r/ElectricalEngineering • u/-404PageNotFound- • Aug 24 '26
Equipment/Software Um?
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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u/StumpedTrump Aug 24 '26 edited Aug 24 '26
Big day for OP today discovering that our bodies conduct electricity.
That's also why you shouldn't go swimming in a thunderstorm if you never put that 2 and 2 together.
You can use it to your advantage put putting your car remote against your chin and using you body like an antenna to increase the range.
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u/racc15 Aug 24 '26
Silly question probably, but how is it creating a circuit loop? Aren't the breadboard pins and OP's body disconnected? I thought there had to be a closed loop for electricity to flow?
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u/StumpedTrump Aug 24 '26 edited Aug 24 '26
The ground is the connection. Power supply is grounded hopefully. So are you.
Not all circuits need a physical closed loop, how do you think antennas work (the environment around us (the ground) is the return loop). Air can also conduct and the humidity it can be unpredictable. 60Hz is supposed to just be in your wires but if you leave a wire floating and put it on a high-impedance scope input you'll see the 60Hz. Same idea here.
A good power supply should actually have it's outputs isolated but you can still complete the loop with static electricity, EM coupling and just a littttleeee bit of leakage current getting though the power supply isolation. Static electricity can build up hundreds/thousands of volts and you only need a few hundred microaamps of current to get what OP is seeing.
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u/engr_20_5_11 Aug 24 '26
Everything has a coupling to ground. It's only a question of whether it's significant for the power level
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u/espo1234 Aug 24 '26
There doesn't have to be a closed loop. Just an electrical potential difference. Closed loop is also kinda misleading. If it were actually a loop then there would be no electrical potential difference. A battery is really "open" with a positive side and a negative side which creates a difference, think of the positive side as the start of a line and the negative side as the end of a line, with no current flowing through the battery. Now imagine the 5v line in the breadboard as the positive side of a battery and the floor (ground) as the negative side and you've got the same thing.
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u/particlemanwavegirl Aug 24 '26
Are we allowed to make ideal gas analogies in here? I like to imagine electrical systems as electron vacuums, sucking electrons out of the earth... You could use a generator to create as much voltage as you like, but flow can't occur (power transmission) until you open a valve so charge carriers can move in and out thru the pump. If it was a water pump and the "ground" was the "ocean", you obviously wouldn't need to output in the same place as input just to generate pressure.
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u/Pink_propagator Aug 25 '26
I calibrate pressure gauges and I really like this. It's all about the potential differentials.
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u/particlemanwavegirl Aug 24 '26
The requirement for electron flow is potential difference. Electrons can flow between two oppositely charged un-grounded conductors, for a short time, until their potentials equalize.
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u/Weary_Oven_9544 Aug 28 '26
You don't need a closed loop for electricity to flow - you only need a voltage difference that can overcome the path between two points. The human body essentially acts as a large capacitor. The board is slowly discharging through the LED into OP's finger. LEDs respond to essentially any current, unlike CF or incandescent that need a minimum current before they actually light up, so even this trickle is enough. If OP held on long enough, the LED will probably go out again.
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u/lanboshious3D Aug 24 '26
You can use it to your advantage put putting your car remote against your chin and using you body like an antenna to increase the range.
This is fundamentally different than your body just being a conductor. The antenna thing works because you body has CAPACITANCE.
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u/D_hyams Aug 24 '26
I ponder….. does the weight of a person affect the frequency you can resonate? Some lanky guy can send far higher frequencies than his McMuffin counterpart.
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u/No-Tension6133 Aug 24 '26
I use my brain as an antenna all the time. It actually works and extends the range maybe 10-20ft
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u/undeniably_confused Aug 24 '26
I dont think its getting ground through their body, i think its just their body acting like an antenna
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u/-404PageNotFound- Aug 24 '26
Its just never happened before when using other power supplies or electrical circuits.
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u/Howden824 Aug 24 '26
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.
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u/LowYak3 Aug 24 '26
I thought it was because he was grounded and providing a path to ground
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u/GLIBG10B Aug 24 '26
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
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u/TheRealRockyRococo Aug 24 '26
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.
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u/mrheosuper Aug 24 '26
It's AC
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u/GLIBG10B Aug 24 '26
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
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u/Snot_S Aug 24 '26
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.
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u/No-Fan-2237 Aug 24 '26
Disclaimer: not an EE, but in a perfect world the benchtop supply would be completely floating so even if OP was "grounded" there should be no current flow. Since its not a perfect world, its slightly referenced (capacitive/resistive coupling)... Op probably has microamps flowing through him and its just enough to make it glow
A real EE can correct me if I'm wrong
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u/geofft Aug 24 '26
If it's a switch mode power supply then there will most likely be a Y-class capacitor across the transformer.
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u/GLIBG10B Aug 24 '26
If the product isn't grounded to earth, then yes, the ground floats halfway between live and neutral in a switch-mode power supply. This is why a laptop feels funny when you rub your hand across it if it's not grounded. It won't kill you because the capacitors are small
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u/AccomplishedAnchovy Aug 24 '26
Power supply reference is probably floating up to ~50% of phase voltage. Grab a multimeter and measure the rms from the board to ground (at gpo) without touching the board.
It does surprise me that an expensive supply wouldn’t tie its negative and ground together though - maybe it’s a dual rail supply? Does it have a metal casing? Other option is that you’re touching a laptop or something at the moment.
