimed specs)
Mode-C Microair T2000SFL (200W nominal at connector) 150-200mA @ 14V

Stand Alone Encoder
AKC-30 Encoder, warm up current ~420mA @ 14VDC operating current 60mA
@ 14VDC, heater off less than 10mA

The Becker ATC 4401-1-175 is probably the most popular transponder in
the USA and my personal favorite. It suffers on paper in that it's
power consumption quoted is a maximum and well above the typical you
will see. For an actively interrogated transponder in real use I'd
budget about 400mA plus encoder for the 4401-1-175. For the
foreseeable future I have no problem installing a Mode-C transponder
in the USA, Europe of course is a different story with Mode-S. I
expect Mode-S transponder prices to fall in the USA, driven by
European adoption and increasing competition from vendors like Trigg.
I'd like to see/play with the Trigg Mode-S. All these transponders are
in a useful power consumption range for typical sailplane
installations. I personally would not use power consumption as the
differentiator to decide between them. But how important that is for
you depends on you glider power budget/flight profile etc. (do the
math).

And as a final reminder, measuring or understanding quoted power
consumption can be tricky, as it depends on the SSR interrogation
rate and may or may not include the encoder draw since the power for
the encoder is typically supplied through the transponder. Some
manufactures may quote a total power consumption with their specified
encoder. Initial encoder power consumption (i.e. what you will measure
if you just turn the unit on) is often dominated by the heater start
up current, which will be a lot higher than typical power
consumption.

Darryl