Resistor and Diode Protected Relays Improve Circuit Longevity and Reliability

[MikePhua]

The Role of Relays in Heavy Equipment Electrical Systems
Relays are electromechanical switches that allow low-current circuits to control high-current loads. In heavy equipment, they are used to manage starter motors, hydraulic solenoids, lighting systems, and auxiliary functions. A relay typically consists of a coil that, when energized, creates a magnetic field to close or open contacts. While simple in concept, relays are subject to electrical transients that can damage surrounding components if not properly protected.
As equipment becomes more electronically integrated, protecting sensitive control modules from voltage spikes has become critical. This is where resistor and diode protected relays come into play. These components are designed to suppress voltage surges generated when the relay coil is de-energized, preventing backflow into the control circuit.
Terminology Notes

• Flyback voltage: A high-voltage spike generated when current through an inductive load (like a relay coil) is suddenly interrupted.
  
• Diode protection: A diode placed across the relay coil to redirect flyback voltage safely.
  
• Resistor protection: A resistor used to dissipate flyback energy as heat, reducing peak voltage.
  
• Polarity sensitive relay: A relay that requires correct voltage orientation due to internal diode configuration.
  

Diode Protected Relays
Diode protected relays use a flyback diode connected in parallel with the coil. When the coil is de-energized, the diode provides a path for the collapsing magnetic field to discharge safely. This prevents voltage spikes from reaching the control module or switch contacts.
Advantages:

• Effective suppression of flyback voltage
  
• Minimal electromagnetic interference (EMI)
  
• Ideal for solid-state control systems
  

Limitations:

• Polarity sensitive—incorrect wiring can damage the diode
  
• Slight delay in coil release due to energy recirculation
  
• Not suitable for AC circuits
  

Diode protected relays are commonly used in modern equipment with electronic control units (ECUs), such as late-model excavators, loaders, and telehandlers.
Resistor Protected Relays
Resistor protected relays use a high-wattage resistor across the coil to absorb and dissipate flyback energy. Unlike diodes, resistors are not polarity sensitive and can be used in circuits where wiring orientation may vary.
Advantages:

• Simple and robust
  
• Compatible with both DC and AC circuits
  
• Faster coil release compared to diode types
  

Limitations:

• Less effective at suppressing high-voltage spikes
  
• Generates heat during operation
  
• May not fully protect sensitive electronics
  

Resistor protected relays are often found in legacy equipment or systems with mechanical switches and minimal electronics.
A Story from the Field
In 2014, a fleet manager in Alberta noticed frequent ECU failures in a batch of newly retrofitted graders. After investigation, the culprit was traced to standard relays without suppression. Each time a hydraulic solenoid was deactivated, a voltage spike traveled back into the control harness, damaging the ECU. Switching to diode protected relays resolved the issue, and no further failures occurred. The manager later stated that “a $5 relay saved us thousands in downtime.”
Choosing the Right Relay for Your Application
When selecting a relay, consider the following:

• Voltage rating: Match to system voltage (12V, 24V, etc.)
  
• Current capacity: Ensure relay contacts can handle load amperage
  
• Coil resistance: Affects activation current and suppression behavior
  
• Protection type: Diode for sensitive electronics, resistor for general use
  
• Environmental sealing: Use sealed relays in dusty or wet conditions
  

For mixed fleets, color-coded or labeled relays can help prevent polarity errors during service.
Installation and Wiring Tips
To ensure proper function:

• Verify polarity before installing diode protected relays
  
• Use fused circuits to prevent overload
  
• Avoid routing control wires near high-current lines
  
• Test relay function with a multimeter before deployment
  
• Replace relays showing signs of heat damage or corrosion
  

Technicians should be trained to recognize relay types and understand their impact on circuit behavior. A miswired diode relay can short the control circuit, while an unprotected relay can damage expensive electronics.
Preventive Maintenance and Troubleshooting
Relays should be inspected during routine electrical checks:

• Listen for clicking sound during activation
  
• Measure coil resistance and contact continuity
  
• Check for melted housings or discolored terminals
  
• Replace relays every 2,000–3,000 hours or as per duty cycle
  

If a circuit shows intermittent failure, test the relay under load and inspect for voltage spikes using an oscilloscope. In high-vibration environments, consider using solid-state relays with built-in suppression.
Final Thoughts
Resistor and diode protected relays are small components with a big impact. In heavy equipment, where electrical reliability is vital, choosing the right relay can prevent costly failures and extend the life of control systems. Whether upgrading a legacy machine or designing a new harness, understanding relay protection is essential. With proper selection, installation, and maintenance, these devices quietly safeguard the heart of your electrical system—one click at a time.