Jerry 5TJ Posted September 5, 2016 Report Share Posted September 5, 2016 2000 Ovation Pitot Heat Question Flying in light icing conditions near Rock Springs WY yesterday with OAT -1C I had pitot heat switched on. And the TKS in operation. Everything going fine, the protected areas were clear and ASI normal. But after a few minutes I for some reason happened to touch the area under the panel lower edge just above my left leg. I felt a metal box or bracket that was quite warm, too warm to touch for more than a few seconds, so I guess it was at least 50-60C. I aimed some air from the left vent at the hot spot. A few minutes later and out of the clouds I turned off the pitot heat. The metal cooled off rapidly. i took a picture of the area. It is an aluminum bracket with some electrical component bolted on the top, non-visible side. Anyone know what this is, or have a schematic for the pitot heat? Is it a thermal switch or current sensor? And should it be so hot in normal operations? The part is marked 802338-501 but that is just the metal bracket number. Quote Link to comment Share on other sites More sharing options...
carusoam Posted September 5, 2016 Report Share Posted September 5, 2016 Looks like some pretty heavy wires attached to it, or is that a photo oddity? Heavy wires have a tendency of delivering more electrons over a period of time. I'm interested in the answer as well... Best regards, -a- Quote Link to comment Share on other sites More sharing options...
Cruiser Posted September 5, 2016 Report Share Posted September 5, 2016 the proper part # is 820338-501 it is a bracket mount for the diode. I think it is part of the heater control curcuit Quote Link to comment Share on other sites More sharing options...
carusoam Posted September 5, 2016 Report Share Posted September 5, 2016 Following that logic... the large current flowing through the resistance of the diode is generating heat. A diode is essentially a one way valve for electrons. It has much higher resistance to electrons flowing one way than the other... Follow-up question... What is the purpose of this particular diode, why is a diode being used in this circuit? There is so much to know about a GA plane... Best regards, -a- Quote Link to comment Share on other sites More sharing options...
Jerry 5TJ Posted September 7, 2016 Author Report Share Posted September 7, 2016 The M20R service manual shows a simplified schematic of pitot heat connections. The PH502-24 pitot assembly is powered through a box called "heat control relay" itself energized by a 20 amp switch-breaker. Mention is made of lights on the annunciator panel but their connections aren't shown. All this is presumably shown on the system schematic but so far I've found them inscrutable on screen. I'll have to take the file to Kinkos and have a large print made. There is no mention of a diode, nor of why this pitot heat would need a diode in operation. Thanks for the part number correction--my typo. Yes, that is the bracket/ heat sink. I will get my head up in there and have a look at this mystery circuit later this week. Quote Link to comment Share on other sites More sharing options...
carusoam Posted September 7, 2016 Report Share Posted September 7, 2016 Dis you see the note in the lower left corner regarding the dropping resistor to keep the stall vane warm. That sounds like a source of heat that might be nearby..? I would have to research if my ship has a heated stall vane...? Or is that for ships with FIKI TKS? Best regards, -a- Quote Link to comment Share on other sites More sharing options...
Jerry 5TJ Posted September 11, 2016 Author Report Share Posted September 11, 2016 The metal bracket which gets warm is the heat sink for a diode bridge. I managed to get a camera up in there today and got a picture of the component. You can see the two wires go to the diode bridge. The diode bridge is NOT shown in the simplified schematic from Mooney. Aircraft +28V power goes to the bridge, and then to the pitot heat switch. From the pitot heat switch there are two circuits, one to the pitot and the other to the heated stall warning vane. It appears to me that the way the bridge is wired there are TWO diode forward drops in the path to both the stall warning heater and the pitot heater. Two Vf drops in a silicon PN junction is 1.4 to ~1.8 volts depending on current. The current specification is about 7 A for the pitot heat but I have no info for the stall warn heat. It could be 10-15 amps total and that times Vf would explain the heat generated. The stall warn dropping resistor (seen on schematic) is not on this heat sink. Quote Link to comment Share on other sites More sharing options...
