High Voltage Capacity
p{text-align: center;}. !http://blog.smalldog.com/images/4948.png!
I’m going to date myself a little bit here, but it’s for a good cause, so I think it’s ok. When I was a kid (at least until the early 2000s) digital cameras still weren’t very prolific. Most of the cameras I had used 35mm film though my parents did give me a little blue toy camera when I was very little that used something called a 110 cartridge. I would also occasionally get disposable cameras at parties and things. One of the things I remember about the disposable cameras was the warning labels about not disassembling the camera because of electric shock risk. Actually, disassembling any camera with a flash will have this risk, but I think people might’ve been more prone to disassembling disposable cameras in an attempt to reuse them. What is the risk though?
The risk specifically comes from how the camera flash is generated. In order to create such a brief, bright flash, a normal battery source cannot be used. Instead a large voltage build up must be created and then quickly dumped into the light source. Batteries of all chemistries are pretty versatile at delivering consistent, controlled power at relatively low voltages. AA cylinder batteries for example are 1.5 volts, 9V block batteries are obviously 9 volts and even big car batteries are only 12 volts. These batteries are so common because they are safe for pretty much anybody.
One of the things that makes batteries so safe is that they cannot deliver large amounts of power quickly. When a battery is part of a circuit it acts not only as a voltage source, but also as a resistor. All batteries have some amount of what’s called “internal resistance”. This puts a hard limit on how much power (voltage x amps) that a battery can deliver without self-consuming due to resistive heating. So what do we do if we need a lot of voltage delivered rapidly?
Typically, and in our example of camera flashes, this is accomplished with capacitors. I’ve talked about capacitors here before and while they’re not as glamorous and exciting as some other electrical stuff, they are very useful and powerful. A capacitor is a bit like a battery in the sense that it stores electrical charges. They can also be charged and discharged thousands of times with little change in capacity. Compared to batteries, capacitors have a very low internal resistance, and ideally, they have none, though in the real world this isn’t the case because all electrical components have some internal resistance. Unlike batteries, capacitors can be charged almost instantly and also unlike batteries the amount of available charge is extremely low. This means that batteries cannot just be swapped out for most types of capacitors, even if it would be cool to fully charge your iPhone from 0% to 100% in 30 seconds.
Capacitors are usually rated for their capacity and maximum voltage. The capacity is measured in farads where one farad is one coulomb of charge stored across a potential of one volt. Typical capacitors are not measured in whole farad units though, because one coulomb of charge across one volt is ??a lot?? of charge. Most consumer-grade capacitors are measured in microfarads (millionths of a farad). A “large” capacitor you could easily buy (not specialized) would be 1000 microfarads. I have some of these at home. Smaller capacity capacitors are typically able to charge to higher voltages. A camera flash mechanism is typically a 100-200 microfarad capacitor charged to between 200 and 300 volts. How dangerous is that?
This is a difficult question to answer, because we’re talking about different units and there are all kinds of conflating factors, but let’s make some assumptions and see what we can learn. Let’s start easy. The typical human body has enough resistance that anything below 50V won’t be able to create enough current in you. At 200V, we’re well above that, but as I said, capacitors don’t store much electrical potential. Stored energy in a capacitor is measured in joules and is defined as 1/2CV^2. 200 squared is 40,000, times our capacitance which is 0.0002 farads equals 8, and finally, divide all that by 2 and you get 4 joules of stored energy. Is this enough to kill?
A defibrillator can deliver up to 360 joules of energy, so our little capacitor is going to be handing out a fraction of that. This probably won’t kill, but it is going to hurt. Notice though that the biggest factor in how much energy a capacitor has is the voltage since the energy stored is a function of the voltage ??squared??. Doubling the voltage to 400V and leaving everything else the same produces a four-fold increase in energy. Increases in voltage, even on low capacity capacitors can quickly increase the potential death factor, especially since increased voltages represent more potential to push current, no matter what the resistance. When dealing with capacitors, always, always, always short them out with a screwdriver or other heavy-duty metal before working near them. Capacitors can stay charged for a long time so your junkyard find of an old stereo, or CRT monitor might still have the power to kill.
Be safe and be knowledgeable!