Caution, High Voltage activity
Introduction
The printed circuit board of a disposable camera flash provides many interesting electronic components. Please do the Camera Dissection before doing this activity.
Material
Disassembly
Put on rubber gloves to protect yourself from
shock hazard.
Wear eye protection to protect against broken pieces of
plastic.
Remove the back from the camera by using the flat blade of the screwdriver to pry it off. Look for a seam in the plastic. The back is held on by snaps, pry them open and you will not have to break plastic.
To Do and Notice
Make sure there is a battery in the camera. It is
usually a 1.5 volt AA cell.
Find the switch which charges the flash, press the switch, or slide
it to the side. Listen for a high pitched tone which rises in pitch.
Watch for a lamp to glow or flash.
If you do not hear the high pitched tone or see the glowing lamp. Reverse the battery and try again. If you still don’t succeed check the battery with your meter or try a new battery.
Inside the back of the camera find the teeth of a wheel which sticks into the holes in the film. Wind this wheel to the right until you hear a click or two.
Press the shutter release.
The shutter should click and the flash should fire.
When you are done using the flash proceed to its destruction.
After firing the flash and before recharging it again,
Remove the battery from the camera.
Expose the camera flash electronics by prying the front off the camera.
Remove the circuit board from the camera. You must wear rubber gloves to do this part of the activity or risk a painful shock.
Locate the largest cylinder on the circuit board. It should be a large capacitor. This capacitor is the primary energy storage capacitor. It contains enough electric charge at a high enough voltage to give you a painful shock. That capacitor is why you are wearing gloves
The cylinder in my camera was labelled 180 mF, 330 V. This translates to 180 microfarads, 330 volts. The voltage tells you the maximum voltage for the capacitor. Higher voltages will destroy the capacitor. Good design practice calls for capacitors to be run at 1/2 their maximum voltages or less. These flash units are not designed for long lives and so run near 300 volts.
Set the voltmeter on the 1000 volt scale and measure the voltage across the capacitor. If the capacitor is discharged you will have to switch to a lower scale, for example the 100 volt scale.
The meter may read 50 volts, even though the battery has been removed and the flash fired. Be careful when working inside electronic equipment. Often capacitors are charged to high voltages even when the device they are in is unplugged and turned off
(See the math root below for more information on capacitors.)
To make the capacitor safe:
Place a 100 ohm resistor, or piece of pencil lead, across the two leads coming from this capacitor. This should discharge the capacitor. Re-measure the voltage across the capacitor.
When the voltage goes to zero the capacitor is discharged.
To keep the capacitor safe connect a clip lead between the two terminals.
Once you have connected the clip lead you may remove your gloves.
Here is the basic recipe for examining any circuit.
First, locate the power source, then examine the inputs and outputs.
Power
Find the power supply for the circuit. In my camera the flash was powered by a 1.5 volt AA battery.
Output
The primary output is easy , a xenon-filled flash tube produces a flash of light. It takes over 10 kilovolts to fire this tube. The electronic circuit produces this high voltage.
Find the flash tube which should be surrounded by a metal reflector.
The secondary output is a neon glow tube, a glass cylinder with two electrodes inside. It glows when the circuit is charged.
Input
There are two inputs.
Pressing one button charges the main capacitor. A high pitch whine is produced by the oscillator driven transformer as the circuit charges. When the capacitor is charged the neon lamp glows.
A second input is the shutter release button. Pressing this button triggers the shutter to open. When the shutter is open fully, it closes a switch which fires the flash.
(Before the shutter can be fired the camera must be wound which is why you rotate the wheel inside the back of the camera.)
What’s Going On?
(Circuit diagram)
Each different manufacturer uses a slightly different circuit design. However you should be able to identify the same components: resistors, capacitors, transistors, etc in each circuit. You should also be able to identify the three stages of each circuit.
1. The first stage of the circuit operates when the charging button is pressed. It boosts the voltage of the 1.5 volt battery to a high voltage. A capacitor and inductor (coil) form an oscillator. The oscillator is fed into a transistor amplifier to boost the current, part of the boosted current is fed back into the oscillator, then the current passes into a transformer where the voltage is increased to a high, AC, voltage.
