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signal ground between two isolated voltage circuits

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jberookc@gmail.com:
I have a design where there are two separate circuits. One circuit operates on 15 volts DC. (Isolated electronic motor control) The other operates on 5.0 volts DC.
The 15 volt electronic motor control has a 5.5 volt output that I do not want to use for isolation reasons.
The 5.0 volt circuit needs to send a low voltage signal (PWM) to the electronic motor control.  Will a single signal wire to the motor control (no ground) work or will the signal wire require a signal ground… And, what measures (precautions) should be taken to avoid ground loops and EMF issues?
I know that this is a bit like asking how long is your well rope due to not knowing anything about the motor control schematic ( and I have no way of knowing), however any comments are welcome.

helius:
Welcome to the site.
A single wire between two points is not an electrical circuit and cannot serve any purpose. All electric circuits must have a loop for current to flow.
If you need to galvanically isolate two circuits while sending a signal from one to the other, there are means to achieve this, in particular, optoisolators and signal transformers.
In your case, I suggest you investigate optoisolators, since they will handle PWM signals. They are used in most switching power supplies in this fashion, to control the primary power from the secondary side, while maintaining galvanic isolation between primary and secondary.
For controlling a motor, you also need some kind of feedback from shaft speed or position and some means to prevent damage if the motor is stalled.

jberookc@gmail.com:
Thank you, helius:

I will check out the optoisolators, thanks again.

jberookc@gmail.com:
As usual the devil is in the details.  The opto's appear to be just the ticket, however, since I am a newbe, I have more issues.  The Motor control is also a BEC or battery elimination circuit designed to use the same battery that powers the motor (15v) and the other electronic circuits (5.5v).  I will not be using this 5.5v BEC power supply from the motor control, I need the two circuits isolated.

There are three wires coming out of the motor control (BEC line) White (signal), Red (5.5v+) and Black (5.5v-).  As I see it, the signal (white wire) is an input. Then there is the issue that you brought up regarding feedback.  Since this situation uses PWM is the feedback issue necessary? 

Also, would grounding the signal ground (isolated 5v circuit) through the opto going to work or cause some other issues?

I will try and send some schematic details;  These will be snippits of DWG files.  Please do not be too harsh regarding this schematic, It is a work in progress.

 

pwlps:

--- Quote from: jberookc@gmail.com on April 16, 2019, 04:39:34 pm ---As usual the devil is in the details.  The opto's appear to be just the ticket, however, since I am a newbe, I have more issues.  The Motor control is also a BEC or battery elimination circuit designed to use the same battery that powers the motor (15v) and the other electronic circuits (5.5v).  I will not be using this 5.5v BEC power supply from the motor control, I need the two circuits isolated.

--- End quote ---

In a similar project with a DC motor, where the control and power circuits were working at different PS voltages and also needed separate and isolated grounds,  I have been using the HCPL2200 optocoupler.  What is nice about this IC is that it has an integrated amplifier so it can translate your 5.5V PWM pulses to 15V PWM pulses without any additional components, just a transistor for the current gain. 

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