Trig TT21 voltage requirements: No problem!
Eric Greenwell
requirements. The short answer is the TT21 functions properly well below
11 volts (as low as 9 volts), as it is a certification requirement. The
following is the reply I received from Andy Davis, Trig's chief
executive officer (posted with permission):
------------------
Dear Eric,
Thank you for your interest in the TT21.
I'll start by saying that although the brochure is meant as "useful
guidance material", the official answer is the one in the installation
manual, and the complete answer is in section 11, the environmental
qualification form, where is says that we are category BX for power
input. All you need to do is pull out your copy of RTCA DO-160F and
those little letters will tell you everything you needed to know.
Since the DO-160F document sells for $425, I'm going to hazard a guess
that you don't actually have a copy, and therefore I will give you a
quick tour of the relevant section.
In our certification category, three voltages of interest to you are
defined, called respectively "normal low voltage", "emergency low
voltage" and "abnormal low voltage". The "normal low voltage" is
specified as 11 volts - and that is where that number comes from in the
manual. The Trig transponder behaves completely normally at 11 volts.
The "emergency low voltage" is specified as 9 volts, and here's the good
news for you - the Trig transponder is designed and tested to continue
working normally in an emergency, so it also works fine at 9 volts. The
"abnormal low voltage" is specified as 6 volts; the transponder is NOT
intended to continue working normally at 6 volts, but it IS intended to
recover gracefully after a temporary blip that drops the voltage to 6
volts and returns it to 9+ volts.
So, cutting through the terminology, what does all this mean? The TT21
will operate normally providing the input voltage is 9 volts or above.
As the input voltage falls below 9 volts, you'll start seeing messages
on the screen that say one of "WARNING: TX Power Low" or "WARNING: PSU
Low". The screen will hold up, and the system will keep trying to tell
you that it's all going horribly wrong down to about 6 volts, but as
soon as you are seeing these messages, you should assume that ATC and
TCAS can no longer see you. The transition from full power output to no
output is fairly abrupt, and if you get one of these messages it means
that the transponder tried but failed to send a reply. It's time to
land - or switch batteries!
I hope this answers your questions.
Regards,
Andy Davis
Trig Avionics Limited
-------------------------
"Be Happy, Don't Worry" - the Trig TT21 is completely compatible with
our 12 volt systems.
--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly
* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* Sections on Mode S, TPAS, ADS-B, Flarm, more
* "A Guide toe Self-launching Sailplane Operation" at www.motorglider.org
Jim White
>on the screen that say one of "WARNING: TX Power Low" or "WARNING: PSU
>Low". The screen will hold up, and the system will keep trying to tell
>you that it's all going horribly wrong down to about 6 volts, but as
>soon as you are seeing these messages, you should assume that ATC and
>TCAS can no longer see you. The transition from full power output to no
>output is fairly abrupt, and if you get one of these messages it means
>that the transponder tried but failed to send a reply. It's time to
>land - or switch batteries!
>
>I hope this answers your questions.
>
>Regards,
>
> Andy Davis
> Trig Avionics Limited
>
>-------------------------
>
It's time to land, switch batteries, or fly in stealth mode!
cernauta
<flyg...@verizon.netto> wrote:
> The Trig transponder behaves completely normally at 11 volts.
I have a question, call it incontrollable curiosity...
Is the output power somehow related to the voltage of the supply?
In other words: with the Trig or any other modern xpdr, could we in
fact reduce the output, and thus the current drain, by deliberately
reducing the voltage of the supplied power?
Aldo Cernezzi
Eric Greenwell
The power regulator in the TT21, as with other modern transponders and
communication transceivers, compensates for a reduced input voltage by
*increasing* the current drain, ensuring the circuitry receives the
correct amount of power. Nominally, it will draw a constant power within
it's voltage range.
--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly
* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* Sections on Mode S, TPAS, ADS-B, Flarm, more
* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org