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What exactly are volts and amps, and what possible interest can
they be to me? |
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I was fortunate
enough to join the BBC over 40 years ago when it was considered
essential that all technical staff had an understanding of the
basics of their craft. As such, we were all duly packed
off on a three month residential training course at Wood Norton
in the Vale of Evesham to learn the ins and outs of dBs, volts,
amps, watts, balanced and un-balanced and anything else that you
never dared admit you knew nothing about.
Sadly, in today's less than
philanthropic world many otherwise perfectly competent
operatives lack a proper grip of these concepts.
Some of you out there may have
been plumbers in a previous life, or have gained some knowledge
of the craft through necessity. The laws of how water
works are a very good way of understanding how electricity
works.
So here follows a "90 second"
guide to basic electricity for plumbers: |
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Volts
are equivalent to water pressure,
ie how much FORCE
there is
Amps
are equivalent to water flow, ie how
many LITRES
have passed
Watts
are equivalent to water capacity, ie
how FAST
you fill a bucket |
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From the above
rough analogy it is hopefully obvious that : |
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Watts =
Volts x Amps |
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ie to fill your
bucket in a reasonable amount of time you need good water
pressure AND a large enough pipe to allow a good flow of water.
You can have very good pressure (volts) but still little water
will flow (amps) if the pipe is too thin (resistance). |
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Ohms = Resistance |
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Diameter of pipe is equivalent
to thickness of wire, ie thinner wire (more resistance) = less
current flow (amps). |
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The relationship between ohms,
volts and amps is our famous Ohm's Law. For more in depth
information about the
Ohm's Law click here. |
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It is obvious that we can control
the flow of water by varying the pressure (volts) AND/OR the
pipe diameter (ohms Ω).
The product of pressure (volts) and flow (amps) gives the
capacity (watts), for example: |
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12 volts @ 2 amps = 24 Watts |
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If you know any two quantities
you can easily work out the third, ie how much current will a 12
volt CRT monitor draw? The answer is 60
÷ 12 = 5 amps. |
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We cannot take the water analogy
too far (electricity will flow uphill!) but it is often
useful to imagine how water would behave in a given situation
and that is generally how electricity will also behave. |
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A popular misconception is that
there are a certain number of volts in an amp. But from
the above it is clear that they are two completely different
properties. Fed with a certain voltage a circuit will draw
as much current (amps) as is defined by it's resistance (ohms
Ω).
You cannot feed a circuit too many amps! |
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Can you convert Watts per hour into Amps per hour? |
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Can you convert Watts per hour into Amps per hour? |
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Yes, you can!
Simply divide the Watt/Hour figure by the battery voltage. |
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For example, a 12
volt battery is 60/12 = 5Ah. That is, this battery
will supply approximately 5 amps for one hour, or 1 amp for five
hours. |
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In reverse, a 14.4
volt lithium battery rated at 6 Ah would = 14.4 x 6 = 86
Watts hours. |
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What
in simple terms is a dB? |
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What
in simple terms is a dB? |
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A dB is a decibel,
that is one-tenth of a Bell. Strictly speaking, a dB is a
ratio of two values. The base level for any application is
often fixed by general agreement so values relative to this
level are simply referred to as + (plus) or - (minus) however
many dB. |
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For example, take a
sound mixer with an output level meter calibrated in dBs.
4 on a PPM (peak programme meter) is equivalent to 0dB.
This level corresponds to a voltage of 0.775 volt. If we
talk of a level of +6dB this is a shorthand for saying
2 x 0.775 volts, ie 1.55 volts. |
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The dB is a logarithmic scale
which is very handy as our ears have a broadly logarithmic
response. As such, each increase of 6dB approximately
doubles the voltage. Note that when referring to power,
and increase of 3dB approximates to doubling the power.
Increasing the power output level of a loudspeaker by 10 times,
ie 10dB, will make it sound about twice as loud. A change
in sound level of 1dB is about the smallest amount that can be
perceived by the human ear. |
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When using
attenuators or pads that are marked in dBs it is important to
realise that the amount of loss in dBs will only be as marked if
the impedances are as stated on the pad. A 20dB pad is
meaningless if it is designed for a 600 ohm circuit and you are
using it on a 50 ohm circuit. |
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dBs are not just
useful for sound levels. They can be used to denote
relative power levels for all sorts of equipment in electronics
and carry a suffix such as dBA or dBu, which have different
agreed reference levels for different applications. |
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What is the difference between a balanced and un-balanced
circuit? |
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What is the difference between a balanced and un-balanced
circuit? |
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An un-balanced circuit has a
single wire that carries the signal information, whereas a
balanced circuit has two signal wires. |
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The advantage of the balanced
circuit is that it is much less prone to interference. |
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In an un-balanced circuit,
although the single signal wire is often screened with an outer
covering of metal braid, this is never perfect and in situations
where there is lots of interference - perhaps from mains
lighting dimmers - the ratio of wanted to unwanted signals can
be enormous. |
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Consider a microphone cable
running alongside a mains dimmer cable. The mains cable
could have voltage peaks of up to 375 volts, yet the microphone
cable may only be graced with a few microvolts, ie several
hundred million times less! Given that you need less than
one-millionth of the interfering signal to be inaudible you are
looking to achieve an isolation of hundredths of billionths of
the interfering signal. Even the best cables only achieve
about 98% optical coverage of the braid so the only practical
solution is the balanced approach. |
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A balanced cable has two signal
conductors twisted together. Any interference appearing on
one conductor will appear the same on the other conductor.
