Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums > AC Power and Grounding

Creating a ground current on purpose for testing

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Jay Barracato:
Recently, I had a band bring in some extra lights (standard par cans) all daisy chained together with ancient power strips. The instant they plugged in all the backline started buzzing. Fortunately for the show, they couldn't get the dimmers working right, so we ended up unplugging them with a return to silence.

It got me wondering, if there was some way to safely purposely place a couple of volts onto the ground, to test how an installed system would react to that situation.(other than saving one of these crappy walmart power strips that musicians plague my stage with)

John Roberts {JR}:
The technical name for the device you are looking for is a "hummer" or more specifically a "pin 1 hummer".

Instead of injecting a voltage that could result in massive current across low impedance grounds and wiring, we inject a known modest mains frequency current and look (listen) for unintended signal corruption (hum).

Making one that is reasonably safe is as simple as repurposing an old AC output voltage wall wart capable of 50-100mA... Use ohms law to calculate what size resistor to use in series to deliver a nominal 50-100mA of 60 Hz current. This should be low enough current to not start a fire, but adequate to identify wiring problems, or poorly designed I/O circuits in products used in your signal path.

JR

PS: perhaps google "pin one problem" to find TMI.


For example a 9VAC wall wart will deliver 90 mA into a 100 ohm resistor, with the other end shorted to ground. That will dissipate roughly .8W so a 1 or 2 watt resistor would be indicated (1 watt will get too hot to touch).  Connect the two ends to different grounds to see if the hum current leaks into the audio. Good product design, and good wiring practices should suppress all or most of that noise. 

Tim McCulloch:
Sokol has a device he built from a resistance soldering PSU and some other bits.  I think he can put up to 2 Amperes 120VAC on an audio line.  You guys might have to share this under the super-secret double knot  spy & college professor license...

Mike Sokol:

--- Quote from: Tim McCulloch on March 28, 2017, 09:41:01 PM ---Sokol has a device he built from a resistance soldering PSU and some other bits.  I think he can put up to 2 Amperes 120VAC on an audio line.  You guys might have to share this under the super-secret double knot  spy & college professor license...

--- End quote ---

Yes I do, and it can create up to 30 amperes of ground loop current. Pretty dangerous to the gear since it can lift a trace on a circuit board, but I use it to identify what can happen to gear with real-world ground loop currents. While the 50 mA limit of the bell transformer noted by JR is a good test to see if your gear has the pin-1 problem, there is no such current limit in actual building wiring, sometimes with hundreds of amperes of current available in industrial situations (yes, I've measured it).

John Roberts {JR}:
Yes, 30A is more than typical branch fuses...and can do damage. The UL test for ground bonding (inside product chassis) uses something like 50A and will indeed vaporize traces if too wimpy (been there done that).

For debugging hum in audio paths, KISS. The wall wart PS are isolated from mains so relatively safe, and 9V generally won't sting you.

JR

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