Electricity for Camera People
First a story. Camera, battery and cable are set up. Camera doesn't run. Battery
is changed, still doesn't run and third battery tried, still no-go. Cable is
changed and all batteries tried, again, still no-go. Call to equipment company
new camera and batteries are ordered as the director fumes. New battery and
camera arrives and works fine by time for lunch. What happened: The first cable
had a short (two wires touching that shouldn't.) The bad cable blew the fuses
in every battery. The good spare cables didn't work because the batteries had
blown fuses. When the good cable was used with the new camera package, everything
worked and would have worked if the assistant knew a little about electricity
and had a VOM (Volt-Ohm- Meter...and knew how to use it). The bad cable is still
A SHORT is when two wires connect when you don't want them to. An OPEN, which
happens more often, is a break in between two wires or connectors that should
be connected. A blown fuse acts like an open and no current flows. An open in
a cable in the story above would have shown up when the cable was changed. Cables
are the most common camera electrical failure. Switches on camera cable also
fail often because DC current is hard on switches.
Some Theory. We don't have to know electron theory or the theory of relativity
to test camera batteries and cables.
Electricity is like water and pipes. Voltage (E) is like water pressure. Amperage
(I) is like volume of water. Wattage (P) is like the total amount of water /
electric power delivered whether at high pressure through a small hose/wire
or low pressure through a large hose/wire. The formula for wattage is P = I
x E or Watts = Volts x Amps. You can start a car with lots of amps but only
12 volts (and a big wire) or run a power saw (about the same wattage) with 120
volts at a few amps (and a small wire). As long as the motor, light or machine
at the end of the line is made for the right voltage it will work if you have
the right voltage, enough amperage available and a large enough wire from the
A fuse is a safety valve that opens when too much current flows as when a short
happens. The fuse blows and protects the battery from damage from trying to
put out too much current. A fuse can also protect a camera if too much voltage
is put into a camera, but not always. Some times fuses blow with a camera jam
and some times for no apparent reason.
Our camera power sources are usually batteries. They are not unlimited sources
and a common problem. A video monitor running on a camera battery will go down
pretty fast. Older tube video taps also ran batteries down fast too. The cameras,
which usually run for seconds or minutes, at most, usually don't run good batteries
down. It's the accessories. A video monitor will run many hours on a car battery,
so will many other 12-volt accessories. Yes, a car battery is heavy, but a battery,
case and charger is cheaper than camera battery rentals. Get car batteries from
an auto supply store not a camera supply company. DON'T ship car batteries by
air, it is dangerous! A good battery will last years.
(See Battery Information)
Cables like water hoses or pipes restrict the flow and the longer and smaller
the cable the less pressure/voltage you get out of the other end.
Wire size. Many camera cables are # 16 gauge because they are more flexible.
For a short cable, # 16 gauge is OK, but a long # 16 gauge wire can loose a
couple of volts and the camera won't come up to speed or run at all. Put 14
volts into a long cable and WITH THE CAMERA RUNNING you might get 12 or so volts
to the camera. Most cameras are OK with 14 volts, but for example one circuit
in the Arri III doesn't like voltages over 15-17 volts. A 14.4-volt Ni Cd battery
just off charge can be up to 17 volts. The camera might not like it and burn
up a voltage regulator. When the camera runs or some time after charge, the
voltage will drop to a safer level.
So, long extension cables should be at least #14 or #12. Note, these wires are
not easy to wire into the Cannon XLR-4 connectors. You can remove some strands
or splice in a short piece of # 16. Any time you make a cable, check that you
have the wires right and not crossed polarity. Reversed polarity can eat some
cameras. Don't use microphone cables, they are much too small. Extension cables
from the electrical department are OK as long as they don't get plugged into
the wall. Label them well. (I think Mega / speaker cable is a hoax and plain
old extension or speaker cable will work as well for much less money.) (Regular
extension cable is also fine, as there is no danger of getting shocked from
12 volts.) Remember pin #4 of Cannon XLR 4 plugs is Positive and Pin #1 is Negative
Some tools: A VOM, Volt-Ohm Meter, is absolutely necessary. The pins on the
prods are usually too large to reach inside a 4 pin XLR female (battery and
camera end of cable) plug. You will have to file or sand the test pin smaller.
