Smart Relay

Electron Speed’s Smart Relay 33 is a variable and robust component for implementation in motorsports systems. The unit transmits and receives CAN messages, provides current limiting on all channels, and custom configurations can be constructed according to varying implementations of the unit.


  • Recommended operating voltage range:  8-18V
  • Maximum Smart Relay case temperature:  100C (212F).
  • Maximum ambient temperature assuming minimal airflow and full rated current flow:  90C (194F)
  • Reverse polarity protection for the Smart Relay (but not for attached devices).  (Will hard switch on to protect itself under reverse polarity)
  • Channel Inputs are looking for a ground sink of 7ma.
  • Channel Inputs are capable of continuous short to Gnd or Vbat
  • Quiescent current with no channels active and waiting for ignition signal: <50uA
  • Maximum, repeated, peak current per channel:  150 amps.
  • Intended steady state current per ratings:  15, 25 or 35 amps.
  • Flyback diodes clamp outputs at -1 volts.
  • Intended for both inductive and resistive loads. (pumps, fans, lights, etc.)

Circuit Protection

The Smart Relay 33 has two fault indication modes. If an overcurrent condition (see chart above) occurs on one of the channels, the channel will shut down and the LED for that channel will blink until the channel is switched off for half of a second. If the overcurrent condition is still present, the channel will continue to trip until the condition is rectified.

If board temperature reaches 100°C, all channels will deactivate in order to protect the device from damage. All LEDs will strobe in sequence until the device reaches temperatures below 100°C.

Channels and Current Limits

Channels for the Smart Relay 33 are programmed with current limits to protect both the unit and your devices from overcurrent events. These current ratings are indicated with labels on each channel. Typical current limits per channel are 15A, 25A, or 35A max. As with all Electron Speed products, Mil Spec wiring and terminals are used. Channels are constructed as follows for each rating:

  • 15A-rated channels are typically built with 14 AWG wire and DT-2S connectors
  • 25A-rated channels are typically built with 12 AWG wire and DTP-2S connectors
  • 35A-rated channels are typically built with 10 AWG wire and unterminated

The status of each channel is indicated by LEDs on the board. The four status modes are:

  • On – steady, active LED
  • Off – LED off
  • Overcurrent – Channel(s) LED blinking
  • Overtemperature – All channels’ LEDs strobing in sequence

Wiring Examples

Simple Race Car with Minimal Accessories

This customer is powering a simple race car with minimal electrical accessories.

Since the Smart Relay is acting as the circuit protection for all of the electronics, the car can be killed with the Smart Relay. The system can be reenergized by waking up the Smart Relay. This is done with pin 7. Multiple kill switches can be wired in series providing easy access for corner workers and the driver. The 12V source is a current-limited, source. This means that it is more safe than the typical installation that uses a ground to trigger the relay. A ground can easily be created by the switch or a wire contacting the chassis during an accident. This results in the inability to turn off the relay.

Pins 2 and 1 are directly connected together to instantly wake the engine controller and data system when the kill switch is toggled on. The rain light would also run off this circuit if equipped.

Pin 3 activates Smart Relay output 2 and energizes the ignition system and related components (fuel injectors, lambda sensor heater, alternator field etc.). This is controlled by a switch connected the Smart Relay’s ground reference (pin 1).

Pin 4 is configured to allow the engine controller run the fuel pump when it is necessary.

Pins 5 and 6 can be wired to a data system giving the crew the data they need to understand the current draw of their system. This may allow for further development of the car’s electronic system based on known electrical loads. Could the alternator or harness be smaller? Since there were no other devices connected to the CAN bus, termination resistors were added to the ends of the harness.

Endurance Race Car Fuel Control System

This customer has installed the Smart Relay 33 near the fuel tank in this typical endurance race car fuel control system.

The engine controller’s fuel pump control output directly grounds Smart Relay 33 pin 2 when the system needs fuel. This activates the output 1 on the Smart Relay and turns on the lift pumps. The driver can choose to activate the main, high-pressure, fuel pump or the reserve pump by also sending the engine controller’s request to pins 3 or 4. Since only one of the high pressure pumps is on at any given time, the heavy gauge wiring from the battery does not need to be sized for both pumps. And, since we are only running the pumps when the engine controller is requesting them, we are safer than direct driver control.

Pins 5 and 6 are wired to a data system to inform the driver or engineer about problems with the fuel system. For example, if the current draw by the high pressure pump is increasing, the filter between the pump and the regulator is likely to be becoming restrictive. If termination resistors are not already exist in the harness or the devices connected, they will need to be added.

Since this car has a separate battery cutoff switch, Pins 7 and 8 are connected together to simplify the wiring. This means the Smart Relay 33 will be ready to energize any output at any time that the battery connection is powered.

Paddle-Shift System

For our in-house paddle shift systems, we use large electrical solenoids to eliminate the need for compressors, regulators and hoses. We use the Smart Relay 33 to control these actuators by wiring it like this example. This safe system requires the powertrain controller to simultaneously be supplying a positive signal to wake up the system and a ground signal to energize an actuator. This guarantees that the transmission will not be engaged (or damaged) by random controller power problems. This includes situations like reprogramming the powertrain controller, checking harnessing during maintenance or shorting the signal wires to ground. It also means that the Smart Relay 33 is drawing the minimum quiescent current when the car is not being driven.

Pin 7 is pulled high (12V) by a powertrain controller output when the shift system is turned on and ready for its first gear engagement. Before this point, accidental pulls of the paddles on the steering wheel will have no effect on the system.

Pin 2 is grounded by the powertrain controller to energize the upshift solenoid. This solenoid draws 60 amps but the short duration of a shift is not long enough to melt any components so this can easily be handled with a channel that is rated for a lower steady state current.

Pin 3 is grounded by the powertrain controller to energize the downshift solenoid. This solenoid draws 60 amps but the short duration of a shift is not long enough to melt any components so this can easily be handled with a channel that is rated for a lower steady state current.

Pin 4 is grounded by the powertrain controller to energize the blip solenoid that opens the throttle for a short period of time to better synchronize downshifts.

Pins 5 and 6 are wired to a data system to inform the driver or engineer about problems with the shift system.

Since the high current draw of the two shift solenoids are never seen at the same time, the power cabling can be sized for only one of the solenoids.

NASA Prototype

Designed for use in an Élan NP01. Click here to learn more about this vehicle.