Full Isolation of micrcontroller

[Korawy]

Hi all...

Im making a stepper motor (40v) driver using Pic micro (5v), and i wish to well isolate the Pic from the motor.

I searched and found that optical isolation is the best solution, the problem is that im powering PIC and the stepper from the same source (AC source), that means that they both will have the same ground, and thus Isolation is not occured.

Am I right?
Do I need Isolated DC-DC Converter beside the opto-isolator to well isolate the PIC? Or Just the optocoupler will isolate the Pic although they share the same ground??

Thanks,,

 

[Ogu Reginald]

That is not a problem, go on and use the optical isolator because it is good for complete isolation of any two circuits.

 

[Korawy]

Yes the optocoupler is good for isolation, but i think isolation will not occur because im using the same ground for both the PIC and the motor.

 

[Ogu Reginald]

It will perfectly work, go ahead and try it. The interfacing here is done through light waves and so there is full isolation between the input and the output.

 

[betwixt]

Ogu Reginald, I think you are missing the point - the motor and micro are working from the same power supply so they are joined anyway.

You could do this with a small DC-DC converter or an isolating transformer, you should be able to connect the grounds together at least as only a difference in voltage across two points can disrupt the PIC.

With good power supply decoupling and maybe additional filtering it may still be possible to connect them without opto-couplers but they are a 100% sure solution if there is any doubt. Be very careful with ground connections, a motor starting up or being electrically braked can pass a very high current so voltage drops in ground lines can be significant. Always ground all the PIC circuits to a single common point and connect that point to the motor driver ground through a single connection path.

Brian.

 

[Ogu Reginald]

Yes you have a point.

 

[Korawy]

Thanks Ogu and betwixt..

what about making two power supplies from the same ac source?!

The input AC is 36-63v ac, so i might use two bridges, and make one for the motor and other for the PIC.

 

[betwixt]

That probably wont make any difference. First, explain why you think you need full isolation, the best solution will depend on your answer.

Brian.

 

[Korawy]

Well, Im making a stepper motor driver, motor rating is 40v 12a, also it will be put in a machine that works with other motors and PLC, So i need a good isolation for my poor micro.

if making two bridges wont solve the problem, then what about making two transformers to transform from 60v ac to 30v or 25v?

May be it's better to make an Isoalted DC-DC converter, but i failed to find an easy circuit for that!!!!

 

[ftsolutions]

The main concern is that you don't want the motor return currents to be passing through your digital microcontroller's ground plane. While you can fully isolate things, with such low power motors it often is not necessary to have the grounds completely isolated as long as you have the motor ground returns go directly back to the power supply return with heavy, low impedance cabling/copper. The microcontroller board will then have its own separate ground return line going back to the power supply with lighter gauge wiring and possibly with EMI suppression 'bead' inductors to add high frequency impedance (not always needed). If the power supply is properly connected, motor current will be returned to the supply's GND connection and not through the PIC controller board - there will be some injected noise depending upon power supply impedance, but often will be acceptable/tolerable. But, if the PIC controller board interface to other parts of the machine where it can also become a current-return path for large currents then there is more need to have separated ground planes. Optically isolating the control signals from PIC -> motor bridge is a good idea too, but again, may not be absolutely necessary. IF this is to become a product, for safety related certifications, then more formal isolation may be required by legislation.

 

[Korawy]

as long as you have the motor ground returns go directly back to the power supply return with heavy, low impedance cabling/copper. The microcontroller board will then have its own separate ground return line going back to the power supply with lighter gauge wiring and possibly with EMI suppression 'bead' inductors to add high frequency impedance (not always needed).

Does this mean to use a PCB with two different types of copper, Or it just means the width of the Traces in a PCB??! and what's meant by "EMI suppression 'bead' inductors to add high frequency impedance "?

But For SOME MAIN reasons, the Optical isolation must be made and if that then the grounds must be seperated!!!!

 

[ftsolutions]

EMi suppression beads - they came in a variety of current capacity and nominal impedance@rated frequency, but they are NOT high-Q inductors.
An example of one particular one is Murata part #BLM21AG121SN1D. google it /look up in electronics distributors like Newark, Farnell, etc.

Are the H bridges and motor power connections on another physical PCB, or on the same PCB as the PIC micro? If it is on a separate PCB, you may not need to optically isolate your signals, as some decoupling is possible through logical wiring practices and the motor currents won't be passing through your micro's ground plane. But, if the H bridge is on the same physical PCB, the cleanest solution is to optically isolate the signals, layout separate ground planes with good isolation practices, and maybe even using heavier (2oz or more) copper on the layers for the H Bridge GND returns. Be sure not to overlay micro's plane layers over the high current motor power layer(s). You may not need to use optical isolation - it may be adequate to use other, moderate level isolation such as the galvanic isolation available in various ICs if going to optical creates too many difficulties if your are driving bridges at very high frequencies. I don't usually resort to opto-isolators unless there are safety certification requirements or I need more than 3000Vrms isolation (lightning protection, high energy industries, 460VAC/600VDC industrial motors, etc).
But, if you go with opto-isolation and proper PCB layout you can be assured of not having any interference issues.

 

[Korawy]

Thanks ftsolution, ur Posts is very helpful.

Unfortunately the H-bridge and the PIC micro are all on the same circuit.

layout separate ground planes with good isolation practices, and maybe even using heavier (2oz or more) copper on the layers for the H Bridge GND returns.

Talking about the ground and power planes, im using only one layer for the Connections (not upper and lower layers),Is this making a problem for me, Or it's better to use two PCB Layers?

 

[ftsolutions]

Keep the motor and H bridge power connections apart from the PIC controller signals area, separated by at least 6mm or as much as you can. Use wide tracks for the motor current traces and try to keep them together as much as you can easily manage.