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Archive for February, 2008
Posted by ccdengr | Feb 24, 2008 @ 05:25 PM | 5,348 Views
After some cogitation, decided that I'd put a Dimension Engineering SportBEC on the Trex. I had an unused ParkBEC, but I figured that its 1.5A limit was a little too tight (after all, if I ever using a lot of servo current, it'll probably be in a situation where I don't need the additional complication of a Spektrum lockout ). The thing to note about a switching regulator is that it'll shut down based only on the current, not as part of thermal shutdown like a linear (see, for example, the National LM25575, rated for 1.5A and will limit at 1.8A). There have been a few reports that the ParkBEC can't handle HS-65 loads.

A CCBEC would have been a little cheaper and is rated for twice as much current (7A with <12V input, but they assume 5 mph airflow), but there have been a few complaints of QC problems with this converter, and I didn't need the hassle of needing a Castlelink to set the output voltage.
Posted by ccdengr | Feb 21, 2008 @ 11:40 PM | 6,399 Views
I put my Trex together for a budget of $300, not counting the Telebee gyro and AR6100 receiver that I had left over from a previous project. So I got a 450S kit with the Align lipo, motor, and 35X ESC with 2A 6V BEC. I figured that the 6V gave me some margin against undervolts on the AR6100.

Doing a simple WCA didn't look too good, though. I have HS-65HBs on the swash (yeah, I know, karbonite gears ) and an HS-81 on the tail. Suppose, per Servormances data, that each HS-65 has a current of 300 mA and the HS-81 is 300 mA at 6V. Then if everything was running at the same time that'd be 1.2A total. Each of the two regulators in the 35X would be sourcing 0.6A; suppose the battery voltage was 11.5V, so each would be dissipating (11.5-6)*0.6 = 3.3W. Suppose the regulators are something like TO-252 LM317s with a thetaJC of 12 C/W and a thetaJA of 103C/W. Suppose through copper board area they get the thetaJA down to 50 C/W (could be even better, but I don't want to guess at the cooling from forced convection coming off the main gear fan). Then each regulator would be 165C above ambient, and we'd be SOL, since they shut down at a junction temp of 150C if not lower.

So I figured some bench testing was in order. Running the receiver and servos off a lab supply, I measured about 60 mA idle, about 300 mA peak with one servo moving, and about 1.1A stirring the collective as fast as I could so all four servos were moving. With a 1A current limit I was flashing the...Continue Reading
Posted by ccdengr | Feb 19, 2008 @ 10:48 PM | 5,005 Views
I'm going through the usual helicrack progression: Havoc Heli (Apr 07), CX2 (Jun 07), HBFP (Jul 07), King 2 (Aug 07), Compy FP (Nov 07) and Trex 450S (Jan 08).

I'm an avionics engineer by trade, and I've become interested in the various behavior and limitations of things like ESCs, HH gyros and Spektrum radios. At one point I was interested in autonomous helicopters, but got sidetracked by how much fun helis are to fly manually.