carusoam Posted September 12, 2016 Report Share Posted September 12, 2016 Nice follow up details, Jerry. Thanks for sharing. Best regards, -a- Quote Link to comment Share on other sites More sharing options...
milotron Posted September 12, 2016 Report Share Posted September 12, 2016 Diodes are often used like this ( one direction of current flow ) to prevent inadvertent energization of other circuits depending on the switching logic used with the relays, especially if that current relay noted for the pitot heat circuit is solid state. With the bracket and everything else related to this, it is clearly meant to be there and not a band aid fix for some other issue. Quote Link to comment Share on other sites More sharing options...
Jerry 5TJ Posted September 12, 2016 Author Report Share Posted September 12, 2016 3 hours ago, milotron said: Diodes are often used like this ( one direction of current flow ) to prevent inadvertent energization of other circuits depending on the switching logic used with the relays, especially if that current relay noted for the pitot heat circuit is solid state. With the bracket and everything else related to this, it is clearly meant to be there and not a band aid fix for some other issue. To this retired EE the diodes in series with the pitot heat "smells" like a design band-aid. The PH502 series is available in 12, 24, and 28 volt versions so perhaps Mooney installed the wrong part and patched up that error with diodes? Maybe the Vf drop vs I curve helps limit inrush to the positive temperature coefficient heating element? The relay coil is not connected to this circuit, and if it were included for back EMF protection they wouldn't need a beefy diode and heat sink. Given the temperature of the heat sink bracket I'd take a professional guess that the thermal design is inadequate. I will take some V and I measurements on this kluge when I get back to the home aerodrome. Quote Link to comment Share on other sites More sharing options...
gun986 Posted May 14, 2021 Report Share Posted May 14, 2021 Did you ever figure this issue out? I have the same thing on my Ovation... oddly enough also figured out in icing over WY. Thanks! Gunnar Quote Link to comment Share on other sites More sharing options...
carusoam Posted May 18, 2021 Report Share Posted May 18, 2021 Let’s see if @Jerry 5TJ is around.... I think Gunnar is looking for some help, you may be familiar... Best regards, -a- Quote Link to comment Share on other sites More sharing options...
Mark89114 Posted May 18, 2021 Report Share Posted May 18, 2021 It must be normal as my 2000 ovation does the same thing. Quote Link to comment Share on other sites More sharing options...
Jerry 5TJ Posted May 18, 2021 Author Report Share Posted May 18, 2021 On 5/14/2021 at 7:09 PM, gun986 said: Did you ever figure this issue out? I have the same thing on my Ovation... oddly enough also figured out in icing over WY. Thanks! Gunnar No, Gunnar, I never figured out why the diodes are in the circuit, or why the heat sink is inadequate. The only time that we notice the hot bracket is in icing conditions with both the pitot and stall warning heat on. Perhaps those diodes are to keep the lower part of the instrument panel ice-free? 1 Quote Link to comment Share on other sites More sharing options...
gun986 Posted May 18, 2021 Report Share Posted May 18, 2021 (edited) 36 minutes ago, Jerry 5TJ said: No, Gunnar, I never figured out why the diodes are in the circuit, or why the heat sink is inadequate. The only time that we notice the hot bracket is in icing conditions with both the pitot and stall warning heat on. Perhaps those diodes are to keep the lower part of the instrument panel ice-free? Hi All, Thanks for the help with this... I ended up replacing the Diode Bridge but that didn't change the amount of heat that bracket absorbs. The good news is the instrument panel continues to be ice-free! Edited May 18, 2021 by gun986 Quote Link to comment Share on other sites More sharing options...
carusoam Posted May 19, 2021 Report Share Posted May 19, 2021 MS word of the day... Kluge... sometimes spelled Kludge.... often used as Kludgy... -a- https://en.wikipedia.org/wiki/Kludge Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.