By applying the signal wires to the opposite ends of a receiving
transformer any interference is cancelled out but the wanted
signal is unaffected since the wires are driven in anti-phase
from the sending transformer. Of course, the cable is
still screened as a first line of defence but balancing the
circuit gives a huge improvement. |
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As a further improvement, star
quad cable has four signal conductors twisted together in pairs
which provides yet another layer of interference reduction.
In modern circuits the transformers referred to above are
increasingly being replaced by electronically balanced
components, largely for cost and size reasons. In some
respects they are superior, but in others the transformer is
still best. Another story for another day! |
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Most professional sound equipment
uses balanced circuits and most domestic equipment uses
un-balanced circuitry for obvious cost reasons. Domestic
equipment rarely has the long cable lengths that can often occur
in the professional arena. Examples of balanced circuits
are XLR3, stereo ¼" jacks wired
balanced mono, HRS10 and Tajimi 12 pin mixer cables.
Examples of un-balanced circuits are BNC, phono and stereo
jacks. |
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Don't forget
that for a balanced circuit to work all parts of the system have
to be balanced. It is no good connecting a balanced XLR
feed via a phono adapter as the whole circuit is now
un-balanced. Connecting any un-balanced circuit to a
balanced one un-balances it. Grounding either signal wire
of a balanced circuit un-balances it (ie connecting the XLR
output of your SQN mixer to a Walkman un-balances the Tajimi 12
output too, unless you use our
Time Code Box,
of course!) |
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Fuses are really simple, straightforward components - aren't
they? |
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Fuses are really simple, straightforward components - aren't
they? |
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Fuses are used to protect
equipment from taking too much current if a fault develops.
By creating a weak link that will interrupt the current quickly
and safely excessive damage might be avoided and the risk of
fire reduced. |
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It is important to know that
fuses do not provide
protection against electrocution. |
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Fuses nearly always blow for a
reason. It is obvious you need the correct physical size
of fuse. Most modern equipment uses 22mm long fuses and
older equipment perhaps 1.25", but there are numerous oddball
sizes too. Increasingly, fuses are mounted direct on PCB's
and don't look anything like a fuse; finding them and
replacing them is a job for a specialist. (This is the
case with the tiny fuse in the Panasonic 7" LCD Monitor which
blows if you power it with reverse polarity). |
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Fuses generally come in current
ratings from 63mA (very small) to 10A (pretty hefty).
Replacing a blown fuse with a larger value one will mostly
result in a more costly repair later and at worst could result
in a risk of fire. |
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Occasionally fuses fail through
fatigue and replacing with the original value will make the
equipment work again normally, but this is a fairly rate
situation. This usually happens on high powered equipment
with high inrush currents like halogen lamps where the fuse
heats and cools each time the lamp is switched on. |
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Fuses come in different speed
ratings. Very fast fuses (FF) are used to protect
semi-conductor equipment but for use with a transformer you
would need a delay fuse (T) to cope with the high inrush
current. It is just as important to have the correct speed
as the correct current rating. Some fuses are ceramic
bodied and sand filled which gives them a very high breaking
ability. Safety could be compromised by replacing with a
glass fuse that might shatter. |
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Most fuses you meet every day
have a voltage rating of 250 volts, which is fine for the
majority of equipment. However, specialist applications in
monitors, TVs, LCD displays and microwave ovens require high
voltage fuses that may be rated up to 10,000 volts. Using
the wrong fuse here could result in a severe fire hazard. |
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Some fuses are designed for a
very low resistance to ensure a small voltage drop, such as
The MONSTER's
10 amp fuse. Replacing with the wrong type will make the
bargraph display on
The BIG One
inaccurate. |
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Like most things in life, there
is much more to fuses than initially meets the eye. |
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Download
in
PDF format
(will open in a new browser) |
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Download
in
PDF format
(will open in a new browser) |
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