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Sherlock Ohms

More Video Hum Bar Problems

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BrainiacV
User Rank
Platinum
Never take their word for it!
BrainiacV   9/23/2014 10:39:54 AM
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I had one installation where I was installing barcode readers for a manual sortation system. Each station had an outlet for the barcode reader. I had received the barcode readers and just before plugging them in, I had been given a new multimeter to be added to my toolkit. Just for grins I inserted the leads into the socket and was surprised to find instead of 110 vac, I had 70 volts instead. I plugged in on of the barcode reader chargers and found instead of 12 vdc, I had 0. If I hadn't tested the outlet, I would have assumed the entire lot of chargers were defective.

That instance taught me to always test my assumptions before proceeding on any future installations.

ChuckMahoney
User Rank
Gold
Re: is the power OK?
ChuckMahoney   9/23/2014 10:12:06 AM
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Absolutely! But ground/neutral swaps can be tested with a simple outlet checker, a Scope is not needed. But there is more electrician-induced danger...bootleg grounds and reverse bootleg grounds. A bootleg ground is where the N & G are tied together right at the outlet (instead of back at the service entrance). A reverse bootleg has N & G tied together, but wired to the HOT. The Hot leg of the outlet is connected to neutral. Again, a scope is not needed (and might even be dangerous if plugged in to the reverse-bootleg) so a non-contact tester is better. Search Google for "bootleg ground" and look for videos and posts by Mike Sokol and Mike Holt...

Battar
User Rank
Platinum
Re: is the power OK?
Battar   9/23/2014 9:47:53 AM
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Not only test the power supply, but test it with a scope, not just a DMM. Many years ago I was faced with total mayhem in a system which was the result of the ground and neutral wires being swapped on a 220/110v transformer feeding it. With a DMM all the DC supplies checked out OK, but with a scope you could see them riding on a sine wave.  

ChuckMahoney
User Rank
Gold
is the power OK?
ChuckMahoney   9/23/2014 9:30:39 AM
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1) this is a problem that young engineers will never see (hum bars in video)

2) Never trust an electrician

3) always check your power source first (AC or DC). Never assume it is correct.

 

Quick story - I had a job in college as a technician (while studying for my BSEE) on some PhD research project. The Prof is showing me the circuit boards and how to test them, emphasizing that they cost $5k each...but none of the boards worked. He was in a panic. After a few minutes of him poking around trying to determine why all of the bipolar transistor Vbe's were 0VDC, I asked "Professor, shouldn't the DC power supplies be turned on?"


He looked up, turned on the DC supplies and continued on without even saying thank you. Lesson learned. Always check power first.





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Sherlock Ohms highlights stories told by engineers who have used their deductive reasoning and technical prowess to troubleshoot and solve the most perplexing engineering mysteries.
Sherlock Ohms highlights stories told by engineers who have used their deductive reasoning and technical prowess to troubleshoot and solve the most perplexing engineering mysteries.
Sherlock Ohms highlights stories told by engineers who have used their deductive reasoning and technical prowess to troubleshoot and solve the most perplexing engineering mysteries.
Sherlock Ohms highlights stories told by engineers who have used their deductive reasoning and technical prowess to troubleshoot and solve the most perplexing engineering mysteries.
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