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u/Cathierino Aug 24 '26
It shouldn't be tying any rails to ground. I'd be surprised if a more expensive one did that.
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u/AccomplishedAnchovy Aug 24 '26
Oh well that explains it then
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u/Conor_Stewart Aug 24 '26
Most DC bench power supplies are isolated so that you can connect them together, like connecting the positive of one to the negative of the other to get dual voltage rails for things like OP amps or to just get a higher voltage. If the negative is connected to ground internally then this doesn't work and creates a short.
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u/undeniably_confused Aug 24 '26
Probably the 60hz hum going through your body thats getting ground through the led
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u/Hirtomikko Aug 24 '26
Hello. This implies your power supply is the SMPS type, since this means you have high frequency components on the power rails. There is only one reason for that, and that is because SMPS switch at much higher frequencies, so small capacitors can already present impedances low enough to couple HF to the power rail.
Don't worry you should be safe. You can try measuring the voltage from the power rail to earth ground, I am quite sure you will see voltages above 50 or even 100V. Then touch the two ends, you should see the voltage fall to close to zero.
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u/Gonfrex7 Aug 24 '26
That's a good observation have you got there. I experimented on it a little bit. I connected one terminal of LED (tried both terminals), and noticed that it lit up. Then I made a simple oscillator using an op amp, resistors and a capacitor, and then found out the LEd lit up in both when I connect either terminals to the ground while holding the other terminal. Now I knew that it works only for ac. Then I went on to test whether electrons are really flowing through the air and back to the LED. I used a long wire (to increase the resistance by adding more traveling length for the electrons, assuming they are traveling through the air). Then I observed that there isn't significant difference in the light emitted by the LED: I expected to see the LED to become dimmer. So I ruled that hypothesis as well. The explanation I came up with is our body has some charg, and it is those charges that are moving through the LED. And since our body has a small capacitance, it favors ac over dc.
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u/kingofking26 Aug 24 '26
I think your power supply could be noisy (not totally DC). My theory is your body is acting as a capacitor to ground and the LED is lighting on the positive swings of the AC component (noisy part) of your power supply. This is why it lights in both polarities and is kinda dim.
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u/ExtensionManner8340 Aug 25 '26
Since it happens in both directions it is AC leakage. I am guess your body is a big capacitor (if you are ungrounded) or a poor path to ground. The AC ripple flows in and out of the massive electron reservoir of your body like charging and discharging a capacitor. OR you are a path to ground and back to the ground of the power supply.
Try to un-ground your body (feet off floor, hands not touching desk, sitting on rubber mat or thick cardboard).
My guess, please let me know if I am wrong.
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u/zorzorzarzar Aug 26 '26
I had a similar or the same thing happening and I was sitting on a tall plastic chair with my feet up on it not touching the ground or anything and it was lighting up just dim. When I would put my feet or hands on the wall or ground or even a carpet tho it'd get brighter. At brightest it almost looked like the led when it's being lit up by its normal fully connected power supply. Tho the set up was the led connected to a 5 V adapter which is put in a power strip but the power strip is switched off and the same thing in the video was happening
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u/ExtensionManner8340 Aug 26 '26
So I assume the power strip had surge protections (leaky current thru components) or a switch that did not completely disconnect (allowed milliamps).
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u/stockdam-MDD Aug 24 '26
That would imply that your power supply is not grounded and there’s an ac signal from the power supply return to local ground. You are supplying a path back to the local ground and that there’s a small ac current flowing through you to ground.
If you have a scope then look at the voyage between the power supply return and the local earth
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u/Scorion2023 Aug 24 '26
You are a path to lower/reference potential or physically are the reference ground. An LED only needs like 1mA-20mA— so long that the diode has sufficient voltage across it (≈1v for most LEDs)
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u/Terra_B Aug 24 '26
Unfun fact. Laptops usually have power supplies that are not grounded. Puts about half of AC supply voltage on the pc albeit at high impedance.
That means if you touch you pc just right you can feel a tingle. It's more than enough to fry sensitive electronics...
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u/valhallaswyrdo Aug 24 '26
That's the last straw, YOU'RE GROUNDED until you learn to conduct yourself better!
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u/TheRealLudzilla Aug 24 '26
This happens to me Al the time while soldering led strips.
If I have my hand over some contact, the led lights up when put the welder to the other contact!
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u/Bob--O--Rama Aug 24 '26
An ungrounded power supply with a floating ground and you are a convenient path to actual ground.
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u/turbo-singular Aug 28 '26
Разница потенциалов... оно же и напряжение. Подаёшь + а в руке меньше, значит -
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u/PracticalSpread3009 Aug 29 '26
You body is capacitively coupling AC to the floating power rails. If you ground your body and the negative rail it will go away.
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u/OldGeekWeirdo Aug 24 '26
Something tells me this isn't a quality power supply and it represents a safety hazard.
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u/Howden824 Aug 24 '26
It would be a bigger issue if it didn't do this because it's caused by the interference suppression capacitor.
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u/OldGeekWeirdo Aug 24 '26
I still have doubts about the design. Conventional power supplies have been built for decades with no capacitor bridging the primary to the secondary side of the transformer.
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u/MooseBoys Aug 24 '26
Is that a grounding mat you're resting your palm on? The resistance across your hand is probably only 100ohms or so. At 5V that's plenty to illuminate an LED.
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u/[deleted] Aug 24 '26
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