Of course that is another way of doing it, but it involves bonding the diode
thermally to the pitot tube, putting a reference diode somewhere in the same
general vicinity of the pitot tube to sense ambient temperature, insuring an
ambient airflow over the reference diode, an opamp to sense the 2.5
millivolts/°C change, and all of that stuff. I prefer dumb when I can get
away with it.

As I recall, the pitot heater gets about 50°C above ambient, so you are
messing around trying to detect a little over a tenth of a volt change,
which isn't rocket science, but trying to explain it in this newsgroup is.

Jim




"Richard Lamb" wrote in message
oups.com...
Hi Jim,

Question..

Why put the diode in series with the pitot heater power.
(Ok, it senses that power if flowing in the heater circuit, but is that
necessary?)

Couldn't the same thing be accomplished by using the diode as a
temperture
sensor (thermometer) to decide if the pitot tube was above a certain
temperature?

No self-heating on the part of the sense diode, and no pitot heat
failure if the diode
opens up (burns up?).

Richard