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Low Voltage Relay Wiring for Off Grid Solar Applications


A relay is an electrically operated switch.  Low voltage relays are of great help in many off grid solar applications for controlling a high-current circuit with a low-current switch.  Current = amps.  The low voltages typically found in off grid systems, 12 or 24 volts DC, have high resistance which requires large wire to travel a significant distance without voltage loss.  A relay can minimize this problem.

For example, a relay can be used to switch a Simple Pump DC gear motor on and off using a pressure switch or float switch that is rated below the peak amp draw of the motor.  The nominal amperage of a 1/5 horsepower 12 volt DC gear motor is between 14 and 15 amps, but the increase in torque of the motor on the pump upstroke can create spikes approaching 25 amps.  If the pump disconnects from the circuit during the upstroke, a 15 amp pressure switch or an 8 amp float switch might be damaged.  Using a relay with a 30 amp or higher rating avoids this potential problem.  Use of the relay also gher rating avoids this potential problem.  Use of the relay also allows the pressure or float switch to be connected from a greater distance using much smaller wire.

Here is a relay connection scheme using a small toggle switch (14) to actuate a 30 amp relay (13) to turn on and off an electrical load (15) which is an LED light bulb powered by a small 12 volt lead acid battery which is out of view top left of the photo.

1. Battery positive (red)

2. Battery negative (black)

3. Load positive (red)

4. Load negative (black)

5. Positive to relay (blue) from battery positive 1

6. Positive to relay coil (white) which is on same terminal as 5

7. Jumper from negative to switch common (black)

8. Jumper (black) from battery negative to load negative

9. Positive feed from the relay to the load positive (yellow) which is normally open (meaning that is only closed when you actuate the switch 14)

10. Positive feed from the relay which is energized when the relay is normally closed (red) not used in this application

11. Jumper wire from switch common to battery negative (black) 7

12. Positive from relay coil to switch positive (black)

13. Relay

14. Toggle switch single pole

15. Load LED light bulb

The switch (14) could be a pressure switch or a float switch which are common in water pumping applications.

The current from the battery (1 and 2) goes to the relay via blue wire (5) and to the load positive via yellow wire (9).  The negative half of the circuit goes from the battery to the load via jumper wire (8).

The relay coil is powered by positive white wire (6). The negative half of the circuit goes from the relay coil black wire (12) and the black jumper wire (11) which connects to the battery negative.  When the toggle switch is closed the relay coil is energized which closes the relay to make a circuit to power the load.

Another application for this relay scenario is to protect the low voltage disconnect function of a small charge controller from amp spikes higher than the charge controller amp rating.  Most small charge controllers with low voltage protection use a MOSFET to disconnect the load.  A MOSFET is a kind of transistor used for switching electronic signals which can be damaged by a voltage surge if the load is disconnected when the load amps are spiking. Wire the white relay coil wire to the positive load connection and the black to the negative.  Run the pump motor through the relay.  If the voltage in your battery drops below the charge controller disconnect voltage, the relay coil will open turning off the power to the pump.  The alternative is to run your pump directly from the battery, but you won't have low voltage protection for the battery.

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