Why are capacitors so dangerous?

Question

There are always lots of warnings on electronics when large capacitors are used (televisions, car audio etc). I have worked with capacitors a little bit and have seen that they can in fact deliver a large shock. However, whenever a capacitor is used, it is only capable of charging up to the voltage of it's power source (12V car battery, voltage from power supply in TV), correct? If so, what makes the capacitor so dangerous to work with?

Answer 1

Several things ...

1) Capacitors can be like a battery, that is they can store energy for a long time, they are active (can be energized) and therefore can shock you.

2) A capacitor can deliver an unknown charge in the form of a current. In a shock it is the current that does the damage. In the wrong situation it does not take a lot of current to kill if it goes through the heart.

3) It is not true that a capacitor can only charge to the level of its power supply. Look at your TV, capacitors that charge to create the voltage in a TV can he hundreds of times more than the 120V wall socket voltage because they can be stepped up with other circuits - typically 20,000 volts.

4) Unless a circuit is designed safely or protected the charge in a capacitor can exist and not bleed off for a very long time. Display tubes are notorious for holding very high volages for a very long time and shocking people badly.

Answer 2

Capacitor Danger

Answer 3

Although you can't be electrocuted by twelve volts, a capacitor with a large charge could hold enough energy to cause burns or start a fire. Also, electrolytics can explode if the polarity is reversed.

In the case of televisions, there is a high voltage supply for the CRT, and the capacitor there can deliver a nasty jolt.

Answer 4

No, a capacitor can contain a charge much greater than its power source.

Quote from Wiki
"Capacitors may retain a charge long after power is removed from a circuit; this charge can cause shocks (sometimes fatal) or damage to connected equipment. For example, even a seemingly innocuous device such as a disposable camera flash unit powered by a 1.5 volt AA battery contains a capacitor which may be charged to over 300 volts. This is easily capable of delivering an extremely painful, and possibly lethal shock."

Answer 5

A capacitor is like a battery.
A battery is a constant current source, a 12volt battery does not have enough voltage to harm you.
A capacitor may not have the current capacity of a battery but at high voltages it can be very deadly.
You could series batteries to higher voltages.
Above 50 or 60 volts they can also be deadly.

Answer 6

The main idea is that it's not the potential difference -or tension- (which is measured in volts) that kills you. It's the current -or intensity- (measured in amperes). Although there may be some differences, depending on the voltage, duration, current, path taken, frequency, etc.; we may have two situations at the same time: In some cases 16 volts might be fatal to a human being when the electricity passes through organs such as heart, if it excedes ~60 milliamperes (AC); while at the same time we can have high voltage currents (>500-1000 V) that may only cause minor burning.

Answer 7

they can hold a charge even though you have your power locked out. that's why you discharge them before you do a lockout/tagout procedure in any work place. Capacitors can hold more than 12 volts and you should consider the amperage as well.

Answer 8

Capacitor:
A capacitor is an electronic device which consists of two plates (electrically conductive material) separated by an insulator. The capacitor's value (its 'capacitance') is largely determined by the total surface area of the plates and the distance between the plates (determined by the insulator's thickness). A capacitor's value is commonly referred to in microfarads, one millionth of a farad. It is expressed in micro farads because the farad is such a large amount of capacitance that it would be impractical to use in most situations

Capacitor and DC voltage:
When a DC voltage source is applied to a capacitor there is an initial surge of current, when the voltage across the terminals of the capacitor is equal to the applied voltage, the current flow stops. When the current stops flowing from the power supply to the capacitor, the capacitor is 'charged'. If the DC source is removed from the capacitor, the capacitor will retain a voltage across its terminals (it will remain charged). The capacitor can be discharged by touching the capacitor's external leads together. When using very large capacitors (1/2 farad or more) in your car, the capacitor partially discharges into the amplifier's power supply when the voltage from the alternator or battery starts to fall. Keep in mind that the discharge is only for a fraction of a second. The capacitor can not act like a battery. It only serves to fill in what would otherwise be very small dips in the supply voltage.

Capacitors and AC voltage:
Generally, if an AC voltage source is connected to a capacitor, the current will flow through the capacitor until the source is removed. There are exceptions to this situation and the A.C. current flow through any capacitor is dependent on the frequency of the applied A.C. signal and the value of the capacitor.

Capacitors may retain a charge long after power is removed from a circuit; this charge can cause shocks (sometimes fatal) or damage to connected equipment. For example, even a seemingly innocuous device such as a disposable camera flash unit powered by a 1.5 volt AA battery contains a capacitor which may be charged to over 300 volts. This is easily capable of delivering an extremely painful, and possibly lethal shock.

Care must be taken to ensure that any large or high-voltage capacitor is properly discharged before servicing the containing equipment. For safety purposes, all large capacitors should be discharged before handling. For board-level capacitors, this is done by placing a bleeder resistor across the terminals, whose resistance is large enough that the leakage current will not affect the circuit, but small enough to discharge the capacitor shortly after power is removed. High-voltage capacitors should be stored with the terminals shorted, since temporarily discharged capacitors can develop potentially dangerous voltages when the terminals are left open-circuited.

Large oil-filled old capacitors must be disposed of properly as some contain polychlorinated biphenyls (PCBs). It is known that waste PCBs can leak into groundwater under landfills. If consumed by drinking contaminated water, PCBs are carcinogenic, even in very tiny amounts. If the capacitor is physically large it is more likely to be dangerous and may require precautions in addition to those described above. New electrical components are no longer produced with PCBs. ("PCB" in electronics usually means printed circuit board, but the above usage is an exception.) Capacitors containing PCB were labelled as containing "Askarel" and several other trade names.

High-voltage

Above and beyond usual hazards associated with working with high voltage, high energy circuits, there are a number of dangers that are specific to high voltage capacitors. High voltage capacitors may catastrophically fail when subjected to voltages or currents beyond their rating, or as they reach their normal end of life. Dielectric or metal interconnection failures may create arcing within oil-filled units that vaporizes dielectric fluid, resulting in case bulging, rupture, or even an explosion that disperses flammable oil, starts fires, and damages nearby equipment. Rigid cased cylindrical glass or plastic cases are more prone to explosive rupture than rectangular cases due to an inability to easily expand under pressure. Capacitors used in RF or sustained high current applications can overheat, especially in the center of the capacitor rolls. The trapped heat may cause rapid interior heating and destruction, even though the outer case remains relatively cool. Capacitors used within high energy capacitor banks can violently explode when a fault in one capacitor causes sudden dumping of energy stored in the rest of the bank into the failing unit. And, high voltage vacuum capacitors can generate soft X-rays even during normal operation. Proper containment, fusing, and preventative maintenance can help to minimize these hazards.

High voltage capacitors can benefit from a pre-charge to limit in-rush currents at power-up of HVDC circuits. This will extend the life of the component and may mitigate high voltage hazards.

Answer 9

A capacitor does exactly what it sounds like. it fills with electricity to its capacity then it releases it all at once so if it is able to store a lot even filling it with a small battery can be deadly

Answer 10

Because they hold a charge even when the power source has been removed. Without the proper care you can get a nasty shock.