SPC3 9 Amp in / 10 Amp out Solar Power Center

A kit with the circuit board and parts for this circuit is available from CirKits.

SPC3 Circuit Board Kit

SPC3 9 Amp in / 10 Amp out Solar Power Center

(C) G. Forrest Cook 2007


The SPC3 is a solar power center, it can handle all of the power functions for a solar charged 12 Volt DC system. The SPC3 contains a 9 amp photovoltaic charge controller, a 10 amp low voltage load disconnect circuit and a pair of built-in white LEDs for area illumination. The low voltage disconnect circuit has a load on-off switch, and a battery low voltage indicator. By using the SPC3 as the center of a solar powered device, long battery life is assured. The SPC3 can be used for a self-contained solar lighting system, it can be used for making solar powered audio and radio devices and much more.

The circuit was designed with the following goals:

Single board smart solar power system with self-contained light.
Analog simplicity with common parts for ease of repair.
High efficiency: low-loss charging and minimal idle current.
Radio-quiet operation, low frequency charge control switching.
Capable of handling resistive, inductive and capacitive loads.


Nominal battery voltage: 12V
Maximum solar panel current: 9 Amps
Maximum load current: 10 Amps, higher power loads can be connected directly to the battery.
Night time battery drain current: approximately 150 micro-amps
Temperature compensation for use in variable climates
Radio-quiet operation

See the full SPC3 kit specifications for more details.

Here are the SPC3 kit alignment instructions.

Charge Controller Theory

The charge controller is shown in the upper half of the schematic. When the PV panel's voltage rises above 12V, current flows through zener diode ZD1 causing transistor Q2 to turn on and send power to voltage regulator VR1. VR1 provides 5 Volt power to the rest of the charge controller circuitry. The charge controller power is supplied by the battery when the sun is shining on the PV panel. During the night, the Q2 circuit turns off and prevents the charge controller from draining battery power.

The upper half of IC1 is the heart of the charge controller. It acts as a comparator/oscillator circuit. When the battery voltage is well below the float voltage setting, IC1 turns on, this causes the LED to turn red and the 4N35 opto-coupler to turn on. The output of the 4N35 activates FET Q1, which connects the solar panel power to the battery through Schottky diode SD1.

When the float voltage is reached, the circuit oscillates just above and below the float setting and battery charging current is switched on and off. The oscillation frequency changes with the battery's state of charge and the available PV panel current. The maximum switching frequency of IC1 is limited due to the hysteresis provided by R11 and the low-pass characteristics of R6,RV1,R8 and C4.

The thermistor TM1 modifies the float voltage to a slightly higher value when the circuit experiences colder temperatures. This thermal compensation improves cold weather battery charging.

The lower half of IC1 always produces the opposite output from the upper half of IC1, this is used for driving the bipolar state indicator, LED1. Shorting the equalize terminals causes the float voltage setting to rise, this is useful for occasionally overcharging (equalizing) a battery.

Schottky diode SD1 prevents battery power from leaking back into the solar panel at night. Diode D1 is wired as a crowbar circuit, if the battery is connected in reverse the fuse will blow, saving the rest of the circuitry from destruction.

Low Voltage Disconnect Theory

The smart switch and low voltage disconnect circuitry is shown in the lower half of the schematic. The circuit operates like a solid-state version of a latching relay. Unlike simple voltage controlled switch devices, when the LVD circuit shuts off, it stays off until it is manually turned back on with a flip of switch S1. This action prevents the load from oscillating off and on due to the rise in battery voltage after the load is disconnected.

When momentary switch S1 is turned on, transistor Q5 is turned on. This activates the comparator circuits formed by IC2d and IC2c. When the circuit is active and battery voltage is above the LVD setpoint, the IC2d comparator output goes low and transistor Q4 is switched off. This allows gate drive voltage to reach Q3, Q3 connects battery power to the load. Resistor R16 bleeds off the gate drive voltage from capacitor C8 when the circuit shuts off.

When momentary switch S1 is turned off, the Q3 gate drive voltage is shorted to ground, causing load power to be switched off.

IC2a and IC2b form a square wave oscillator that produces a frequency of approximately 350 Hz with a voltage swing from 0-12V. The output of the oscillator is fed to the D3,D4,D5 voltage multiplier circuit to produce the Q3 gate drive voltage, this is typically around 18.3V.

Once the LVD circuit has been turned on, diode D7 feeds the load power back to the circuit to keep it active. Power to LED3 and LED4, the two white LEDs is routed through R17, a current limiting resistor.

