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| Gregor99 | Elite Veteran - Location: Western Wa - |
The Spektrum AR6100 only pulls 24ma. Also note that many low end BECs rate thier max current at 2S. With 3S the max current goes down, by quite a lot. This is because the built-in BECs are linear and working harder the more have to reduce the voltage. 3S has higher input voltage so the BEC has to work harder. Switching BECs are much better and not prone this limitation.
So when we say things like we had to retire the 3a BEC and move to a 10a BEC, its not because we are drawing close to 10a. Its because the 3a BEC is overated and is acually good up to 1a, and we are pulling 1.5a. It just so happens that the 10a CC BEC is a good buy even though its MASSIVE overkill. When in reality you could probably safely run with a 2a ParkBEC that is a switching BEC and could safely supply a full 2 amps. I'm currently using the $12, 3a HobbyWing and its working very well. Its small, inexpensive and easily configurable. I'm running it at 6v, which gives my servos another .4ms of speed. A lot cheaper than buying new servos that are .4 faster.
When my 3a BEC hit thermal overload at 1.3amps, I looked into the matter in some detail. Read this thread for all the specifics.
http://www.runryder.com/helicopter/t405751p1/
Here's an excerpt from the with the relevant details.
The idle current is the device powered up but no Tx input and no load applied. The loaded current is the device with Tx input and the movement almost stopped by manually holding the servo arm. BEC voltage from the ESC is 5 volts.
Device.....................Current (Idle)....Current (Loaded)
================================================================
AR6100 (rx)....................................24ma.............24ma
2100t (gyro)...................................66ma..............66ma
HiTec HS-81 Analog tail servo....36ma............425ma
Eflight S75 Analog Servo...............6ma............125ma
JR S285 Digital Servo.................11ma............275ma
Interestingly the HiTec results are quite a bit higher than listed on the servo city page (8ma and 220ma respectively). Maybe mine is bad. It still works but the tail has never been all that perfect. Since this one servo is consuming about 30% of the entire BEC current load, I might be able to help things by upgrading to a better servo. If only I could find the current draw for the new LogicTech. RcCarguy, care to measure? Its moot a point anyway since a proper BEC will solve this in a much more complete way.
Now on to comparing my old setup to my new setup. As you can see the new servos have contributed quite a bit to overloading my ESC's BEC. I had guessed at the difference and thought it would be closer. I had heard once that digital servos draw less current. Well, I think it really depends on the servos. The HiTec analog tail servo draws almost twice as much as the digital JRs. But the el cheap-O Eflights draw less than half of the digital servos. Interestingly in the current competition, the S75s come out on top. That's not an endorsement, just and interesting observation. So to summarize here are the numbers comparing the two setups on my heli.
Setup with the S75 Servos
Idle..........144ma
Loaded...890ma
Setup with the S285 Servos
Idle..........159ma
Loaded...1340ma (1.34 amps)
Next up, what to do about this. I am with Tryan and would prefer to keep the BEC and ESC together. There's less wiring, its easier to mount and keep it away from the other electronics. However, that means replacing my ESC which is more costly than getting a stand alone BEC. Plus, there's no guarentee that a new ESC will operate at its rated BEC current and I'll be in the same state I'm in now. For that matter, we don't know if a standalone BEC will operate at its rated current so its a gamble either way. I think Tryan is on to something with the Align comments and I suspect thier ESC ratings are not quite the same work of fiction that we see with ELE. I'm tempted to dump the ELE and go for CC or an Align. But, it may always be good to have a seperate BEC if nothing more than a little current insurance.
In the BEC catagory I've narrowed it down to these:
Castle Creations 10amp. Overkill yes. Still the price is pretty low considering its a CC product. Output voltage from 4.8 to 9 using the Castle Link. If you have one already, great. Otherwise its not good option unless you are wanting to invest in the CC line.
http://www.readyheli.com/Castle_Cre..._p/csem0005.htm
Align 3a - If it's really 3a then it will be plenty. However Align uses a linear supply which makes it less efficient, more prone to under performance at higher voltage levels, but it will be electronically quieter. Output voltage is 5.8V unless you use the included step down adapter which is needed for servos such as the Hitec HS-50 and Futaba S3154.
http://www.readyheli.com/K10336TA_3..._p/k10336ta.htm
HobbyWing 3a - Great value for a switching BEC. If it really is 3a or even 2a that will be all I need. Output: 5V/3A and 6V/3A switchable (how? it doesn't say. Doug can you tell us?)
http://www.helidirect.com/product_i...roducts_id=4405
ParkBec 1.5a - This looks nice but unless its underated, I'll be very close to the maximum current with my setup. The MFG helps reduce the BEC setup by having you plug the ESC into the BEC. Most likely the RED pin from the ESC is just disconnected inside the BEC. This is not a huge deal for me as I don't mind taking the red pin out of the ESC plug, had to anyway to run the current tests. Its just a little more plug and play. Output is fixed its at 6v so it won't be suitable for Hitec HS-50,Futaba S3154 or any other servo that cannot operate at 6v.
http://www.helidirect.com/product_i...roducts_id=4536
SportBec 3.5a - The next model up from the ParkBec. This is overkill for what I need but there is a product from this MFG that is between the two. Output is selectable between 5V/6V using a small slide switch.
http://www.helidirect.com/product_i...roducts_id=4550
Canton MO backyard flying club
West Coast Chapter |
| 07-25-2008 11:04 PM | | | |
