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Feb 21, 2008, 11:40 PM
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Align BL-35X worst-case analysis


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 overload indicator but everything stayed up (clearly there is some fine structure in current use at timescales below that of my DMM.) When I plugged into a 1/4A supply, I could get the AR6100 to undervolt (and learned I don't have fast reconnect, sigh.)

Not sure what to conclude. If we do a flight test and the ESC isn't super hot, then we might assume that the TO-252s are only 20C hotter than the heat sink. It'd be unlikely that I would ever move the sticks that fast in actual flight, and the real average current use has got to be much less than an amp (else Trexes would be falling out of the sky every day.) On the other hand, the safe play would be to get a 3A switching BEC. Of course, the safest play would be to never fly at all.
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Feb 22, 2008, 02:53 AM
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Jack Crossfire's Avatar
The story on HS-65's is here:

https://www.rcgroups.com/forums/showthread.php?t=584445

HS-65MG's on the cyclic need over 3A sustained & peak over 5A. After a crash due to servo lockup, got The Castle Creations BEC & had no problems. Carbonite gears just break in flight & shouldn't be flown at all.

Most of the thousands of T-Rexes in the world R probably flown once, crashed, & put away.
Mar 26, 2008, 03:48 PM
LaurenceGough's Avatar
That's a joke right? HS-65's need up to 5A?

http://www.readyheli.com/ProductDeta...EC%2D35020%2D6

Won't cut it then? Are you sure....?
Mar 26, 2008, 04:07 PM
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Originally Posted by Technex
That's a joke right? HS-65's need up to 5A?
It has been claimed that the stall current of a 65 is over an amp; I didn't test that. I installed a 3A ParkBEC in my Trex and haven't had any problems so far. The 2A Medusa BEC would probably be OK, but it's that much less margin.
Mar 26, 2008, 05:11 PM
LaurenceGough's Avatar
Could you read up here: https://www.rcgroups.com/forums/showthread.php?t=839771

Thank you.


Is there any real world proof that it can be that high?! And besides it would never stall in flight...
Mar 26, 2008, 06:10 PM
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Quote:
Originally Posted by Technex
Is there any real world proof that it can be that high?! And besides it would never stall in flight...
It could stall in flight if you get binding. I think the max current in the thread referenced by Jack above is from somebody at Hitec. I know a few people have claimed to have problems with a 1.5A ParkBEC. I think the CCBEC is cheaper than the Medusa and rated to 5A, so why take chances?


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