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with Kurt Küpper*
* Kurt Küpper is director of Aquavolt
Electric Boat Parts. Tel: 02 9417 8455
So what else would you like to know? You are welcome to send in questions about
boat electrics or suggestions for topics that you would like explained.
29E Babbage Rd, Roseville Chase 2069
Tel: 02 9417 8455
Fax 02 9417 8423
WHISPER QUIET FANS
While stocks last
3-speed white gimballed
fan with timer 12V
For full details see website
What are relays, and why do we need
them? This is best explained by
On a flybridge cruiser one wants to be
able to control the anchor winch with a
switch on the flybridge. This would mean
that the wire from the battery would need to
be run from the batteries all the way up to
the flybridge, and then all the way down to
the winch. As a winch draws a lot of power,
thick wire needs to be used. The longer the
distance between the battery and the winch,
the thicker the wire has to be to prevent
voltage drop due to electrical resistance.
This is thus a far from ideal situation, as
such thick wire is expensive and heavy.
The solution to the problem is to run the
wire directly from the battery to the winch
along the shortest practical route, and to fit
a remotely controlled switch into this wire
to turn the winch on and off. This remotely
controlled switch is called a relay.
A second circuit is used to switch the
relay on and off, i.e. power is run from the
battery through a switch on the flybridge
to the control terminals on the relay. As
the power needed to switch the relay is a
tiny fraction of the power required to run
the winch, only very thin wire is needed
for this circuit.
The above example is a fairly heavy
application, where currents in the hundreds
of Amps are switched. Such heavy duty
relays are usually called solenoids or
contactors. But there are many applications
where smaller relays are used to switch
much lower currents of say 10A to 20A, e.g.
for floodlights, toilets, pumps etc.
HOW DOES A RELAY WORK?
Most relays use an electro magnet
to open and close the relay contacts.
When current is passed through the
electromagnet, a hinged metal lever called
an armature is drawn to the electromagnet.
The armature mechanically switches the
contacts in the heavy-duty circuit (also
called the switched circuit). The armature
is also attached to a spring, which pulls the
armature back to its original position when
the electromagnet is switched off again, so
the contacts too are switched back to their
original position. These are called electro-
In its basic form, the switched contacts
are open when the electro magnet is not
energised, and they are closed when the
magnet is energised. This is known as a
normally open (NO) contact.
But there are also situations where it is
desirable to have a set of contacts that are
closed when the magnet is not energised, and
that open when the magnet is energised. Put
another way, switching the relay on causes
the switched circuit to be turned off. This is
known as a normally closed (NC) contact.
Change-over or double-throw relays
control two circuits. They have one normally-
open contact and one normally-closed
contact with a common terminal. The
common terminal is therefore switched from
the one circuit to the other whenever the
relay is energised or de-energised.
Relays can be placed in circuits in
order to switch other circuits on or off.
By appropriate use of normally open and
normally closed relay contacts, one can
ensure that another circuit is always on when
the controlling circuit is on, or the other
circuit would always be off, or unable to
be activated, if the controlling circuit is on.
The latter is typically used where there
is a safety requirement that two electrical
devices should not be able to be operated
at the same time.
In their simplest form relays have four
contacts – two for the control circuit that
switches the coil on or off, and two for the
switched circuit. The terminal by which the
power is fed from the battery to the relay is
known as the common contact, the load is
connected to the switched contact.
There also are five contact relays. Usually
these are change-over relays, which switch
between a normally open and a normally
closed output, allowing power to be changed
between two different loads, depending
on whether the relay is activated or not.
However, there are also five contact relays
that have two normally closed switched
contacts, so that two separate loads can
be switched by one relay. Two loads, each
drawing close to the rated current for the
relay and that if switched off one contact
would together have exceeded the current
rating, can thus be switched at the same
time with one relay without exceeding the
Other variations include double pole
relays, which are effectively two relays that
switch together off one control circuit, and
relays with integrated fuse holders and
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