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Hella Relays | ||||||||
2 East Locust Street, Ground Floor, Fleetwood, PA 19522, 9AM-7PM, M-F Eastern Time |
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Hella Relays
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If
you have the German Part Number for a Hella Relay that starts
with either 4RA or 4RD, please click
here for
a cross-reference chart. |
What
is a relay? |
Briefly: a remote-control switch in which an electro-magnetically produced force operates the contacts through which the main power current flows. |
Why use a relay? |
Leads
are resistances. Resistances cause voltage drop, which means that consumer
performance is reduced. One of the basic rules of electricity states
that the electrical resistance of a lead depends on its length and
cross-sectional area. The longer and thinner it is, the greater the
resistance will be. Higher resistance means less voltage at the far
end of the lead, i.e. where the power user is. If the power user is
a bulb, a little less voltage means a lot less light, as illustrated
in the table below: |
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Important |
Not
Good |
The
Best Solution |
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The importance of optimum operating current and the negative effect voltage drop can have, on the luminous intensity of a bulb, can be seen from the table above. |
Power current must travel from battery to headlamps - via the dashboard switch using a large gauge cable.
Result: voltage drop causing reduction in
headlamp performance. |
The power current is routed over the shortest distance between battery and headlamp via the relay. A control current lead of only 0.75 sq. mm (current consumption for this lead less than 150 mA!) is required between dashboard switch and relay. |
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What sort of relays are there? |
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| SPST Normally-Open | SPST Normally-Closed | SPDT Change-Over |
A normally-open relay closes an electrical circuit when current is passed through it's coil. Normally open relays are required for all types of vehicle lighting, horns, fanfares, heating circuits, fan motors and air-conditioning equipment.
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A
normally-closed relay opens an electrical circuit, i.e. it
interrupts the flow of current. This type of relay is used,
for example, to automatically disconnect headlights and radio
when the engine is being started since for these few seconds
the starter motor needs all the "power" it can get
from the battery.
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A change-over relay switches current from one user to another. Change-over relays are used for switching from the horn to fanfare, from forward to backward movement of the sliding roof, from opening to closing electrically operated windows and for two-stage users such as fans and some rear window heating systems.
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Eliminating voltage
spikes: |
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Voltage spikes from 300V to 500V can occur momentarily when a relay is switched off. Sensitive electronic equipment can be damaged or malfunctions can occur if these spikes each the vehicle electrical network without suppression. A relay with a resistor reduces these voltage spikes to less than 100 Volts. A relay with a diode eliminates them completely. The correct polarity of the connections is marked on the diode protected relay. |
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Relay with Resistor. |
Relay with Diode. |
Relay with Resistor and Diode |
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Relay with Dual 87 |
Integral Fuse |
Dual SPST Relay |
The relay has two High Current output terminals on the relay so you don't have to splice the wires from two lamps together before connecting to the relay |
Some Relays have a Fuse built into them so you don't have to have a separate fuse holder. |
Some Relays are actually two relays in one housing. |
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Service Life of a Relay |
A
relay's service life varies depending upon the amperage used. For
instance, a relay rated at 40 amps but operated at 20 amps should last
twice the service life of the same relay operated at 40 amps. If
the rated service life is 100,000 operations at 40 amps, it shoulds
last for 200,000 operations at 20 amps. |
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There are various pin layouts as shown below: |
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| Pin Out A | Pin Out B | Pin Out B1 | Pin Out B2 |
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| Pin Out B3 | 280 QC Pin Out C | Micro Pin Out D | Dual SPST Pin Out L |
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Susquehanna
MotorSports, 2 East Locust Street, Ground Floor, Fleetwood, PA 19522, 9AM-7PM, M-F Eastern Time Toll Free 866-480-2159, Phone 610-944-3233, Fax 610-944-3234, e-mail |