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ESC cutoff - 12/4/2002 10:39:39 PM   
00hex



Posts: 256
Joined: 6/7/2002
From: MD
Status: offline 		I am designing a circuit that includes an ESC. At what point should the ESC shut off the motor to protect the (7 or 8 cell) battery? It seems like it should be around 1.2V/cell but the high current may pull the voltage down somewhat.
I looked at a few ESCs at Tower Hobbies but most of them don't say. For the Great Planes C-5 and C-10 it's 3.7 volts - that sounds WAY too low! (could be a mistake)
       Post #: 1

ESC cutoff - 12/6/2002 2:36:00 AM   
DGREVE



Posts: 10
Joined: 10/13/2002
From: TX
Status: offline 		I believe the esc cuts off motor at 4.8 volts, leaving the rest of battery power for control servos.

(in reply to 00hex)
       Post #: 2

ESC cutoff - 12/6/2002 6:18:39 PM   
00hex



Posts: 256
Joined: 6/7/2002
From: MD
Status: offline 		Hmmm... that sounds better. The voltage will probably go back above 5V without the motor drawing all that current from it (I think it will anyway). BECs seem to frequently use 5 volt linear regulators, which need at least a little extra voltage to work properly (5.1, 5.2 is probably OK). Without this they will work, but they won't supply a proper 5 volts. On the other hand servos and receivers are used to having a power supply which would drive more complicated electronics crazy.

(in reply to DGREVE)
       Post #: 3

ESC cutoff - 1/26/2003 1:28:22 PM   
PK



Posts: 7
Joined: 1/25/2003
From: AU
Status: offline 		Hi 00hex,
Batteries should not be droped in voltage to below aprox .9volts per cell; therefore for a 7cell pack your cut off should operate around the 6.5volt mark, and 8 cells aprox 7.2 volts. This will also give you the extra voltage needed for the regulator. The LM7805 will need 7 volts to suply the 5 volts for the reciever and is alright to be used for 8 cells. You will need a low dropout version like the LM2940ct-5 if you want to use it for 7 cells.
Hope this helps
Phil

(in reply to 00hex)
       Post #: 4

ESC cutoff - 1/26/2003 4:16:19 PM   
00hex



Posts: 256
Joined: 6/7/2002
From: MD
Status: offline 		Thanks for the info PK!

I already had it set to 7 volts just to be safe (using 7 cells). I think I'll leave it because i don't know how precise it is. The ESC's electronics have a LP2986 i think and the servos run off a 2940, both low dropout. The 2986 is a nice little 8 SOIC that takes up half the space of a 7805 even with a couple of capacitors.

(in reply to PK)
       Post #: 5

ESC cutoff - 1/26/2003 8:51:12 PM   
FHHuber



Posts: 4931
Joined: 1/1/2003
From: gone, , USA
Status: offline 		I'm not sure why the commercial ESC's use a 4.8 cutoff... it really is low for most applications. It might be that these units CAN be used with 4 cell packs at times, and they want the motor to run in those cases.

If It were up to me... I'd have a DIP switch set (or multiposition dial switch) to select the number of cells in the pack being used, then you could have the low voltage motor cutout be correct for every pack you use. It would add a bit of weight and complexity to the ESC... Since you are making your own ESC, this is something to consider trying.

Note on powered glider rigs... the ESC's low voltage cut-out doesn't prevent you from draining a pack completely. If you get up in a thermal and stay up long after the motor cuts out, you can drain the pack right on down to essentially 0.0 (and lose control...) The motor cut-out means LAND NOW you are already into the danger zone for the power curve of the battery. (this will still be true with your 7.0 v cut-out...)

(in reply to 00hex)
       Post #: 6

ESC cutoff - 1/27/2003 1:50:15 AM   
PK



Posts: 7
Joined: 1/25/2003
From: AU
Status: offline 		Hi 00hex,
Sounds good, but why did you need two regulators? (Ie 1 for esc & 1 for servos). Is the rest of the esc surface mount too. I have built a few but I haven't had a go at surface mount yet. Sounds interesting. I would love to give it a go, If you are willing to share your design, can I get a copy? My e-mail is stuff@dodo.com.au
best regards Phil

(in reply to FHHuber)
       Post #: 7

ESC cutoff - 1/27/2003 7:20:11 PM   
00hex



Posts: 256
Joined: 6/7/2002
From: MD
Status: offline 		There's a bunch of other stuff not just an ESC. There is a 433 MHz receiver (instead of an ordinary R/C one), a couple of Atmel AVR microcontrollers, and an accelerometer (tilt sensor). Once I get the rest working I will add a transmitter for telemetry and maybe a GPS. These parts, the accelerometer especially, require a very stable and noise-free power supply. I don't have a schematic for the whole thing, but I copied the main bit of the ESC off a website somewhere. I'll see if I can find it.
Almost all of it is surface-mounted. I used 0603 caps and resistors wherever possible - about 1/16" by 1/32". The connectors, 1 capacitor, and the MOSFET are the only through-hole parts.
The ESC is working nicely but the rest needs work - radio range is around 30 feet! I think I might know why at least (software is doing something silly).

(in reply to PK)
       Post #: 8

ESC cutoff - 1/28/2003 9:10:37 AM   
PK



Posts: 7
Joined: 1/25/2003
From: AU
Status: offline 		00hex, sounds very technical. Its a bit out of my league. What are you using it for?
Phil.

(in reply to 00hex)
       Post #: 9

ESC cutoff - 1/28/2003 5:40:29 PM   
00hex



Posts: 256
Joined: 6/7/2002
From: MD
Status: offline 		It's an autopilot. When it crashes I can say "that wasn't me, it crashed itself!"

Here's how the ESC works:
The + wire of the motor goes to the + side of the battery. The motor's negative side is connected to the MOSFET drain. The S (sink? I forget) pin goes to ground. The gate receives the PWM signal through an optional 50 ohm resistor. There is also a 33K resistor from gate to ground.
Generating the PWM signal is the hardest part. The easiest way is to use a microcontroller.

(in reply to PK)
       Post #: 10

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