12V SSS Solar Charge Control Circuit

6A, 12V SSS Based Solar Charge Control

This is a Solid State Switch upgrade to the 6A, 12V solar charge regulator. Solid state switching provides longer life, smaller size and higher efficiency than a relay. In addition, the clock frequency may be doubled in order to better track battery load /charging requirements. The parts cost is only slightly higher. One drawback is the loss of the secondary output feature. For more information go the above link.

The application for this type of charge control is one in which the battery capacity is large in respect to the charging current; e.g. 6A solar panel charging a 60AH battery in which it takes perhaps a full day’s sunshine to fully charge the battery. For much smaller batteries, the high charging rate and relatively high battery internal resistance results in excessive terminal voltage so that the control immediately interrupts charging –the end result is that the battery cannot charge fully –a linear charge regulator is more appropriate in such cases.

Schematic of the Solid State Switch Solar Charge Controller

12V SSS Solar Charge Control Circuit

Bill of materials


Eliminate the relay life issue

While relay life is probably OK, it is definitely limited. Check out the following Omron relay endurance specification. Mechanically, it is good for 100million operations. With a 10A, 250VAC load, it is rated for only 36,000 operations. Note that a form C contact is used, so the form A (normally open contact) specification does not apply. I believe (guess) that it is good for at least 400,000 operations in the 6A solar charge regulator application –this equates to approximately 1year’s operation. So you can see the advantage of going solid state.

Omron G5LE specification

Omron G5LE Endurance Spec

Driving the MOSFET gate

Obtaining high speed MOSFET operation can be a challenge, but at such a low frequency (0.067HZ), driving the gate is a piece of cake. MOSFET turn-off is generally the issue, so I obtained an oscillograph. Turn-off occurs within about 80uS, and the active portion of the gate voltage is indicated by the step on the transition. R14 is what discharges the MOSFET gate capacitance, including the Miller capacitance in which the change in drain voltage is coupled to the gate thus slowing the transition. The gate turns on via R9 and the gate voltage is limited via zener D3 to 10V –otherwise the full solar panel voltage could appear between the gate and source and potentially damage the MOSFET.




To set the Max Voltage Adjust potentiometer (R12), start with it turned CW, monitor the voltage and wait for the battery terminal voltage to reach the desired voltage (e.g. 14.5V). At this point, turn R12 CCW until the LED extinguishes. When the LED comes on again at the next clock cycle, recheck the voltage at which it extinguishes.


For the future

  • 6A, 6V relay based solar charge control
  • 6A, 6V SSS based solar charge control
  • MPPT buck converter solar charge control
  • Selection guide for the various solar charge controls

Undocumented words and idioms (for our ESL friends)

piece of cake –idiom –easy, or easily accomplished


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