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Circuit debugging contest

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integra

New Member
hi, I'm looking for questions that can be asked in a circuit debugging/circuit design contest. Circuits that do not use micro processors/controllers are preffered although the suggestions regarding the others are also welcome. It would be great if the resources available both on the net and books are mentioned. Thanks in advance.
 

spuffock

Member
I've had this one for real.
A board containing a dozen big FPGA chips blows 2A fuses in the 5V power line.
A low ohmmeter says .7 ohms 5V to ground.
A track short finder doesn't home in , the resistance is about the same everywhere.
What now?
Solution follows in a couple of hours.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
spuffock said:
I've had this one for real.
A board containing a dozen big FPGA chips blows 2A fuses in the 5V power line.
A low ohmmeter says .7 ohms 5V to ground.
A track short finder doesn't home in , the resistance is about the same everywhere.
What now?
Solution follows in a couple of hours.

If it's got any tantalum capacitors on the board, replace those and check again! :D
 

Pommie

Well-Known Member
Most Helpful Member
A simple one,

A transistor that is switching a relay fails, it is replaced but fails again. The relay coil is measured and found to be the correct resistance.

The answer is almost certainly the clamping diode across the relay. edit - gone open circuit.

Mike.
 

ericgibbs

Well-Known Member
Most Helpful Member
A common type of fault.

Q1: When I connect my PIC's UART via a line driver ic to my PC, I can only get it to work when I connect my oscilloscope.

A1: Most likely cause, the PIC's ground line is not connected thru to the PC's RS232 connector.
 

spuffock

Member
Nigel,that'd get most of them with those symptoms, but not this one. It was in fact one of the FPGA's. The answer I favour is this: Connect a car headlamp bulb across the fuse, switch on, replace what gets hot.
The new recruit I helped with this one went on to earn the nickname "blowlamp" Naz. I' m proud of him.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Got to love light bulbs!, I always keep a 60W mains bulb with two wires soldered to it at work, and I've still got an old front fog light here from a car I had back int he 70's - comes in handy every now and again.
 

RadioRon

Well-Known Member
spuffock said:
Nigel,that'd get most of them with those symptoms, but not this one. It was in fact one of the FPGA's. The answer I favour is this: Connect a car headlamp bulb across the fuse, switch on, replace what gets hot.
The new recruit I helped with this one went on to earn the nickname "blowlamp" Naz. I' m proud of him.

That reminds me of my early days in technical school. During an early lab using TTL integrated circuits, a classmate named Dwayne Parish suddenly shouted to anyone within earshot of his workbench, "hey look at this". Several of us rushed over and looked at the circuit that he had just put together. The plastic top of his TTL IC had blown off and there was a bright red glowing line zig-zagging its way from pin 7 to pin 14. You could clearly see the path it followed through the die itself, convoluted as it was. Dwayne sheepishly admitted that he had plugged the IC into the board upside down, prompting the big bang. We subsequently dubbed this method of finding the critical fault path from supply to ground the "Dwayne Parish critical path technique" and saw fit to remind Dwayne of his notoriety at every opportunity by mentioning this as often as possible, with a laugh of course.
 

Sceadwian

Banned
Even the teachers are coming here for their school work now!
 
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