Ohm's Law and Death

Question

So I bought an Arduino today, my high school grades were terrible but I decided to dive into electronic engineering anyway.

I am a total noob so please forgive me. So Ohm's law states v = R * I, therefore V/R=I. If I have 3.3 V output from an Arduino that shall give me 0.1 amp. and 10 ohm resistor will produce a current of 0.33 amp.

"Any electrical device used on a house wiring circuit can, under certain conditions, transmit a fatal current. While any amount of current over 10 milliamps (0.01 amp) is capable of producing painful to severe shock, currents between 100 and 200 mA (0.1 to 0.2 amp) are lethal."

Why am I still alive?

Part 2.

Scenario: I connected a 33 ohm resistor to 3.3 V Arduino output, according to a plan that shall produce 0.1 amp, but my multimeter reads 0.01 amps.

Again please forgive me, it is my first day of electronic engineering.

Your body has resistance so high that 3.3 volts on the skin will not injure you. On the other hand, 3.3 volts into two electrodes into your brain will be a very bad experience. — analogsystemsrf, Oct 15 '18 at 17:34
100 mA through your body is likely fatal. Did you route 100 mA through your body, or just through some device you were investigating? — The Photon, Oct 15 '18 at 17:34
How are you measuring current? Remember current must be measured *in series*, not *in parallel*. Also, your arduino is not going to be able to source 100mA. That would require that the very fine bond wires within the chip be able to handle significant heating. Generally they are internally current-limited to prevent damage to the chip. — DerStrom8, Oct 15 '18 at 17:38
If your multimeter has a resistance range, you can try measuring the resistance between your hands. Different people will have different resistances; I just tried and got about 3 MΩ. — Andrew Morton, Oct 15 '18 at 17:42
@AndrewMorton Thanks for the reply, my meter prob sucks cause it shows just 0.0L — Anton Stafeyev, Oct 15 '18 at 17:52
@AntonStafeyev One thing to note when measuring something is to make sure that the measuring device is working first. For example, when using a multimeter to measure resistance, touching the probes together should give a reading very close to 0 Ω. It doesn't have to be exact, just good enough. If you mention the make and model of your multimeter when asking a question, we would have a better idea of what sort of measurements we could ask you to make. — Andrew Morton, Oct 15 '18 at 18:37

score 1 · Answer 1 · answered Oct 15 '18 at 18:18

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. A simple current measurement.

Find a multimeter.
Switch it to the most sensitive DC current measurement range. Plug the leads into mA and COM.
Connect one lead to +3.3 V.
Hold the other lead in your left hand.
Touch the 3.3 V common or ground terminal with your right hand.
You will not be dead so you can take a current reading from the multimeter.
Let go the leads and write down the reading.
Is the current greater than 100 mA?

Let us know what you find.

Plug the leads into V and COM and switch back to volts. If you forget to do this before you take your next voltage measurement you may blow the current shunt fuse or destroy the meter.

score 0 · Answer 2 · answered Oct 15 '18 at 17:40

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Your Arduino can only provide so much current. Obviously, it's not infinite, so you can put a 10 ohm resistor across it, but what will happen is the voltage will drop as your trying to overdraw the pin, and you will likely burn it out. Off the top of my head, an Arduino uno pin can output about 10mA?

As for dangerous voltage, it is 35V DC and 50V AC which is the safe working limit. Anything above that is deemed dangerous voltages. They in themselves will not kill you but if they are able to provide lots of current the power passing through you could. The electrical signal will cause your muscles to contract, so if you have a weak heart - that voltage could stop it.

answered Oct 15 '18 at 17:40

Curious Diode

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Thanks for oyur reply man. another question that i had is why do i get different reading and math results on 33ohm resistor. it is in part 2 of my question – Anton Stafeyev Oct 15 '18 at 17:53
As said, you are over-drawing the maximum current limit of your Arduino. It cannot deliver 0.1A. Think of it like this, your holding your arm out at 90 degrees. The height of your arm is voltage. You can load your arm by holding weight in your hand which is current. If you put more weight that you can support you start lowering your arm. By putting too much weight you can damage your shoulder which is the system. Such you can never hold weight again – Curious Diode Oct 15 '18 at 18:00

score 0 · Answer 3 · answered Oct 15 '18 at 17:53

Why i am still alive ?

There are a few factors.

Firstly can kill doesn't mean will kill. The route the current takes through the body is important (electricity generally kills by disrupting the heart) as is the sensitivity of the individual. Many people get shocks off the mains each year, unfortunately we don't know exactly how many because most of those shocks are never recorded anywhere.

Secondly resistors are not the only thing that have resistance, human skin is normally quite high resistance. So under normal circumstances (dry indoor environment, no deliberate attempt to make a low impedance connection to the body) voltages up to about 50V are considered safe to touch.

Scenario: i connected 33ohm resistor to 3.3v Arduino output, according to a plan that shall produce 0.1 Amp, but my multy-meter reads 0.01 Amps.

Because the resistor is not the only thing limiting the current. You have the drive transistor in the arduino, your resistor and the current sense input on your multimeter in series. In particular the outputs of the ardunino are not built to drive that much current. If you take a second multimeter and measure the voltage across the resistor you will probablly find it is much less than 3.3 volts.

score 0 · Answer 4 · answered Oct 15 '18 at 18:08

Current must flow in a loop.

The current that flows through that loop will be a function of all resistances that the current must travel through. Your skin has a relatively high resistance so little current flows due to a 3.3V potential.

If you put a 9V battery on your tongue you will feel something. If you place a 9 V battery on your arm likely nothing. As the resistance of your tongues is much lower.

A GPIO pin of an ardunio has its own output resistance.

A multi-meter has a series resistance when used as an ammeter.

Ohm's Law and Death

4 Answers4