2. A diode followed by the capacitor of stage 2 then turns the AC into a high DC voltage, HV, over 300 volts. A neon lamp glows when the voltage HV exceeds 80 volts. The large capacitor discharges through the xenon flash tube when the flash is triggered by stage 3.
3. A switch is closed by the shutter which puts a voltage step, HV, across a transformer. The transformer boosts this voltage pulse to over ten thousand volts, VHV. A wire brings this VHV voltage to a place close to the flash tube. The very high voltage ionizes the gas in the flash tube, making it a conductor. The high voltage from the capacitor then drives a current through the flash tube heating the gas and causing it to give off a flash of light.
Find and identify examples of the following items usually found on your circuit board
1. A xenon flash tube. This tube contains low pressure xenon gas which is an insulator unless it is ionized. When it is ionized by bringing a very high voltage near it, the tube becomes a conductor. Xenon gas gives off spectral lines in colors which appear white to the human eye and to daylight film.(mini-etc. Can you cause the xenon tube to flash using a charge and carry apparatus?)
2. A neon glow tube. The tube in the camera flash is a special high voltage neon tube which requires over 100 volts to make it glow. It contains low pressure neon gas and two electrodes. The electrode connected to the lower voltage (the negative electrode) glows. It only requires 10-12 amps to make a neon glow tube glow. Neon glow tubes are very useful in electrostatics experiments. Save them from your disassembled cameras.
3. Capacitors there may be two kinds of capacitors, disk capacitors (or lentil bean shaped capacitors) and cylindrical capacitors. The largest cylinders on the printed circuit board are capacitors. They often have their capacitance written on them and their maximum voltage rating.
Capacitance is measured in farads. The capacitors you will find on the circuit board will be a few hundred microfarads, mF, at most. When a DC voltage is applied to an empty capacitor, current flows through the capacitor and each side of the capacitor becomes charged. The two sides have equal and opposite charges. See the math root. When the capacitor is fully charged the current stops flowing.
4. Resistors The board has many small cylinders. The small cylinders that are resistors have colored lines wrapped around them. These colored lines are the resistor color code. The electrical current that flows through a resistor, I, is proportional to the voltage, V, across the resistor, R. This is known as ohm’s law.
5. Diode, Small cylinders with a single line wrapped around one end are diodes. Diode identification numbers often start with “1N”. Electric current will flow through the diode only in one direction, only when the the end with the line is at a lower voltage than the other end. The diode in this circuit is used together with a capacitor to turn AC into DC.
6. Transistor, usually a black plastic half cylinder with three legs. Transistor identification numbers often begin with 2N. The current between two of the legs, the collector and emitter, is controlled by the current that flows into the third leg, the base. The transistor can be modelled as a valve for electric current. A valve that is in turn controlled by electric current.
7. Transformer. These packages often have visible coils of wire. Thin wires lead from the package to the circuit board. The transformers have three or more legs. Transformers convert one AC voltage to another. The coils of wire in the transformers also behave as coils with inductance. An inductor combined with a capacitor will oscillate when it is hit with an abrupt change in voltage or current such as the closing of a switch.
8. The printed circuit board . Components are inserted through holes in the circuit board and soldered to copper conductors. The circuit board starts out covered with copper. It is then placed in an acid bath which etches away the copper leaving behind the pattern of conducting strips.
The voltage, V, across the capacitor is proportional to the charge, Q, on each plate of the capacitor, the constant of proportionality is the capacitance, C.
So the charged capacitor on my flash contained
180 mF x 100 V =18,000 mC, micro-coulombs.
Etc
To avoid electric shocks electrical engineers say, when working on high voltage, over 24 volt, circuits, ”keep one hand in your pocket.” The most dangerous shocks produce electric current through your heart, for example, when flowing from one hand to the other. By using only one hand you reduce the risks from electric shock.
If the screwdriver blade is placed across the main capacitor leads, a large spark will result. The vaporized metal from the screwdriver blade is not beneficial to health. In addition, the ultraviolet light from watching the flash repeatedly can sunburn your corneas leading to painful “snowblindness.” This is why we use a 100 ohm resistor or pencil lead to discharge the capacitor.
Scientific
Explorations with Paul
Doherty 22 June 99