Relay Function and Operations

Relay Function and Operations:



A relay is a simple electromagnetic switch used in many circuits in transit bus electrical systems. The main relay function is as a remote-control switch, where a component control by a relay can be energized by switch far away. In operation, the relay uses a small current control circuit to control a larger current circuit. Transit bus relays are know as “change over relays”. In the near future relays will become obsolete on bus electrical systems; for the time being there are millions still in operation on transit buses. For the purpose of this blog, we will focus on 12V relays; there are however many types of relays with many current carrying capacities.




Most relays in transit bus circuits are Bosch 5 terminal; 85, 86, 30, 87, 87a.

12v relay, 24v relay, relay specs
Relay Specifications

The best place to find the specifications for the relay you are dealing with is on the relay itself. Most good quality relays will have a diagram and specifications included in the housing.

Relay Specification Case
Relay Specification Case




During operation, current flows through two separate paths in the relay; the control and switched circuit. The control side is the low amperage side of the relay, the switched side is the high amperage side of the relay.


The control circuit   85 & 86 contains the relay’s electromagnetic coil.
It is typically controlled by a switch in the current path between the power source (source control) and the coil or between the coil and ground (ground control). When the control circuit is complete the relay coil creates a magnetic field that attracts the metallic switch and changes over the contacts.

The switched circuit contains two sets of contacts.
⇒NO (Normally Open) contacts 30 & 87
⇒NC (Normally Closed) contacts 30 & 87a
When the relay coil is energized, it moves the NO contacts closed and moves the NC contacts open.

–NO (Normally open contacts) – close when relay coil energizes.
NC (Normally closed contacts) – open when relay coil energizes.

relya drawing, relay schematic, control side, switched side
Relay Circuits



Schematic Drawings:

The relay is always shown in the OFF state on schematic drawings. The standard schematic drawing symbol for a relay varies between manufacturers. The representation below is a generic sample.

OFF State

Relay Terminals:

Terminal 30 – is the switch common terminal, this terminal will be used to supply power or ground to terminal 87a when relay is OFF and 87 when relay is ON. Most applications provide power through this terminal.

Terminal 87a – is connected to terminal 30 when the relay is OFF (not energized).

Terminal 87 – is connected to terminal 30 when the relay is ON (energized).

Terminal 86 – is the power supply for the coil. It is very important to observe polarity when using relays that have a diode. Connecting 86 to ground will create a short circuit that can blow fuses, burn the relay diode or worst yet, damage an ECU or module. Resister type relays are not polarity sensitive and 86 can be ground.

Terminal 85 – is the ground for the relay coil.

relay terminals, bottom view relay



Resistor Suppressed and Diode Suppressed Relays:

Relay coils like any other coil create a magnetic field that collapses when the coil current is interrupted. This collapsing magnetic field creates a voltage spike in the 100’s of volts out of the negative side of the coil. To suppress or quench this spike a resistor or diode is place in parallel to the coil. The resister or diode gives the voltage spike an alternate path to dissipate the voltage spike away from electrical components. If the spikes are not suppressed, damage to electronic components may occur.

diode relay, resistor relay
Suppressed Relay Diagrams



Relay Testing:

There are two ways to check the condition of a relay. The best method is to check under operation by measuring voltage, the other method is to check resistance.

Measuring Resistance: use a DMM or ohmmeter to measure resistance.

Coil Resistance –
12 volt = 85 ± 5Ω
24 volt = 255 ± 15Ω
Note: Due to variances from different relay manufacturers, coil resistance values vary. Values shown reflect Bosch relays, refer to manufacturer for correct specs.

30 to 87a– should read continuity, less that 1 ohm of resistance. High resistance, indicates the contacts are corroded and pitted.

30 to 87 – should read open (OL), any resistance here indicates the contacts are touching due to corrosion, pitting, or a weak spring.

Testing Resistance
Testing Resistance


Measuring Voltage: use a DMM or voltmeter to measure voltage.

Terminal 30 – there should be source voltage available at this terminal.

Terminal 86 – there should be source voltage available at this terminal when the circuit is energized.

Terminal 85 – there should be ground (no voltage) present at this terminal when the circuit is energized.

Terminal 87 – there should be source voltage at this terminal when the circuit is energized. This voltage is the supply voltage to the load.

Terminal 87a – there should be no voltage at this terminal when the circuit is energized.

Relay Testing Voltage
Relay Testing Voltage



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