Relays and Solenoids

What is a Relay?

Electrically operated switch that uses small control current to switch larger load current.

Why use relays:

• Protect switches from high current
• Allow remote control of circuits
• Enable logic control (multiple inputs)
• Reduce voltage drop in long wire runs

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Electromechanical Relays

Basic Construction

        ┌─────────────────┐
        │    ┌─────┐      │
  30 ───┤    │Coil │      ├─── 87 (NO)
        │    │     │      │
  85 ───┤    └──┬──┘      ├─── 87a (NC)
        │       │         │
  86 ───┤    Armature     │
        │       │         │
        │    ───┴───      │
        │    Common       │
        └───────┬─────────┘
                │
               30 (Common)

Terminal Numbering (ISO)

Terminal	Function
30	Common (battery power in)
85	Coil negative (ground)
86	Coil positive (trigger)
87	Normally Open contact
87a	Normally Closed contact

Relay Types

SPST (Single Pole Single Throw)

• 4 terminals: 30, 85, 86, 87
• One circuit, on or off
• Most common motorcycle relay

Example: Auxiliary Light Circuit with SPST Relay

    BATTERY (+12V)
         │
         │
    ┌────┴────┐
    │  FUSE   │
    │  15A    │
    └────┬────┘
         │
         ├─────────────────────────────────┐
         │                                 │
         │                                 │
    ┌────┴────┐                       ┌────┴────┐
    │         │                       │         │
    │   30    │                       │   86    │
    │ (Power) │                       │ (Coil+) │
    │         │                       │         │
    │  RELAY  │                       │  RELAY  │
    │  SPST   │                       │  SPST   │
    │         │                       │         │
    │   87    │                       │   85    │
    │  (Load) │                       │ (Coil-) │
    │         │                       │         │
    └────┬────┘                       └────┬────┘
         │                                 │
         │                                 │
    ┌────┴────┐                       ┌────┴────┐
    │   AUX   │                       │ SWITCH  │
    │  LIGHT  │                       │ (Dash)  │
    │  55W    │                       └────┬────┘
    └────┬────┘                            │
         │                                 │
         └──────────────┬──────────────────┘
                        │
                   ─────┴─────
                     GROUND
                    (Chassis)

Current flow when switch ON:

1. Switch closes → coil energizes (86→85 to ground)
2. Relay contacts close (30→87)
3. Full battery current flows to light
4. Switch handles ~150mA (coil), not 4.6A (light)

SPDT (Single Pole Double Throw)

• 5 terminals: 30, 85, 86, 87, 87a
• Switches between two circuits
• Used for: high/low beam, turn signals

Example: High/Low Beam Headlight with SPDT Relay

    BATTERY (+12V)
         │
         │
    ┌────┴────┐
    │  FUSE   │
    │  15A    │
    └────┬────┘
         │
         │
    ┌────┴────┐
    │         │
    │   30    │
    │ (Common)│
    │         │
    │  RELAY  │
    │  SPDT   │
    │         │
    ├─────────┤
    │         │
    │87a   87 │
    │(NC) (NO)│
    └──┬───┬──┘
       │   │
       │   │
       │   └──────────────────┐
       │                      │
  ┌────┴────┐            ┌────┴────┐
  │   LOW   │            │  HIGH   │
  │  BEAM   │            │  BEAM   │
  │  55W    │            │  60W    │
  └────┬────┘            └────┬────┘
       │                      │
       └──────────┬───────────┘
                  │
             ─────┴─────
               GROUND


    COIL CONTROL (separate circuit):

    +12V (Ignition)
         │
    ┌────┴────┐
    │  HIGH   │
    │  BEAM   │
    │ SWITCH  │
    └────┬────┘
         │
    ┌────┴────┐
    │   86    │
    │ (Coil+) │
    │  RELAY  │
    │   85    │
    │ (Coil-) │
    └────┬────┘
         │
    ─────┴─────
      GROUND

Operation:

• Switch OFF (coil de-energized): 30→87a = Low beam ON
• Switch ON (coil energized): 30→87 = High beam ON
• Only one beam active at a time

