Query regarding interference in soldering interconnecting wires

Dear Friends,

I would be glad if you can shed some light on the following problem:

Problem Statement:

I have used AT 89S52 microcontroller for my project. I have made many
circuits using AT89S52 microcontroller. In all the
circuits Ihave faced a peculiar problem.

I have always used a single sided PCB for all my circuits. when I solder
all the components on the PCB and test the circuit, my circuit works
absolutely fine. but when I solder interconnecting wires, for example the
wire from PCB to START switch or from PCB to stop switch, my circuit faces
some sort of interference/ pick up/ harmonics. When I remove all the
soldered wires from the PCB, the circuit again functions properly. when I
turn on a tubelight with choke in my room or press the voltmeter button to
take readings I come across interference. what I mean by interference/pick
up/harmonics is as under.

1. the controller jumps execution from one sub routine to other sub routine.
2. the controller turns on the relay and enters in an infinite loop.
3. sometimes the controller does not execute instructions as per program.
i.e gives eratic output .

Can you provide some images of your PCB?
What’s the length of the interconnection wires?

Hi,

Yes, common problem, use good quality screened cable tied to ground, and ferrites rings or similar at the micro end.
Keep the interconnect cables as short as possible.

Tubes and their chokes are well know for interference, have even seen them affect desktop pc systems; so ideally keep them and their main cables well apart.

i have attached the photo of my pcb. There are 9 wires coming out of the PCB. all the wires are about 1 meter long. the wires are 0.5 mm square multicore wires.

sudippusalkar said:

i have attached the photo of my pcb. There are 9 wires coming out of the PCB. all the wires are about 1 meter long. the wires are 0.5 mm square multicore wires.

Click to expand...

Hi,

Think you have really answered your own question.

You have the mains power wires going all over the top of the pcb.
The pcb is next to a contact breaker.
The signal wires to the left are I assume the switches on a long unshielded cable,
The white devices on the front of the case, again, unshielded cables, also running alongside the ac cables to or from the small transformer.

The micro does not stand a chance.

What current is the contactor switching and what devices is it powering ?
Does it have any contact suppressors anywhere.


I know very little about all detailed methods of circuitry shielding etc, but its seems the first place to start is to rewire so all those mains cables are as far away from the pcb as possible, the remote signal cables, however short, should all be shielded and fitted with ferrites or similar.

Perhaps even having the pcb in a separate grounded metal box from the mains devices, though some people say this does not work ??

All that before you look at your actual pcb layout to see if it has been designed to minimise interference pickup, a subject which I'm sure others can explain better than me.

hi,
thanks for the feedback. i agree that there are indeed many line wires and control wires coming out of the pcb. the contactor is used for switching three phase motors operating at 415 v.
but here lies the true problem statement.
the use of long control wires is imminent and there is no way of reducing their lengths owing to fabrication vows.
heavy wires carrying line currents have to pass over the PCB since they pass through the CTs as seen in the photo.
moreover, use of shielded cables is also not possible due to its thickness.
hence, the only solution lies in designing a circuit not prone to interference due to wires. can you please throw some light on use of ferrites. their value and connection diagram...

There is a lot of opportunity for 50/60 cycle hum to get to your control circuitry.
This may or may not be the same as the interference you speak of.

Guidelines to reduce exposure to mains hum include:
* using shielded coax wiring to carry signals
* filtering of power supplies
* use of grounded steel enclosures to separate sensitive devices from mains-carrying devices
etc.

Also consider you may have a ground loop somewhere. This can generate interference.

sudippusalkar said:

hi,
thanks for the feedback. i agree that there are indeed many line wires and control wires coming out of the pcb. the contactor is used for switching three phase motors operating at 415 v.
but here lies the true problem statement.
the use of long control wires is imminent and there is no way of reducing their lengths owing to fabrication vows.
heavy wires carrying line currents have to pass over the PCB since they pass through the CTs as seen in the photo.
moreover, use of shielded cables is also not possible due to its thickness.
hence, the only solution lies in designing a circuit not prone to interference due to wires. can you please throw some light on use of ferrites. their value and connection diagram...

Click to expand...

Hi,

With all that power floating around its hard to say exactly what is causing your problems.

I know from experience, even short input leads to the micros can cause problems and that should be your first area to cover.

Cannot believe you cannot find some shielded cable thin enough for your applications, there are so many around.
As to the exact selection of suppression parts, its a complex subject that I know little about.

You can thread the cables though Ferrite rings, placing then at the micro end of the cable, see the pdf datasheet for these products.
https://uk.farnell.com/fair-rite/2643250402/ferrite-bead-cylindrical-111ohm/dp/2113012

Also you could try these.
https://uk.farnell.com/jsp/search/productdetail.jsp?sku=9527370&CMP=i-9162


Your biggest problem is still identifying exactly what else is causing your problems, until you define what the cause is you cannot really offer a cure.
I would think arcing fom the contactor during its switching will be one area, do you have any contact suppressors fitted.
You need the proper parts that wil take those kind of voltages, not cheap.

As you can see from Brads reply there are so many sources of interference.

If you are using this for a commercial solution perphaps worth getting professional guidance on overcoming these problems.
( assuming you have not been given the project to cure all these problems they know exist in a circuit like this )

hello,
i agree that shielded wires can help me. i will re-wire the entire circuit with shielded wires and see if i get any results. this might take a day or two...