Verification that Heated Pitot is working

Question to you electrical designer gurus:

Here is what I would like to do and Jim Wier suggested that I post this
here. I want an indicator on my Cozy homebuilt airplane to show me
that my heated pitot is actually working when I turn on the switch on
the instrument panel. So, the light will go out (with the switch still
in the on position) if the heated pitot stops working for some reasons
(but not because it tripped the circuit breaker). So, how can I build
such a device or circuit? I would appreciate specific parts or
identification of parts as I am an amateur. This is a 12 volt DC
system and the heating element draws 7 amps. I know I can not use a
LED in series because it would blow the milli-second I turned the unit
on. I know I can't use a light or lamp in parallel because it would
not indicate if the heated pitot was on or off. So that is my dilemma.
Please help.

Reply to:

iflycozy wrote:

Question to you electrical designer gurus:

Here is what I would like to do and Jim Wier suggested that I post this
here. I want an indicator on my Cozy homebuilt airplane to show me
that my heated pitot is actually working when I turn on the switch on
the instrument panel. So, the light will go out (with the switch still
in the on position) if the heated pitot stops working for some reasons
(but not because it tripped the circuit breaker). So, how can I build
such a device or circuit? I would appreciate specific parts or
identification of parts as I am an amateur. This is a 12 volt DC
system and the heating element draws 7 amps. I know I can not use a
LED in series because it would blow the milli-second I turned the unit
on. I know I can't use a light or lamp in parallel because it would
not indicate if the heated pitot was on or off. So that is my dilemma.
Please help.

Reply to:



Possible thermister bonded to the pitot with a second one insulated from it
but with basically the same air flow over it. Use a differential opamp to
measure the difference of output from each thermister use the opamp to
drive a simple transister amp to drive a led or lamp. I'm at work so I
can't look up specific parts and you'd need to do some testing anyway.
John

"iflycozy" wrote ...
I want an indicator on my Cozy homebuilt airplane to show me
that my heated pitot is actually working when I turn on the switch on
the instrument panel.


The answer is easy, you just look at the ammeter. If it jumps when you turn
on the switch, the heater works.

The far more important question is: why would anyone fly a cozy into icing
conditions? Disturbing the airflow over a canard with ice is just asking
for controllability problems.

Rich

Richard Isakson wrote:


The far more important question is: why would anyone fly a cozy into icing
conditions? Disturbing the airflow over a canard with ice is just asking
for controllability problems.

Rich

Why would you have a heated pitot on a Cessna 172 or a piper arrow then?
Neither of THEM are certified for entry into known icing conditions, but
its nice to have the ability to safely fly OUT of them if you get INTO them.

Dave

"Dave S" wrote ...
Why would you have a heated pitot on a Cessna 172 or a piper arrow then?
Neither of THEM are certified for entry into known icing conditions, but
its nice to have the ability to safely fly OUT of them if you get INTO
them.

Dave,

A 172 can take a bit of icing and get away with it. Trust me and please
don't ask any questions I won't answer. In selecting a canard type
aircraft, the owner has to realize that there are some huge differences in
the way the horizontal controls react to airflow disturbances. In the case
of icing, he needs to take extreme measures to avoid those conditions. Even
if that means grounding the airplane for an extended period. That's what he
bought into when he got the airplane.

Having said that, if anyone knows of a canard icing study that says
differently please point me to it.

Rich

Richard Isakson wrote:

Having said that, if anyone knows of a canard icing study that says
differently please point me to it.

While I won't disagree with you that icing conditions are certainly to
be avoided in canard aircraft, probably more than in conventional
aircraft, I will say that I know two folks that have flown in icing
conditions, collected some ice on the canard, and had minor issues.

One of those people is me, during my instrument lessons near
Schenectady, NY in April of this year. I got about 1/16" - 1/8" of ice
on the canard, and my stall speed went up about 10 Kts. I flew around
under the clouds for one circle of the pattern, the ice melted, and we
landed. Of course, having the stall speed go up and not realizing it is
far less dangerous in a canard, since I can't stall the whole plane or
spin it - I noticed the speed difference because the nose started
bobbing at 90 mph in the downwind to base turn.

Another aquaintance with a COZY has had ice up to 1/8" - 1/4" on his
canard, and he says about the same thing.

That being said, I still agree that icing conditions are to be
strenuously avoided - 1/8" of ice is certainly not a lot, and I wouldn't
want to be experimenting with more.

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2005

The parts you will need are a 10 amp silicon diode, a 10 kilohm quarter watt
resistor, a small PNP transistor, a LED of the appropriate color, and a 470
ohm quarter watt resistor. All these parts are available from Mouser
Electronics, DigiKey Electronics, and the rest of the usual suspects.

The silicon (NOT schottky, plain silicon) diode goes in series with the wire
going to the pitot, anode to switch, cathode to pitot. THe diode will
probably require a heat sink.

THe emitter of the PNP transistor goes directly to the anode of the diode.

One lead of the 10K resistor goes to the cathode of the diode. THe other
lead of the resistor goes to the base of the transistor.

One lead of the 470 ohm resistor goes to the collector of the PNP
transistor. The other lead of the resistor goes to the anode of the LED.
The cathode of the LED goes to airframe ground.

HOW IT WORKS: When the pitot switch is turned on, 7 amps through the diode
will result in about 1 volt drop across the diode. A PNP transistor turns on
with 0.5 volts from base to emitter, so the transistor will turn on through
the 10K current limiting resistor. When the transistor turns on, it flows
current through the 470 ohm current limiting resistor to the LED.

If the pitot heater fails there will not be any significant voltage drop
across the diode and the PNP transistor will not turn on and the LED will
not light.

If you can't find a suitable 10 amp diode, a 0.1 ohm 10 watt resistor will
do the same job and will probably be cheaper. In either case, I suspect
that the pitot heater will be more reliable than the diode OR the resistor,
so a false heater-out alarm will be the predominant failure mode.

Be careful where you mount the diode or the resistor. Under normal
operating conditions, they will get hotter than hell.

Jim







"iflycozy" wrote in message
ups.com...
Question to you electrical designer gurus:

Here is what I would like to do and Jim Wier suggested that I post this
here. I want an indicator on my Cozy homebuilt airplane to show me
that my heated pitot is actually working when I turn on the switch on
the instrument panel. So, the light will go out (with the switch still
in the on position) if the heated pitot stops working for some reasons
(but not because it tripped the circuit breaker). So, how can I build
such a device or circuit? I would appreciate specific parts or
identification of parts as I am an amateur. This is a 12 volt DC
system and the heating element draws 7 amps. I know I can not use a
LED in series because it would blow the milli-second I turned the unit
on. I know I can't use a light or lamp in parallel because it would
not indicate if the heated pitot was on or off. So that is my dilemma.
Please help.

Reply to:

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

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

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