DPDT (Double Pole Double Throw)

• 8 terminals
• Two independent SPDT in one package
• Used for: hazard flasher, polarity reversal

Common Motorcycle Relays

Function	Type	Coil Current	Contact Rating
Starter	SPST	150-300mA	100-200A
Headlight	SPST/SPDT	150mA	20-30A
Fuel pump	SPST	150mA	10-15A
Horn	SPST	150mA	15-20A
Turn signal flasher	SPDT	100mA	10-20A
Cooling fan	SPST	150mA	20-30A

---

Solid-State Relays (SSR)

How SSR Works

Uses semiconductor (MOSFET or triac) instead of mechanical contacts.

Control side: Optocoupler isolates input Load side: MOSFET/triac switches power

SSR vs Electromechanical

Feature	Electromechanical	Solid-State
Switching speed	5-15ms	<1ms
Lifespan	100K-1M cycles	Unlimited
Noise (EMI)	Arc generates noise	No arc, quiet
Audible click	Yes	No
Voltage drop	<0.1V	0.3-1V (generates heat)
Inrush current	Handles well	May need oversizing
Cost	Low	Higher
Size	Larger	Compact

When to Use SSR

Prefer SSR for:

• High-frequency switching (PWM control)
• Vibration-sensitive applications
• Long-life requirements
• Silent operation

Prefer electromechanical for:

• High inrush loads (motors, bulbs)
• Cost-sensitive applications
• Voltage drop critical
• Occasional switching only

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Solenoids

What is a Solenoid?

Electromagnetic actuator that converts electrical energy to linear motion.

Starter Solenoid

Functions:

1. Engages starter gear with ring gear (mechanical)
2. Closes high-current contacts (electrical)

Construction:

                    ┌──────────────────┐
   Pull-in coil ────┤      ████████    │
                    │      ║    ║      │
  Hold-in coil ─────┤      ║Plunger    │
                    │      ║    ║      │
                    │   ───╨────╨───   │
                    │    Contacts      │
                    └────┬───────┬─────┘
                         │       │
                    Battery   Starter
                    (+)       Motor

Motorcycle Starter Solenoid Circuit (Combined Relay Type)

Common motorcycle solenoid combines fuse + relay in one unit.

Solenoid terminals:

• B (Battery): Direct connection to battery positive
• M (Motor): Direct connection to starter motor
• Spade terminals: Coil + fused 12V output (internally connected)
• Coil: ~3.8Ω energizing coil

    BATTERY (+12V)
         │
         │ Heavy gauge cable
         │
    ┌────┴────────────────────────────────────────┐
    │    B              STARTER SOLENOID          │
    │  (Battery)                                  │
    │    │                                        │
    │    │       ┌─────────────────────────┐      │
    │    │       │                         │      │
    │    ├───────┤►  30A FUSE  ├───────────┼──────┼──► +12V Output
    │    │       │   (Built-in)            │      │   (Spade terminals)
    │    │       └─────────────────────────┘      │
    │    │                                        │
    │    │       ┌─────────────────────────┐      │
    │    │       │                         │      │
    │    │       │      ┌───────┐ Coil     │      │
    │    ├───────┤──────│ 3.8Ω  │◄─────────┼──────┼──►  From Control Circuit
    │    │       │      └───┬───┘ Terminal │      │     (Starter Button)
    │    │       │          │              │      │
    │    │       │       Plunger           │      │
    │    │       │          │              │      │
    │    │       │      ════╧════          │      │
    │    │       │     Main Contacts       │      │
    │    │       │        │   │            │      │
    │    └───────┤────────┘   │            │      │
    │            │            │            │      │
    │    ┌───────┤────────────┘            │      │
    │    │       └─────────────────────────┘      │
    │    M                                        │
    │  (Motor)                                    │
    └────┬────────────────────────────────────────┘
         │
         │ Heavy gauge cable
         │
    ┌────┴────┐
    │ STARTER │
    │  MOTOR  │
    └────┬────┘
         │
    ─────┴─────
      GROUND
     (Engine)


