Download Photographic Flash The Impressive Properties of Capacitors

Student’s Guide
Photographic Flash
The Impressive Properties of Capacitors
Olivier Tardif-Paradis
Mathieu Riopel
Cégep Garneau
Centre de démonstration en sciences physiques (support for the development of
demonstrations)
Impact of a drop of water photographed with a very short exposure
made possible by the use of a flash
Source: http://commons.wikimedia.org/wiki/File:2006-02-13_Dropimpact.jpg
APP/Student Guide
Immortalizing the World in Action
Background
The first photographs date back to the early 19th century. Despite the success of the first photographic
procedures for immortalizing the image of a still subject, it was impossible to take an image of a subject in
motion. At that time, the main obstacle to “instant” photos was the long exposure required: an incredible
amount of light was needed to make the sensitive surface of
the photographic plate react, which demanded a lot of time,
from several minutes to several hours. As technological
improvements were made, exposure times became shorter
and shorter. By 1880, exposure times of 1/100 of a second
could be achieved using certain shutters and materials that
were more light-sensitive. Even at these very short exposure
times, a lot of light had to reach the recording surface. The
need arose for a brief but extremely intense source of light.
It was not until 1887 that photographers could make use of
the invention of the first photographic flashes. At that time,
they used magnesium powder, which was burnt in an open
recipient. This manual procedure was relatively dangerous
because of the extreme flammability of magnesium. In 1930,
a safer procedure came to the market: the flashbulb. These
Figure 1: In 1880, these Huron-Wendat, in
bulbs contained an aluminium wire in an oxygen environment. Wendake, Québec, had to stay very still while
their photo was taken. Notice the woman in the
The metal was ignited by an electric current. After a single
centre, who is blurry because she moved during
use, with the metal burnt, the bulb was thrown away.
the exposure.
Flashbulbs were eventually abandoned by the public during
the 1960s when a new affordable and less voluminous
technology appeared: the electronic flash. The electronic
flash uses a reusable xenon bulb and capacitors. When the shutter’s synchronization contact closes the
electric flash circuit, the capacitors rapidly release a large amount of electric energy at a high voltage,
ionizing the xenon in the bulb. The ionized gas becomes a conductor, releasing an intense light. A
photographic flash can produce an intense light for a very short period of time (about 1/1000 of a second).
This flash is generally used to illuminate a scene that is too dark or to take photos of rapid movement.
To understand the role of capacitors in the function of an electronic camera flash, you will design a device
that creates a luminous flash comparable to photographic flash, using a source of continuous voltage,
capacitors and an electric bulb.
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Designing an Effective Flash
The system of an electronic flash is relatively complex. It includes capacitors,
resistances, diodes, transformers, batteries and a discharge bulb. For this
problem, we will simplify the system, considering only the following basic
components: capacitors, a bulb and a source of electricity.
Keeping this simplification in mind, we are asking you to design an electronic
flash that produces maximum luminous intensity but minimizes the length of the
discharge, to achieve the current standard of less than 1/1000 of a second.
You will need the following materials:
Figure 2: Electronic
camera flash, 2005 model

A source of electric voltage that can provide a maximum electrical
potential difference of 100 volts.

Four capacitors with a capacity of 200 F, to the terminals of which a maximum electrical
potential difference of 120 volts can be applied.
o
If the electrical potential difference applied to the capacitor’s terminals is higher than 120
volts, the electric field may attain the value of the disruptive electric field of the dielectric
between the two surfaces of the capacitor. If this occurs, the dielectric will become a
conductor; an intense electric current will travel between its walls, leaving nothing but a
useless, burnt-out capacitor.
An incandescent electric bulb with a resistance of 16 ohms.

You need a maximum power of 2500 watts at the bulb terminals for it to light with optimal
intensity without burning out. If the power supplied to the bulb is higher than this limit, the bulb will
likely burn out and stop working.

You need a minimum power of 1300 watts at the bulb terminals for it to light properly. Below
this power, the bulb will not emit enough light for the requirements of the flash.
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Three-step Cycle
List all the relevant information you gathered when you read the problem. Based on this information, state
what you need to know to solve the problem. As you discover new information, you should summarize
and update the relevant information you have gathered and ask new questions.
List the Following:
What We Know
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To Determine
Summary
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Questions
1) What configuration will you use to charge your system from the source of voltage, to maximize the
energy stored in each capacitor? Calculate the total energy stored in the chosen circuit.
A few questions to head you in the right direction.
a. What parameters influence the energy stored in a capacitor?
b. How is the voltage at the terminals of each capacitor affected by the choice of a series, mixed or
parallel charge circuit?
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2) Assuming that the capacitors have been charged by the circuit selected in question 1, you must
now configure the discharge circuit. What discharge circuit configuration uses the fewest capacitors
while still allowing the system to be functional without burning out the capacitors?
A few questions to head you in the right direction.
a. How does limiting the power of the electric bulb affect the quantity of capacitors required in the
circuit?
b. What physical parameters do you have to limit to avoid burning out a capacitor when mounting the
discharge circuit?
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3) What configuration will you use to discharge your system into the bulb, knowing that the flash will
have to release as much energy as possible without exceeding the maximum power of the bulb and
without exceeding a duration of 1/1000 of a second?
A few questions to head you in the right direction
a. What do you need to maximize in the discharge circuit if you want to maximize the energy in the
flash?
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b. What equation describes the graph of the power consumed by the electric bulb as a
function of time in an RC circuit?
c.
Without exceeding the discharge time of 1/1000 of a second, how does a given configuration of
capacitors maximize the energy dissipated by the flash? In this case what will be the duration of
the flash?
Did You Know?
A xenon flash can only function at an electrical potential difference of between 200 and 400 volts. But it
can only light if it is subjected to a spike in electrical potential difference of more or less 3000 volts. When
the shutter release is pressed, the 300 volts from the capacitor pass through a transformer with a turns
ratio of 10, producing an electrical potential difference spike of 3000 volts, allowing the flash to light. The
concepts related to how transformers work are explained in college-level physics books.
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