Sep 11, 2011

Volts Kill

I hear people say all the time, “voltage is not as dangerous as current”. If you are like me, when you hear this you immediately think “shouldn’t natural selection have taken care of these weak minded dregs by now?”, but later come to find that, alas, they still exist by the thousands. So to any of my normal readers and those who may have stumbled upon this blog by happenstance let me say this once and for all: voltage creates current flow! Do not ever assume that because something is “low-voltage” that is cannot deal you any significant damage. For the record, most industry products are considered “low-voltage” if they run off of less than 48 volts, which is enough to cause you discomfort. It also depends heavily on the voltage source of interest. Constant voltage sources will deliver a current according to Ohm’s law. Capacitors or other energy storage devices, on the other hand, are capable of delivering massive amounts of current quickly despite a “low voltage” on the capacitor.

Figure 1. Capacitor Discharge into a CD, Courtesy of Ben at Buxtronix
So how does a statement like this become so widely accepted? Well, like many scientific myths there is a grain of truth embedded in the nonsense. The truth is that human skin is generally quite tolerant of certain voltages because its resistance is high enough that the voltage source cannot provide, or drive, enough current through us to cause physical harm. The amount of current passing through your body is what will ultimately kill you so in that sense it’s true that current is the most dangerous aspect of working with electricity. However, there can be no current flow without a voltage source to push the current through a channel. That’s the end of the argument if there ever was one. But just to prove my point, let’s attempt some science.

Ohm’s law applies to humans as well as electronic components, but humans are not purely resistive. We are capable of building up charge like a capacitor and then discharging that energy through our skin. For instance, when you rub your socks along the carpet or a balloon against your hair you are building up charges in your body. To release the charges, you need to touch a grounded piece of metal, creating a static shock.  In order to model this effect in humans, researchers usually use what’s called an RC circuit. I will cover capacitors and RC circuits more in future blogs so don’t get weighed down in how these circuits operate. I will give you the highlights.

The human charge equivalent circuit is pretty simple: a 100pF capacitor and a 1.5kohm resistor in series like in Figure 2. The capacitor is capable of charging up to the source voltage, but it is only 100 picofarads meaning it stores very little energy at low voltages. In fact, we can figure out exactly how much energy is stored in this capacitor with a simple formula: E = 0.5*C*V2.
 Figure 2. Human static charge model

Let’s assume that you have charged yourself up to 10,000 volts (10 kV) by building up static charges on your body – and, yes, this voltage is typical of a strong static shock. Using the formula we get a total energy of 5 millijoules. For reference, you have radiated about 500 joules away as heat since you started reading that last sentence. I have also taken a picture of a low voltage 100pF capacitor so you can see exactly what sort of charge tank we are dealing with here.

Figure 3. Scale of a low-voltage 100pF capacitor

*****WORD OF WARNING*******
Here is where it is important to make the distinction between voltage and charge in a capacitor. The amount of energy that a capacitor can hold will increase with capacitance and it is critical that you understand this if you ever plan on handling larger capacitors. For example, let’s assume that a 100 farad capacitor is charged to 2 volts. Noobs may think, “only 2 volts, pfff, what’s the big deal?”. Here’s the big deal: its 100 farads of capacitance. Using the same equation as above, we find that this capacitor when fully charged will hold 200 joules, which is 40,000 times more energy than humans store in our bodies during a decent static shock. THIS AMOUNT OF CHARGE CAN KILL YOU!!!! If you were to discharge this capacitor through your body it would be equivalent to getting a shock from a defibrillator, which would mean game over noob.

How is it possible then that 5mJ of energy could be responsible for the sparks we see when we get a static shock? Hey, why do you ask so many freaking questions? (Sigh)… If I must explain… it has to do with air’s dielectric field strength. All you really need to remember is that voltage is like an electrical pressure, and with enough pressure you can knock down damn near anything. In the electrical sense, this means that with a high enough voltage you can get just about any material to conduct electricity, including air. Air’s dielectric field strength is rated anywhere from 10 – 30kV/cm depending on atmospheric conditions. That means if you had a 10kV voltage source (like your charged body) and a conductor (like a grounded piece of metal) 1 centimeter apart you would be able to ionize the air between the two and create a conductive path for electrons. This dielectric field strength is why you don’t get static shocks from across the room.

But there is another factor in this equation that we haven’t really considered:  time. The length of time you are exposed to a certain voltage has a huge impact on whether you survive or not. When you discharge yourself into a ground, there is a large initial current that flows through your body. Fortunately for lovers of shag this surge of current lasts only a fraction of a second and decays rapidly in magnitude (such is the nature of RC circuits). I have illustrated the expected current flow of our example static shock in Figure 4.

Figure 4. Example static shock discharge curve

You can see that at the very first instant a current path is created between you and ground, a current in excess of 6.5 amps is flowing out of your body. However, just 150 nanoseconds later the current drops to less than 2.5 amps. After just 1.5 microseconds, the current will drop below 300 microamps and you won’t feel anything anymore. While the initial current drain was large, the overall energy dissipated was small and it was dissipated quickly so no harm was done to the individual.

This may be the case in with static shocks and voltage, but what about electric circuitry? In electric circuits there generally isn’t a finite amount of charge stored somewhere that will dissipate once you touch it. Instead, a continuous current will flow through your body causing severe pain, cardiac arrest, muscle paralysis, and eventually death assuming the current is large enough. Again, though we are worried about continuous current flow and its magnitude, the voltage is ultimately what will push the current through your body.

Just to show some stuff blowing up, check out the video below. Ben Buxton of Buxtronix works with high voltages and large capacitances on a fairly regular basis. In the video, he uses a 0.25 microfarad capacitor charged to 23kV, which is about 150 joules according to our earlier equation. Check out the best way to blank a CD.

That ended up being a lot more information than I wanted to give in this post. For clarity, let’s review the important notes again:
  1. Voltage, charge, and current are all tied together. Saying one is more deadly than another is ridiculous.
  2. Energy storage devices like capacitors differ from generic electric circuitry because they are capable of delivering a massive amount of energy into a load (like you) very quickly.
  3. The amount of energy a capacitor can hold is dictated by the capacitance and the voltage. A 100 picofarad capacitor at 10kV has much less energy than a 100 farad capacitor at 2 volts.
I will never discourage someone from starting to work with electronics and high current applications. However, the hobbyists out there need to understand the nature of charge before you get into anything serious or you could up severely hurting yourself and/or someone else. Mistakes will happen, but make sure you have taken the precautions to be able to play another day noobs.


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