If you have the German Relay Number, starting with either 4RD or 4RA, please click here for a cross reference list.
Please Click Here to choose Relays by Type and Voltage.
Hella manufactures many relays and not all versions are available individually; some can only be purchased in bulk. In addition, not all relays Hella makes are available to the after-market. |
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?
Wire presents a resistance to the flow of current. A resistance causes a voltage drop. What starts out at 12 volts at one end of a wire may be somewhat less at the other end, which results in a loss of performance. One of the basic rules of electricity states that the electrical resistance of a wire 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 wire, 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
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. |
Not Good
Current must travel from the battery to the headlamps via the dashboard switch.
Result: a long run of wire with a voltage drop causing a reduction in headlamp performance and possible overloading of the switch.
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The Best Solution
The current is routed over the shortest distance between the battery and the headlamps via the relay. A control very small control wire is required between the dashboard switch and the relay. |
What sort of relays are there? |
SPST Normally-Open |
SPST
Normally-Closed |
SPDT Change-Over |
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Almost all SPST Relays are this type. A normally-open relay completes 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. |
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.
Hella does not offer this type of Relay. An SPDT Relay can be used to perform this function by using the 87a as the 87 pin and not using the 87 pin at all. |
A change-over relay switches current from one user to another.
Change-over relays are used for switching 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:
Relays with a Resistor or a Diode.
Voltage spikes from 300V to 500V can occur momentarily when a relay is switched. Sensitive electronic equipment can be damaged or malfunctions can occur if these spikes reach 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. |
Dual 87:
This relay has two High Current output terminals so you don't have to splice the wires from two lamps together before connecting to the relay (Configuration B2 below).
Integral Fuse:
Some Relays have a Fuse built into them so you don't have to have a separate fuse holder. |
Pin
Layout:
The most common pin layouts are shown below: |
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A |
B |
B1 |
B2 |
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B3 |
C 280
QC |
D Micro
Relay |
L |
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| If you have the German Relay
Number, starting with either 4RD or 4RA,
please click here for a cross
reference list. |
Relay Wiring:
Click here for a guide to wire your Auxiliary Lamps
Click here for a guide to wire your Headlamps
Click here for a guide to wire your Horns
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| If you have the German Relay Number, starting
with either 4RD or 4RA,
please click here for a cross reference
list. |