Regulator VR2 provides a reference voltage for the oscillator circuit and for the battery voltage comparators IC2c and IC2d.

IC2c is the low battery sense comparator. When the battery reaches 0.6V above the shutoff point, the IC2c output goes high and turns on the yellow LED. Resistor R21 sets the difference between the low voltage indication and the power shutoff.

Diode D8 protects the circuit from reverse voltage spikes when driving an inductive load such as a relay or motor.

Charge Controller Alignment

Connect the PV panel and battery to the circuit, put the PV in full sun. Turn the float voltage setting fully clockwise, the dual color LED should turn red. Connect a volt meter across the battery and monitor the battery voltage. The battery voltage should gradually rise while the sun shines.

Leave the circuit connected until the battery voltage has reached or exceeded the desired full-charged setting, this is typically around 13.8V at room temperature. Turn the float voltage setting counter clockwise until the LED alternates red and green. Tweak the setting until the LED blinks and the battery voltage is where you want it to be at the full state. When the LED is alternating red/green, it is normal for the battery voltage to vary by about 10mV.

If a battery pack with a float voltage setting below 13V is used with the SPC3 (NiMH for example), zener diode ZD1 should be changed to a 1N4740 (10V) and resistor R6 should be changed to 250K.

Low Voltage Disconnect Alignment

Disconnect the battery from the SPC3 and connect a variable voltage power supply that can produce 10 to 15 Volts across the SPC3 battery connection. Observe the correct polarity. Set the variable supply to 11.6 Volts. Turn the LVD Setpoint pot fully counter-clockwise. Turn the power switch on. The White power LEDs should turn on. Turn the pot clockwise until the yellow low voltage LED just turns on. Monitor the supply voltage while you decrease it, at around 11V, the LVD should turn off the white LEDs. Repeat the adjustment if you wish to fine-tune the shutoff voltage.


Connect a 12V rechargeable battery, photovoltaic panel, and (optionally) a load to the circuit.

As sun shines on the solar panel, the circuit will pass charging current to the battery. The dual color LED will turn red while solar charging is taking place. When the battery voltage rises to the full setpoint, the charging current will be periodically cut off and the LED will alternate red and green.

The load can be operated during daytime charging and at night.

The on/off switch controls the load like a normal power switch. If the battery voltage drops to 0.6V above the shutoff point, the yellow LED will light up. If the battery voltage drops further, the circuit will shut the load off, preventing deep discharge of the battery. This greatly extends the life of the battery.

SPC3 Revisions

The SPC3-a and SPC3-b1 kit versions should have the following modification: R4 has been changed from 47K to 22K and R25 (270K) has been added across pins 4 and 6 of OC1 (4N35). This fixes a problem where the charge controller can lock on (R/G LED stays green). The problem is more likely to occur if there is residual solder flux on the circuit board. The photo below shows the placement of the 270K resistor. For existing kits, it is easier to solder a 39K resistor across R4 (47K) instead of repacing R4 with a 22K part. SPC3 kits sold after May 16, 2013 will include this revision.

SPC3 R25 modification

SPC3 Circuit Extensions

Automatic LVD turn-on

The SPC3 LVD function is automatic, when the battery voltage drops below the LVD setpoint, the load is turned off. The basic SPC3 circuit requires a manual operation of the switch to turn the load back on after the battery recharges and the voltage rises. For some applications, it may be desirable to have the circuit turn back on automatically. The following circuit can be used to perform this task. It is a simple voltage comparator circuit. When the battery voltage, which is tapped off after the SPC3 fuse from the PV+ line, rises above an adjustable setpoint the op-amp turns on the 2N3904 transistor. The transistor connects the SPC3 On switch to ground, turning on the SPC3 load control circuitry. The turn-on adjustment should be set to activate the circuit when the battery voltage rises somewhat above the SPC3 LVD setpoint to prevent oscillation.

SPC3 Turn-on circuit

Support for higher voltage PV panels

The SPC3 is limited to use with PV panels with a maximum output voltage of 20V. If you would like to use the SPC3 with a panel that has an open-circuit voltage from 20V to 26V, you should perform the SPC3-b1 High Voltage PV panel modification.

SPC3 Kits For Sale

A kit version of the SPC3 solar power center circuit is available from CirKits.com, buying the kit will save you time locating parts and wiring the circuit.

Back to FC's Solar Circuits page.