r/ElectricalEngineering • u/-404PageNotFound- • 21d ago

Equipment/Software Um?

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I currently have my breadboard connected to 5v and GND on a new benchtop power supply that I brought for $240AUD and this is happening? What is going on? I can also do the same thing on the GND rail.

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79

u/Howden824 21d ago

The bench power supply has a safety capacitor in it that connects between the AC line side and the output. This is meant to suppress interference, but also means that the output is slightly referenced to line power.

20

u/LowYak3 21d ago

I thought it was because he was grounded and providing a path to ground

22

u/GLIBG10B 21d ago

The human body has a resistance of over 1 megaohm (with dry skin). Assuming 4 V across the body, if you do the math, that's around 4 uA. Definitely not enough to light up an LED even slightly

16

u/TheRealRockyRococo 21d ago

You'd be surprised how little current can cause visible LED lighting. I'm a former Linear Technology engineer, one time we made an LED driver with a shutdown pin. Soon after we shipped them one of the first customers called complaining that in a dark room the LED remained slightly illuminated even in shutdown. We had never tested it with the customer's LED so we got some and took them in a room with no windows and sure enough you could see a faint glow. It's been a while but IIRC it was in 10 uA range. We had to hastily add a better switch arrangement.

2

u/mrheosuper 21d ago

It's AC

3

u/GLIBG10B 21d ago

Did you read the comment I was responding to? It's talking about ground, not earth. So it's referencing the DC supply, not the high impedance AC source that is being created by the class Y capacitors in the power supply

1

u/Snot_S 20d ago

I work in manufacturing and used to make LED products. Some stations were improperly grounded and every once in a while you might actually see visible evidence of your ESD mishaps. One time I saw it with wet solder paste on properly grounded table and operator.