Electricity 1

Relays

When Measuring Amperage

Break the circuit and complete the circuit with the meter leads

When measuring measuring resistance

The power must be off to measure

Mega (M)

1000000

Kilo (k)

1000

Milli (m)

.001

Micro (u)

.000001

Ohms law determines

current flow or resistance

watts law determines

power consumption or output

ohms law

watts law

What is a relay

A relay is a switch that uses a small amount of current to switch a larger amount of current

why use a relay

eliminate the use of heavy conductors inside the cab of a machine, and that allows for safer operation

A relay has a

control circuit and a load circuit

Inside the relay, an electromagnetic coil

pulls a set of contacts closed

The relay’s control circuit can be switched by supplying a

power or ground to the relay

What happens when the coils in a relay are de-energized

a large voltage spike is produced

The rapidly collapsing magnetic field will move across

the coil conductors, inducing over 200 volts

30

high power feed

85

relay coil ground

86

relay coil feed (trigger wire)

87

high power output (normally open contact)

87a

high power output (normally closed contact)

the function of 30

it is a switch

Locating a relay closer to a load requiring large amounts of current

eliminates voltage drop caused by resistance from long runs of wires

The control circuit is supplied current through a switch

which often is opened or closed by an ECM or the machine operator

If an ECM supplies a positive voltage to control the relay

it is termed a pull-up circuit

If an ECM supplies a ground

it is termed a pull-down circuit

(ISO) relays

International Organization for Standardization

The rapidly collapsing magnetic field

will move across the coil conductors, inducing over 200 volts

A resistor or a diode Is used to

suppress voltage spike

The relay is standardized to be used across

all equipment manufacturers, having standard pin numbers, functions, and dimensions to fit into a power distribution box

Mini ISO relays in a power distribution box. Current suppressed relays use a diode, resistor, and sometimes a capacitor to

minimize a voltage spike produced through self-induction.

Electrical damage can occur when

the relay control circuit shares a voltage supply with the ignition switch or sensitive electronic component

What provides an alternate pathway for the voltage spike to dissipate when the magnetic field collapses

A diode or resistor connected in parallel across the coil