Make sure the test pins don't touch when testing a plug, it will blow fuses!
With a VOM you can check for shorts and opens in cables. You can check battery
voltage. Making an in-line test jumper can be very useful. Wire a short camera
cable with pin sockets wired to each side to plug in your VOM to check voltage
at the camera with the camera running and while stopped. If there is a big difference
in voltage running and stopped, the power cable may be too small, too long or
there is a bad connection some where in the battery or cable. A wire stripper,
60/40 small solder, solder paste and a soldering iron is also useful for repairs
in the field.
Some problems: Most VOM' s don't like to see voltage on an OHMs or AMPs settings.
Make sure your VOM switch is set right before you measure. Start with a higher
setting before lowering the setting. (Read the instruction manual.)
A VOM can show enough voltage from a battery and / or cable that will not run
a camera. If there is a weak connection (almost open) you can read voltage by
not get enough current to run a camera.
A remote Arri II with long cable and Arri Motor. First of all, Arri wild motors
run better with the rheostat (speed control) most of the way up and a battery
voltage adjusted to the speed you want. ((You need to be able to tap fewer cells
in the battery.) Some Arri II's will run at 24 FPS at 12 volts. To run speed
you need more voltage and 16 volts or more with longer cables. The 24 FPS Governor
Controlled Arri II motors should not be run much over 16 volts at the camera.
18 or 20 volts at a vary long #16 cable might be OK if the camera is getting
about 17 volts at the camera end of a cable. (Note: the old rheostat Arri II
High Speed motors have pretty bad speed control at lower speeds. They will work
better if the rheostat is set high (least resistance) and voltage reduced.)
Some theory: The most important formula for electricity is I = E / R or Current
= Voltage divided by Resistance. or R = E / I or E = I x R
Let's say you have a 100 feet of # 16 cable with a 12 volts battery on one end
and a camera on the other end that draws 3 amps. If the cable has 0.8 ohms resistance,
I x R or 3 x 0.8 = 2.4 volts dropped in the cable. A 50-foot microphone cable
would loose 5 volts. (Sound deals with very low currents and can use small wire.)
A 100 foot of #12 gauge wire would loose only 1.2 volts.
Some do's and don'ts:
Don't deep discharge a lead acid battery. (Recharge them immediately after use.)
Don't run any battery dead with a light. Film and video cameras shut off before
damage is done, but lights will run a battery completely dead.
Do check battery voltage with camera running. A battery just off charge will
have a higher voltage.
XLR plugs on batteries and cables corrode and the female plugs get loose with
use. Replace them if questionable.
Do carry the right spare fuses.
Some war stories: On my first job in Alaska with 8.4-volt Arri 16 S batteries
we has one cell open. I shorted the cell and the battery worked fine on 7.4
volts. When we were without AC power to charge the batteries, I used a 10-ohm
resister between our car battery to charge the camera batteries. R = E / I or
4 volts / .4 amps = 10 ohms. The difference between 12 and 8 is 4 volts. A 16-hour
charge current for 4 amp hour batteries is .4 amps. I needed at least a 2-watt
resistor. P = I x E or 4 volts x .4 amps = 1.6 watts. I learned electronics
in the Navy. I built all my own batteries and chargers for years.
The opening story happened twice on high budget shoots that I heard of like
the film being loaded in magazines backward for 2 days running and a grip noticing
the problem. (He made a big thing of it on the set but not many fiends.) (The
lab had said the film was 4 stops underexposed.) The guilty parties are all
doing well in the biz. The cameraman, one of the most famous, never looked in
the camera; he trusts the assistant and lets him do his job.
An assistant heard that batteries should be deep discharged. He built a light
globe discharger and ruined the lead acid batteries by running them flat. (Fortunately
lead acid batteries are cheaper to replace.)
A utility plugged 110-volt battery chargers into 220, but stopped when he smelled
smoke and fortunately didn't cook all the chargers.
The assistant put all the batteries on charge in the bathroom and turned off
the lights and the battery chargers too, which were on the same switch.
© Copyright 1999-2004 Ron Dexter. All Rights Reserved.