Any idea why the txpdr is on the non-essential bus? Seems in this day and age it should be on the essential bus should the # 1 alt and batt become inop. Any input?

I think (and could easily be wrong) that it sucks a lot of power and would greatly shorten the time available before blackout.

Remember that the transponder uses a fairly powerful transmitter - which is why it draws a lot of power.

On the SR22, the #2 alternator and battery are both small. I believe that concerns over the alternator output and battery life had to do with the decision. Garmin lists the power requirements for the 327 transponder as only 15 watts, but to know for sure you would have to know the draw of the other itmes and the exact output from the #2 alternator and the capacity of the #2 battery.

I believe that is correct. The current draw is too high for Alt 2.

The SR20 has the transponder on the essential bus. The electrical load chart shows the transponder as having 0.1A draw - insignificant.

Although the transponder has a high peak power output, the transmission time is very short, so the average power consumption is quite low (and the transponder’s power supply stores enough energy that the peak power consumption would not be reflected in current draw from the bus).

So, it’s puzzling why the transponder is not on the SR22 essential bus.

It seems odd that the transponder’s current draw is so low given how hot it gets.

I don’t find it gets hot. More than likely yours is just being warmed by all the other gear around it. The radio stack collectively emits a bit of heat.

In my experience and what I’ve been told by avionics shops is that the transponder does run hot.