    COIL CONTROL CIRCUIT:
    ══════════════════════

    +12V (Ignition ON)
         │
         ├───────────────┐
         │               │
    ┌────┴────┐     ┌────┴────┐
    │ CLUTCH  │     │NEUTRAL  │
    │ SWITCH  │     │ SWITCH  │
    │(Pulled) │     │(In N)   │
    └────┬────┘     └────┬────┘
         │               │
         └───────┬───────┘
                 │
                 │◄── Either condition
                 │    allows starting
                 │
            ┌────┴────┐
            │ STARTER │◄── Handlebar
            │ BUTTON  │    button
            └────┬────┘
                 │
            ┌────┴────┐
            │  Coil   │
            │Terminal │
            │  (86)   │
            │         │
            │ SOLENOID│
            │         │
            │  Coil   │
            │Terminal │
            │  (85)   │
            └────┬────┘
                 │
            ─────┴─────
              GROUND

Operation sequence:

1. Key ON → Ignition power available
2. Clutch pulled (or in neutral) → Safety circuit complete
3. Starter button pressed → Coil energizes (~3.2A at 3.8Ω)
4. Plunger pulls in → Main contacts close (B to M)
5. High current flows → Battery to starter motor (~80-150A)
6. Engine cranks → Release button to stop

Wire gauge requirements:

• Battery to B terminal: 8-10 AWG minimum
• M terminal to starter: 8-10 AWG minimum
• Control circuit: 16-18 AWG sufficient

Testing Starter Solenoid

Audible Click Test

1. Turn key to start
2. Listen for click from solenoid
3. Click = solenoid activating (contacts may be worn)
4. No click = trigger circuit or solenoid coil issue

Voltage Test

1. Measure voltage at solenoid trigger terminal (small wire)
2. Turn key to start
3. Should see 10-12V

Voltage	Meaning
12V, no click	Solenoid coil failed
0V	Ignition switch, starter relay, or wiring issue
Low voltage	Voltage drop in trigger circuit

Contact Test

1. Bypass solenoid: jumper battery terminal to starter terminal
2. Caution: Bike may start if in gear
3. Starter spins = solenoid contacts worn/burnt
4. Starter doesn't spin = starter motor issue

Other Solenoid Applications

Application	Function
Fuel injector	Opens to spray fuel (fast-acting)
Idle air control	Adjusts idle speed
EVAP purge valve	Controls fuel vapor flow
Clutch switch	Some bikes have clutch interlock

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Relay/Solenoid Testing

Coil Resistance Test

1. Set multimeter to Ω
2. Measure across coil terminals (85-86 on standard relay)

Reading	Meaning
50-120Ω typical	Good coil
OL (infinite)	Open coil - failed
Very low (<10Ω)	Shorted coil

Contact Continuity Test

De-energized (relay OFF):

1. Measure 30 to 87: Should be OL (open)
2. Measure 30 to 87a: Should be near 0Ω (closed)

Energized (apply 12V to coil):

1. Measure 30 to 87: Should be near 0Ω (closed)
2. Measure 30 to 87a: Should be OL (open)

Voltage Drop Test (In-Circuit)

1. Relay installed and operating
2. Measure voltage across terminals 30 and 87
3. Should be <0.5V under load

Reading	Meaning
<0.2V	Good contacts
0.2-0.5V	Contacts wearing
>0.5V	Replace relay

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Flyback Diodes

The Problem: Inductive Kick

When relay/solenoid coil de-energizes:

• Collapsing magnetic field induces voltage spike
• Can be 100V+ in reverse polarity
• Damages transistors, ECU outputs

The Solution: Flyback Diode

Diode across coil provides path for collapsing field current.

        ┌────────┐
   86 ──┤  Coil  ├── 85
        │        │
        │  ──┬── │
        └────┼───┘
             │
         ▼ Diode
             │
        ─────┴─────

Diode orientation: Cathode (stripe) toward positive (86)

Built-In Protection

Modern relays often include:

• Internal flyback diode
• Look for diode symbol on relay
• Check datasheet

ECU-controlled outputs:

• ECU drivers have internal protection
• Additional external diode still recommended for reliability