I was trying to visualize a beginner circuit problem on my breadboard and I plugged in my circuit, touched a resistor and got a nasty shock. I am a cs student playing with circuits to build some logic gates so I really am a complete beginner.


Why did this happen? How can I prevent this from happening in the future?

 

My guess would be that your Chinese power adapter is not isolated from line voltage. Even though I see a UL label, it could be a forgery. Impossible to tell without a teardown or having additional information about your environment.

 

You show a 5V power supply which has no earth pin.
There are three possibilities why this might have shocked you.
1. The supply is faulty or badly designed and is not isolating you from the supply as it should.
2. Even a good, properly isolated, supply will have some leakage from the main supply if it is not earthed. This can you give you a shock which you can feel, but not enough to really hurt.
3. As it is not earthed it, or you, may have built up a static electric voltage, resulting in a shock when touched.

 

Maybe the resistor was extremely hot.

 

What I see is 3 parallel resistors for ~8Ω @ 5V so you have ~3W going through those 1/2W? resistors... Yeah they are hot!

 

In short you appear to have made certain oversights.

 

What I see is 3 parallel resistors for ~8Ω @ 5V so you have ~3W going through those 1/2W? resistors... Yeah they are hot!

Right, and the 10 ohm itself will pass 0.5A which makes it 2.5W on that resistor alone. You can see the discoloration on it already, it is ready to burn up soon.

 

The shock came from your computer. Every computer power supply I have paid attention to uses "Y" capacitors from Line to ground and Neutral to ground. The purpose of the capacitors is to bypass (ground) radio frequencies that would otherwise get onto the power lines. I have noticed that the capacitor values are chosen to cause the greatest amount of AC leakage permitted by the relevant safety regulation. This current is less than 1 milliamp but it is enough to make me (as a former boss would say) "do da dance". It may hurt but it won't kill you.

I ground my desktop computer when I am using it to talk to circuits on my workbench so that I don't have to put up with painful shocks.

If your projects do not require a computer you will be better off in the long run if you use a separate power supply or batteries to power your projects.

 

Desktop computers in metal cases have the cases earthed. That would more-than-likely mean that the circuitry was earthed as well, because the shield to a USB connector would be earthed etc.
The Class-Y capacitors to earth don't cause high voltage on the case because the case is earthed.
It's laptops that are the problem, especially those with double-insulated power supplies. The interference suppression capacitor is then between rectified-mains-positive and output negative, which generally allows the output negative to charge up to about half mains voltage with an output current of about 500uA.

A far bigger problem is home entertainment systems where a number of items are connected together via the screens of screened leads. If all of the connected items are double insulated, then the 500uA is multiplied by the number of items connected together, and it can cause quite a tingle.

 

What did the shock feel like? Was it like a pulsing feeling that lasted as long as you were in contact, or a single sharp jolt when you first touched it? Or something else?

Bob

 

Welcome to AAC.

We realize you said "Shock". However, you should not be able to feel anything from a 5 volt power supply. Unless, like others have said, the supply is bad or something is not properly grounded.

To understand what you're working with - simplify your wiring. All three resistors (100Ω, 100Ω & 10Ω) are all in parallel. they can all be placed in adjacent sockets with just two jumpers, one from positive and the other from negative. The three resistors form a parallel resistance of 8 1/3Ω (as @SamR already said). Assuming a rounded value of 8 ohms, your circuit is conducting 600 mA (0.6 amps). Those resistors look like maybe 1/2 watt resistors. The three in parallel should handle 1.5 watts (assuming they ARE 1/2 watt resistors). Your power factor is 5 x 0.6 which comes to 3 watts. Unless you used ONE WATT resistors, you are overloading their capacity to dissipate heat. They all will be hot. Especially the 10Ω resistor.

When you say you got shocked - I'm betting, like maybe others are, you got burned, not shocked.

I mentioned simplifying your circuits. Start by using a more standard color code. Generally red is positive and black is negative. That alone can help you visualize your connections and spot any errors more quickly. Something I prefer to do when breadboarding is to use the negative rail on one side of the board and the positive rail on the opposite side of the board. It helps me to not accidentally connect a positive connection to the wrong place.

 

This is how I would have set up the breadboard:

Or even better still; connect the power source directly to the resistors where the jumpers are shown. The "above" way requires only two red jumper leads and two black. Those from the supply and those connecting power to the resistors. But you could have gotten away with just two jumpers, one red and one black. There's no need to use the power rails in this case. It's advantageous when connecting IC's and transistors and such but in this case not necessary. The use of the yellow and green jumper in your setup can cause confusion. What do they represent? In your case - nothing more than colored conductors. But using colors can help identify different parts or functions of circuits. When it comes to color codes - there's no "Right" way to do it. But good practice is to keep things universal and always the same setup when possible.

 

I was trying to visualize a beginner circuit problem on my breadboard and I plugged in my circuit, touched a resistor and got a nasty shock. I am a cs student playing with circuits to build some logic gates so I really am a complete beginner.


Why did this happen? How can I prevent this from happening in the future?

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View attachment 237741

I would say- be careful. Respect electricity or it will bite you. _YOU_ are responsible for your safety, don't rely upon some 'ideal' concept of in-built safety when dealing with electricity.

As to 'why?' - you touched a live circuit. Don't do that. Always disconnect power from your circuit if you're changing parts- failure to do so (at a minimum) can create an arc between the breadboard and a component when you remove or insert it- potentially ESD damaging that part (and others).

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

 

It certainly was not a shock from 5 volts, but as others have stated, it could be from the 5 volt supply, since that is probably intended to charge a phone battery where a non-isolated supply would not be a problem, usually.
I doubt that it came from the computer, since I see no connection to the computer, although it could have been the source of the ground return from your mains voltage shock.

 

Are you sure the "shock" wasn't you touching the very hot resistor?

 

If the OP did not disconnect things yet, he should turn again power ON for exactly 15 seconds. After that he should turn it OFF and with a previously selected tender finger touch again the 10 Ohms resistor to confirm the (thermal) shock. Cursing is definitely allowed and even more, recommended. Checking the accompanying smell could add one more tool to his bag.

Please report the outcome.

Still remember a TL431 literally melting in front of me.

 

Haven't heard back from the TS since Friday (three days ago). Wondering if he figured it out on his own and hasn't come back, or maybe forgot how to reach AAC. Nevertheless, there's no further use in advising the TS if they're not participating on the forum.

 

There have been several notorious stories where badly designed USB power cubes have electrocuted and killed people.

 

Once again, the small cubes are intended for charging isolated phones that are not in use at the time. That application does not require isolation. But any other use would be very unsafe.