I just wanted to share the latest developments on the voltage control front.
First, I calculated the effects that different voltages applied to the heating element would have, then I proceeded to test the concept.
The original 12 volts provided by the battery generated 80 watts at the element, and 6.6 amperes, this heated the pitot to over 700℉.
By my calculations, 6 volts across the heating element would create 20 watts to be dissipated as heat, and a current of 3.3 amperes. To achieve this voltage I decided to use an additional identical heating element as a resistor, then check the temperatures.
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| Pitot heat still a bit high |
20 watts still turned into 300℉ at the probe, this was much better already, but an even lower voltage was required.
Digging through my parts bin I found a bigger resistance, capable of dropping the voltage even further to around 2.3 volts, good for about 2.7 watts at the tube.
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| Voltage reduced even further |
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| Pitot heat much cooler now |
This dropped the temperature even further to an almost cool 110℉. I am obviously zeroing in to the proper voltage value, and proved that the concept is sound. There is only problem with the resistance approach, it is very wasteful of the little electricity produced on board the airplane. Furthermore, the resistance used to control the voltage also heats up, and I’d be trading heat at the Pitot tube, for heat into the cockpit needing to be dissipated at the resistor.
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| While the Pitot heat in the nose is at 110℉, the resistor in the cockpit is at 277℉. |
The proper approach to voltage control in my case, is to build a step down voltage regulator.
Voltage regulators are “a dime a dozen”, but voltage regulators capable of handling a high current, are not all that common. The one I designed is built around a monolithic integrated circuit rated up to 3 amps, more than enough for what I am trying to do.
Unfortunately the components I needed are not popular enough to be carried at the local Radio Shack (guess how I know), so I ordered them online at mouser.com, and a week later I was able to finally test my new approach.
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| This looks like fun! |
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| Success at last! |
Testing with two 9 volts batteries in series
Testing with my car's 12 volts battery
The next step will be to produce a prototype circuit board, and test it against the heating element that will actually warm up the Pitot tube.
Another approach to controlling the heating would be just switching on and off the heating element with a certain duty cycle at a low-ish frequency(PWM control), that might be simpler than a switching regulator. A heater doesn't really care if it gets 20% power all the time or 100% power 20% of the time.
ReplyDeleteHey Carlo, you might enjoy the latest development... http://longezproject.blogspot.com/2016/06/voltage-regulator-part-5.html
DeleteThat's a very interesting idea Carlo. Clearly, I am not an electrical engineer, and I am experimenting a bit.
ReplyDeleteOne thing that I would like to achieve though, is minimizing the current I require at any one time. My electrical system will be tailored to the equipment on board, and limited in its capacity. Switching on the heat at full blast even part time, would draw the most current (around 7 Amps), even though it would switch off periodically. This load would have to be factored into the design by sizing the alternator, the circuit breaker, and the wiring accordingly. Ideally, I would like to use around 3 Amps, if possible.
Generally, the more the Amps I need, the bigger and heavier the alternator I'll have to buy, and all in a part of the plane I'd like to keep light, due to weight distribution issues.
But your idea has merit, and I shall investigate it further if my power supply concept miscarries.
Thanks for your comment.