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Topic: PCB short circuits (Read 1 time) previous topic - next topic

Dave_Burrows

Hello,

Using plans I found on-line, I built a PCB DC motor controller from scratch. Then, I ordered parts to make 3 more. When I connect power to one of the later 3, a small wisp of smoke rises, and a bright line appears on the PCB surface as a trace burns out. So far, I have made 3 repairs with another yet to make. My question is to know how to find out which component is causing the shorts? I have a multi-meter, but I really don't know how to use it very well. Is there a very thorough tutorial that might help?

Peace,

Dave

codlink

You would need the Continuity setting on your meter.

(Second half)
http://www.youtube.com/watch?v=F0Y1XGcjlL0

http://learn.adafruit.com/multimeters/continuity

You can also post your schematic and get some feedback here.
//LiNK

MarkT

If you have some circuits that work and one that doesnt, you can use differential analysis - compare the
values between working and non-working boards to identify the area with problems.

Also shorts are often visible if you take a good magnifier and check all the joints in good lighting - sometimes
a poor solder joint or a solder-splash is to blame.

BTW when testing a newly built circuit you should apply power using a good current-limited regulated DC power supply,
setting the current limit really low and checking that excessive currents aren't flowing - otherwise any problem
will destroy things as you've observed.
[ I won't respond to messages, use the forum please ]

GilchristT


BTW when testing a newly built circuit you should apply power using a good current-limited regulated DC power supply,
setting the current limit really low and checking that excessive currents aren't flowing - otherwise any problem
will destroy things as you've observed.


Good general tip, never thought of doing that before.

Constantin

Also, the better PCB fabs like itead will offer testing services for either a small fee or free. I always select 100% testing and have yet to have a problem.

oric_dan

When you do a manual pcb layout, it's not uncommon to have a layout error. When I
get a new pcb, I usually test as follows:

1. use ohmmeter to check that main ground and power busses are not shorted [this
   is always the first check].
2. use ohmmeter to check that all ground and power buss points are connected properly.
3. check that power and ground continuity to all chips is correct.

Then, build up one ckt at a time, checking along the way:

4. build, test 5V regulator power supply.
5. check power correct at main buss nodes and chip pins.
6. repeat #4 and #5 for 3.3V regulator, etc.
7. add main processor cktry, plug in chip, and test.
8. build up rest of board, and test, one section at a time.

The advantage of this procedure is that, if something doesn't work right, it's usually
whatever cktry you just added.

9. same idea works for hardware development as for software development.

Dave_Burrows

Thanks, all! I kept waiting to get an email telling me a new post had been made. When I didn't, I assumed there weren't any. Thanks very much for your excellent responses.

I have a multi-meter, and in checking circuits on my PCB, I found that there was no continuity in one of three 10K resisters. I'll pick one up at Radio Shack in a bit. I was wondering, though,

  • could that alone be the reasons for the shorts

  • could that alone be responsible for no power to the motor?


afremont

Not saying it couldn't happen, cuz I'm sure it could, but I've never picked up a resistor and had it be open circuit.  Maybe I've been lucky or haven't picked enough up yet (I doubt that part).  It might be a fatality from your board issues.  Oric_dan gave excellent advice on checking new boards.
Experience, it's what you get when you were expecting something else.

pito

Quote
Also, the better PCB fabs like itead will offer testing services for either a small fee or free. I always select 100% testing and have yet to have a problem.

I would highly recommend you to check your boards from itead even they claim they do electrical check.. ;)

Retroplayer

#9
Apr 13, 2013, 11:58 pm Last Edit: Apr 14, 2013, 12:07 am by Retroplayer Reason: 1

Thanks, all! I kept waiting to get an email telling me a new post had been made. When I didn't, I assumed there weren't any. Thanks very much for your excellent responses.

I have a multi-meter, and in checking circuits on my PCB, I found that there was no continuity in one of three 10K resisters. I'll pick one up at Radio Shack in a bit. I was wondering, though,

  • could that alone be the reasons for the shorts

  • could that alone be responsible for no power to the motor?




Using the continuity setting, you wouldn't get continuity on a working 10K resistor. Those (typically) only register low resistance around 50 ohms or below. Now, if you measured contuinity across the other two resistors, then you short is either in those resistors or whatever is connected in parallel with them.

Also, a resistor blowing open would generally not cause the event you described unless it were part of a current divider (resistors in parallel), in which case your damage would cascade until all resistors in the divider (in parallel) were blown open. It would do this before blowing open a trace.


Note: Assuming that you mean that you are using the continuity setting on a meter and not just the resistance setting. "Continuity" typically means a low resistance connection, like a piece of wire. It is meant to test for opens. It's not technically correct really, but it is how the term is used in practice.

Dave_Burrows

#10
May 08, 2013, 01:00 am Last Edit: May 08, 2013, 01:07 am by Dave_Burrows Reason: 1
I put this project on the back burner for a little while so I could try to learn more about electronics. I did learn a little. I make my living as an artist, so making things of all kinds holds my interest. Making my own PCBs is now on my gonna-try-that list. The small machines for which these PCBs are intended are nearly completed. I'm anxious to put them to work, as they will allow me to do things I can't right now. These PCBs are all that's left after I do some final sanding.

I have a double sided copper clad board on it's way in case I need to replace the two boards with burned traces. The first PCB I made works. Another has 4 repairs, and doesn't work, a third has 1 unrepaired trace, and doesn't work, and the last hasn't been tested yet.  All are still assembled with components soldered in place.

I wish I knew someone close by who was good with these things. It's easier to learn directly from a person with whom a conversation in real time can be made.  I'm wondering if I should desolder these two faulty boards to try to isolate the problem?

These are the directions I followed:  http://www.dreamingrobots.com/ElectricEelWheel/BuildersGuideElectronics_v1200/

These are the custom boards I ordered:  https://www.batchpcb.com/pcbs/102800

Images of Eagle board, and schematic:




Eagle files attached

Thanks so much, you all.

There's so much here to digest, it's hard to know where to start. Maybe what would help most at this point is for someone to sort of take me step by step through the process of learning what's going on with these boards of mine.

I have a box of salvaged PCBs, mostly from several laser printers I deconstructed, from which I want to reclaim the components. There are many dozens of boards; large, and small. I can't afford a solder pot, have considered making one with a 3" cast iron skillet on a portable electrical burner, or using a heat gun; I'm not really sold on the solder suckers I've seen. That in mind, I could buy / make whatever to use to remove these components, but do I remove them all at once, or one at a time? I'd also be interested in hearing your recommendations for a desoldering tool. Thanks again.

Peace,

Dave

PS: I appreciate this demo on repairing burned traces:
http://www.youtube.com/watch?v=ozH2F3AX8BY

Any thoughts?

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