From here
Graham wrote:
For what it’s worth (or not), the main barrier to putting a Mode S transponder into the PA-17 (no proper electrical system, just a 12v battery powering a single com radio) is the requirement to operate it at all times. I would want to turn it off to conserve the battery when well away from controlled airspace. Thus Mode S is essentially unworkable for us.
I would expect a mode S transponder to use less energy than a mode C one, as it doesn’t need to reply to every interrogation?
The battery I use is only 7Ah. I don’t know what a Mode S transponder draws, but I shouldn’t imagine it is trivial.
Airborne_Again wrote:
I would expect a mode S transponder to use less energy than a mode C one, as it doesn’t need to reply to every interrogation?
Mwah. A mode S transponder will have to reply to the broadcast requests plus requests targetted to it. Plus, I believe it is mandatory for a mode S to also squawk mode A/C?
GTX330 is under 2A normally. 24V…
Would probably look at the small form-factor Funke offering to match our radio. Doubt there is much of an appetite for it though.
I’ve got a controlled airspace crossing negative transponder before now, they just have to think a bit and pay attention!
lionel wrote:
Mwah. A mode S transponder will have to reply to the broadcast requests plus requests targetted to it.
Yes, but the broadcast requests should be a whole lot fewer than with mode A/C,
Plus, I believe it is mandatory for a mode S to also squawk mode A/C?
That’s true, and when mode S isn’t compulsory the radar has to send mode A/C interrogations as well. If mode S is required, then there is no need to interrogate mode A/C.
Graham wrote:
The battery I use is only 7Ah. I don’t know what a Mode S transponder draws, but I shouldn’t imagine it is trivial.
The Garmin GTX335 on a 12v system requires a 3 amp circuit breaker. The maintenance manual for the GTX335 gives the (non-GPS version) a typical current of 570mA and maximum of 860mA on a 12 volt system. With the built-in GPS and ADS-B out this rises to a max of 1.22 amps. Given a battery in good condition you should be able to power something like a GTX335 for your full fuel endurance. There are mode-S transponders with less powerful transmitters (the GTX335 peaks at 200W TX power, from memory), which should have a lower typical/max current values.
alioth wrote:
The Garmin GTX335 on a 12v system requires a 3 amp circuit breaker. The maintenance manual for the GTX335 gives the (non-GPS version) a typical current of 570mA and maximum of 860mA on a 12 volt system. With the built-in GPS and ADS-B out this rises to a max of 1.22 amps. Given a battery in good condition you should be able to power something like a GTX335 for your full fuel endurance. There are mode-S transponders with less powerful transmitters (the GTX335 peaks at 200W TX power, from memory), which should have a lower typical/max current values.
This is true, but the lead-acid batteries used are not really designed to be deep-cycled and won’t last long if they are. I also don’t really want to be carrying a charger about with me to put the battery on to charge before the return flight. My battery just gets a minimal dent from the radio each time I fly and then when I get home it goes on the smart trickle charger. This is close enough to the intended use of the battery (motorcycle or burglar alarm backup) that it will last.
If I were to have a radio and transponder in a plane without a generator, I’d switch to a LiFePO4 battery.
The Trig TT21 (12V) draws less than 0.3 A active on average. The transmitting power is 130W, which should mean around 10A if I’m not mistaken (sounds like a lot though ?)
