Lighting & Signalling modules
23
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What is a relay?
A relay is an electrically controlled disconnector switch, used to control a power circuit using a lower
current control circuit.
Justify the use of a relay for the main and dipped beam lighting circuits:
The power absorbed by these consumers would lead to the switches and wiring being over dimensioned
for the current levels if the control and power circuits were not separated.
What happens if one adds an additional two 55W driving lights, linked to the existing headlamps
(of 55W) without a separate power circuit?
The current will be doubled; however the power and earth-return circuits are not designed to handle this
power. There will therefore be a drop of voltage through the conductors, leading to heating and a voltage
drop at the consumer terminals, which will not work at their optimum level, with ensuing operational
problems.
Calculations for wire cross-sections:
The aim of this study is to show the cross-section of the wires to be used to connect an additional set of
driving lights, assuming they are to be controlled from a relay in the vicinity of the battery, approximately
1.2m from the right hand light.
The earth from the bulbs will be connected 20 cm from the bulbs, for this reason we can assume the
same cross section as used for the bulb supply.
The characteristics of the bulb: minimal voltage 12.5V
Power absorbed at 12.5V: 55W
Calculate, using the previous method, the cross-section of the supply wire to prevent a voltage
drop of greater than 0.5 V.
I at 12.5V = 4.4A
U = IR therefore R = U/I giving an R
max
= 0.11.
R =
* L / S giving S = L
/ R = 1.87 mm
2
S = (
* D
2
) / 4 giving D = (4 S /
)
0.5
= 1.54 mm
The corresponding diameter is 1.54 mm.
What can be deduced from the positioning of the relays?
The position should be chosen to minimalize the length of the power cabling.
Showing the resistance of a wire:
Connect the earths from the bulbs on the DTM7002G&D modules, link to a selector switch for the
parallel bulbs, using wires of 2 mm
2
, and set up a power supply battery.
Prepare two wires with a different cross section, e.g.: 0.4 mm & 2 mm, with a length of 2 m.
Connect the consumers successively with the two wires after having first connected their terminals to a
voltmeter.
