Parasitic Voltage From Motor Controller Inputs

[eric.williams.neu]

Hi All,

I've noticed in a few of my applications that motor controllers (I've seen this happen with RC servos and larger DC motor controllers) that as much as 2.5V can be fed back to my power supply circuits, coming from the microcontroller, through the signal line(s), through the motor controller, and back the power line (5V in all of the applications I've been working with). It's enough power to light up my status LEDs (indicating 5V power) even when there is no source 5V.

I've thought about using diodes (from 5V supply to 5V input to motor controller/servo) to provide reverse current protection, but I'm worried the diode drop might interfere with the onboard logic as the control signals will still be coming from the 5V microcontroller.

My question is 2-fold: should I worry about the diode drop? And if so, is there another way to provide the "one-way valve" effect that I'm looking for?

Sincerely,
Eric

[Soeren]

Hi,

I've noticed in a few of my applications that motor controllers (I've seen this happen with RC servos and larger DC motor controllers) that as much as 2.5V can be fed back to my power supply circuits, coming from the microcontroller, through the signal line(s), through the motor controller, and back the power line (5V in all of the applications I've been working with). It's enough power to light up my status LEDs (indicating 5V power) even when there is no source 5V.

How did you establish the path of the voltage?

I've thought about using diodes (from 5V supply to 5V input to motor controller/servo) to provide reverse current protection, but I'm worried the diode drop might interfere with the onboard logic as the control signals will still be coming from the 5V microcontroller.

You just claimed that it came "through the signal line(s)" and if it does, they'd need diodes as well.

My question is 2-fold: should I worry about the diode drop? And if so, is there another way to provide the "one-way valve" effect that I'm looking for?

First, make sure there really is a problem, then establish exactly where it is and whether it... Well, is a problem (i.e. has any ill effects).
Then a solution can be selected.

[eric.williams.neu]

How did you establish the path of the voltage?

When main power is disconnected from the peripheral 5V LDO, but the Arduino PWM lines are still connected to the motor controller, and the motor controller is still connected to the output of the 5V LDO, my 5V power LEDs still light. When either the motor controller is disconnected from the 5V rail or the PWM lines are disconnected from the Arduino, the LEDs turn off (the correct state, as there is no main power to the 5V LDO). Also, I've experimented with the diode installed such that current can only flow from the LDO to the motor controller, and this also shuts off the LEDs.

You just claimed that it came "through the signal line(s)" and if it does, they'd need diodes as well.

As described above, I'm only concerned with power being bled back into the peripheral power board, not the motor controller itself. So if the diode blocks voltage back to the board, even though the motor controller is still being ghost powered by the Arduino PWM lines, I'll be happy. As such, the signal lines would not need diodes.

First, make sure there really is a problem, then establish exactly where it is and whether it... Well, is a problem (i.e. has any ill effects).
Then a solution can be selected.

That was precisely the implication of the question, I just forgot to mention it . While it's mostly an asthetic/debugging problem (I won't be sure of whether the 5V peripheral power is being properly generated or it is simply feedback from the motor controller), I'm also worried about the other peripherals attached to the 5V rail being powered (i.e, servos, ultrasonic sensors). I don't have a definite idea of whether or not this will cause undesirable behavior, but I was mostly curious about whether this is a common problem (it must be) and how to avoid it.

Thank you for taking the time to look at this and the in-depth reply; this was a great introductory experience to this forum, the atmosphere here seems quite helpful.

[Soeren]

Hi,

[] I was mostly curious about whether this is a common problem (it must be) and how to avoid it.

I haven't seen it, so it's a little hard to whip out an easy solution, especially since your description is a bit foggy - I'm still not sure what you define as a peripheral board (which apparently isn't the controller, nor the ESC)?
Adding a drawing would probably help quite a bit, a schematic of the interface that has the problem would help as well and so would jotting down the voltmeter readings that you have (or should have, at least) taken.

An LED is not a sure indicator of 5V and doesn't show whether something is just ghosted or if you have a large capacitance holding power for a while. It can be lots of things and you'll only get good answers if you present good questions, meaning that you need to be our eyes, as we cannot see what the circuit looks like or what you have done - They neither teach mind reading nor ESP at the engineering faculties.

Thank you for taking the time to look at this and the in-depth reply; this was a great introductory experience to this forum, the atmosphere here seems quite helpful.

We can only reply as in-depth as the information allows. The better you describe the problem, the more helpful we can be.

[Fr0stAngel]

When main power is disconnected from the peripheral 5V LDO, but the Arduino PWM lines are still connected to the motor controller, and the motor controller is still connected to the output of the 5V LDO, my 5V power LEDs still light. When either the motor controller is disconnected from the 5V rail or the PWM lines are disconnected from the Arduino, the LEDs turn off (the correct state, as there is no main power to the 5V LDO). Also, I've experimented with the diode installed such that current can only flow from the LDO to the motor controller, and this also shuts off the LEDs.

LEDs are diodes, ( they light up in only one direction, they don't respond to reverse voltages) so my guess would be to check the connections on the PCB ( same sort of thing happened to me, i had accidently  had current flow between two traces that shouldn't have been, cause i messed up a bit in soldering).

So, use a multi meter to check the traces, if nothing there is wrong then perhaps there is need for diodes as motor circuits are prone to back emfs.

[eric.williams.neu]

We can only reply as in-depth as the information allows. The better you describe the problem, the more helpful we can be.

I believe the internet ate my tone: there was no sarcasm or malice there. I was genuinely happy to see that you took the time to break down my post and respond point by point. My apologies for any confusion.

As for the schematic, hopefully this image (I don't have a formal diagram yet) will help:
https://picasaweb.google.com/lh/photo/x1hnuJUfSuaWRNxvY0jGSg?feat=directlink

[Soeren]

Hi,

I believe the internet ate my tone:

Oh well, it's under the bridge.

As for the schematic, hopefully this image (I don't have a formal diagram yet) will help:

Yes.
Since I have no clue as to the innards of any of the boxes, the easiest suggestion I can come up with is like you suggested yourself: A diode in the 5V power line.

But to do this, you need to up the output a diode drop, by placing a diode between the ground pin of the 5V LDO and ground (forward biased, i.e. cathode to ground).
However, this may need a change of output capacitor, should it become unstable - LDO's are notorious for being a bit picky about how they're used.

If you build the power section yourself, you could always change it for a regular 5V (i.e. non-LDO) regulator.

There's no point in using an LDO anyway, as you have 24V in. LDO's are for situations where you have an input voltage of around 6V-7V.

If anything, it would be better to replace it by a switcher, as you loose 3.8W for each 1W that you use, making its efficiency less than 21%!