Re: T Rex build question
- From: "Doug McLaren" <dougmc+usenet-20070801@xxxxxxxxxxx>
- Date: 01 Aug 2007 14:56:04 GMT
In article <9CVri.18777$rH6.8404@xxxxxxxxxxxx>,
Chuck <Chuckk2nospam@xxxxxxx> wrote:
| There are potential issues with receiver and servo power from an
| esc. Voltage-- Some servos don't like 6 vdc, and are happiest on
| about 5 vdc
Actually, all (except maybe a few of those that go with 1 cell lipo RX
packs) servos are *perfectly* happy with 5 or 6 volts. The reason
that 6 volts has a bad rap is that when you take 5 NiCd cells and put
them in a pack, if you multiply 1.2 volts * 5, you get 6 volts, but in
reality when the pack is fully charged you get 1.4 volts * 5, or 7
volts, and that's when some servos have problems. Most are fine, but
a few have problems right at seven volts.
Now, the vast majority of ESCs out there that provide power to the RX
have a voltage regulator that drops the voltage down to 5 or 6 volts.
The only exceptions are a few car ESCs that assume that your radio
gear can handle the 8.4 volts (1.4 volts * 6) that the battery can put
out. But I've never seen an airplane/helicopter ESC that provided
power to the radio gear that didn't have a voltage regulator or
something else to drop the voltage if it actually did provide power to
the radio gear. I guess it would be possible for something designed
for 5 or 6 cell NiCd/NiMH or 2 cell LiPo packs, but I've never seen
one.
| High servo current draw when there are more than 3 servos, or one or
| more of the servos is a high speed version (example gyro servo).
.... or one of the servos is stalled. Stiff linkages are very bad
things for BEC circuits!
| The max current draw can exceed the safe current capability of the
| esc (usually about 1.5 amp for the receiver and servo voltage)
That varies from ESC to ESC. It's mostly a matter of cooling.
| If a LiPo battery is used for power, and has a built in safety
| shutdown, you may loose control if it actuates due to low battery
| voltage or high current draw.
That actually has nothing to do with LiPos, unless you've actually
found some used for R/C usage where the shutoff is actually in the
battery itself -- which is very rare. Many ESCs do have shutdown
circuits that will cut power if the temperature goes too high, but as
far as I know they all cut power only to the motor, not to the RX. If
your servos are drawing too much power and that causes the BEC part to
emit too much heat, then the motor will shut off as that heat affects
the rest of the ESC, but the BEC will not, and eventually it may
release it's magic smoke. Yes, a protection circuit could be added,
but turning off your RX generally means a crash, so they'd rather push
the ESC a little too hard and risk damaging it than cause a certain
crash.
The reason that most BECs have so much trouble with high voltages and
high currents is the way they work. Most BECs use simple voltage
regulators and waste a lot of power. If your servos are drawing one
amp, and your pack puts out 12 volts, and your BEC puts out 5 volts,
then you're drawing 1 amp from the battery, and (7 volts * 1 amp) of
heat is being generated in the BEC, which is a lot for something so
small.
Things like the Ultimate BEC use a DC-DC converter and are a lot more
efficient. In the case given above, if the servos draw 1 amp at 5
volts, the UBEC will draw about 0.4 amps from the battery at 12 volts
and create little heat.
| > Forgive me if this is a stupid question, I'm just putting together
| > a T Rex 450 and was just starting to connect the radio gear to
| > test the servo movements, when if dawned on me I haven't set up a
| > power supply for the receiver which is a JR R770, but when I plug
| > in the esc it appears to power up the receiver, is this correct or
| > should I be connecting in a separate power direct to the batt port
| > of the receiver ?.
It's not a stupid question at all.
The ESC's BEC may be adequate as a power supply for the RX and servos.
But if it gets too hot, you might want to cut the red wire from the
ESC to the RX pack, and either use a separate RX pack or get something
like a UBEC. The UBEC is only one of many different brands of DC-DC
converter meant for R/C use. Really, it's a little switching power
supply, like the one in your computer.
Of course, a separate RX pack is more weight, and another battery that
needs charging and one you can easily forget to charge, especially
since it won't need charging after every flight. I'd suggest just
getting something like a UBEC if you need to go this route. But your
ESC's built in BEC may very well be adequate for a smaller helicopter
like a Trex 450 -- but it's going to be close.
--
Doug McLaren, dougmc@xxxxxxxxxxx
You can only live once, but if you do it right, once is enough.
.
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