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Opto isolating BESC30-R3 PWM

(Etienne Demers) #1

Hi guys,

I am wondering if you have ever tried to opto isolate the pwm signal going to BESC30?

I am presently working on a design where the power and control side will be isolated with an ISO DC-DC converter and I realized that the BESC-30 signal will be reconnecting my 2 “grounds”.

Cheers,
E.

(Jacob) #2

We have not tried it @etienne, but it should work.

(Etienne Demers) #3

Would you happen to know how much amp per pwm output it can push?

This is my intended design

(Jacob) #4

I’ve got no idea, but the schematic is online. My guess would be ~5mA.

(Etienne Demers) #5

Hello,

Did some digging around.

There are 2 ICs with 8 outputs each. Max output is 50mA

One of the ICs has 8 connected, the other has 6.

Your 5mA estimate was pretty good.

I changes my resistors to 1K. this means, taking the max values I would be using 5.5V - 1.2V / 1000ohm = 4.3mA for a max total of 34.4mA…

Just putting the info to conclude the post.

Cheers,
E.

1 Like
(Rusty) #6

Hi Etienne,

We actually did have some opto-isolators on a revision of our BlueESC at one point. We did run into one major issue.

The switching speed of optoisolators is fairly slow, especially when you don’t drive them with much current. The Pixhawk autopilot has current limiting resistors on each PWM line to protect those lines. I believe the resistors are 1k and the PWM voltage is 3.3v, so the drive current is 3.3mA max. With that amount of current, the switching was very slow and changed the length of PWM pulses such that the range of signals was skewed. If you connected it to a device that had more current, the range would be normal.

We ended up removing it because we couldn’t manage to get predictable performance in all situations.

Hope that helps!

-Rusty

1 Like
(Etienne Demers) #7

Thanks rustry, I didn’t see any resistors in the diagram.

The opto couplers I looked at has 80KHz top speed. Dark current in the nano amps. I’ll have to run some tests I guess :slight_smile: