Troubleshooting the SCC3 Solar Charge Controller

(c) 2012-2016, Document revision: June 29, 2016

[SCC3-e1 closeup]
SCC3-e1 board

Here are some tips for getting your SCC3 kit up and running.

You may want to refer to the SCC3 Schematic page, be sure to read the SCC3 FAQ document.

Common Assembly Problems

The SCC3 kit has been around for a number of years, here is a list of common assembly mistakes that have been observed:

Live Testing Procedure

For the live test, you should have a 12V battery that is charged to at least 11V and either a solar panel in the sun or a current-limited DC power supply that can put out around 18V DC. A useful PV panel simulator can be built with an 18V DC power supply and a 22 ohm, 10 watt resistor (or similar) in series with the supply's + lead. You will need a volt meter, and optionally, an oscilloscope.

Remember, if the wiring from a battery is short-circuited by coming into contact with loose wires, jewelry or other metal objects, the resulting high currents can cause fires. Be especially careful when working with large batteries. The SCC3 board should be securely mounted to a stable surface and the battery wiring should be secured so that it does not move.

The PV voltage should be around 18V when it is disconnected from the SCC3 and facing the sun. Different panel types will have different open circuit voltages. The open circuit PV voltage must be several volts higher than the desired battery float voltage for charging to occur. The voltage difference between the PV and the battery allows the battery charging current to flow.

Connect the battery to the SCC3 battery terminals and the PV panel or power supply to the SCC3 PV input terminals. Turn the equalize switch off. Connect the negative (-) lead of the volt meter to the SCC3 battery's negative (-) lead for all voltage tests. Verify that there is 11V to 13V on the SCC3 battery + terminal and a higher voltage on the PV input terminal. With a functioning SCC3, the PV input terminal will be pulled down to a voltage that is from .5V to 1V higher than the battery voltage, this value depends on the amount of PV current that is flowing into the battery.

Check the SCC3 power activation circuit IC4. The output of this comparator circuit, IC4 pin 6, should rise to about 1V below the battery voltage as soon as the PV voltage is greater than the battery voltage. The IC4 VCC power input (pin 7) should read about .2V less than the battery voltage.

After verifying proper operation of the power activaction circuitry, move on to the 5V regulator, IC3. There should be 5V on the + side of C1, this voltage is also available on pin 8 of IC1. If the +5V power is present, the LED should be lit, either red or alternating red/green (orange). If the 5V line is too low, check the orientation of D2 and verify the correct values of R10, R11,R12, R13, R16 and R22. Make sure IC4 is a 741 type and it is installed correctly. [8 Pin DIP pinout]

Verify that the oscillator circuit (IC1b) is working. IC1 pin 5 should be at 2.5V and the output of IC1a, pin 7, should have a 150 Hz square wave signal that varies from 0 to 5V. Pin 7 should show a reading of around 2.5V on a DC meter, it is recommended that you use an oscilloscope to check this signal.

Check the voltage sensing op-amp circuit, IC1a. Pin 2 of IC1 should be around 2.4V and steady. Pin 3 of IC21 should also be around 2.4V, but the voltage varies with the PV input voltage, the VR1 setting and the equalize switch position. If the voltages look normal, now would be a good time to adjust VR1 and try setting the circuit's float voltage setting. The IC1a output, pin 1, should be either 0V or oscillating between 0V and 5V depending on the input pin voltages. If any of the voltages around IC1a are incorrect, check for the correct resistor values around IC1.

If the LED stays solid red or solid green, Q1, Q3 or Q4 may have been interchanged.

SCC3-e1 revisions

This revision applies to SCC3-e1 kits that were shipped prior to November 19, 2012: Resistors R10 and R11 should be changed from 100K to 10K to improve the circuit's start-up at sunrise. Instead of removing the 100K resistors, you may find it easier to solder 10K resistors across the 100K resistors, the exact value is not critical as long as both resistors are the same.

A batch of red/green LEDs with reverse polarity was included in kits from the fall of 2012 until July, 2013. The LED colors will be reversed (green for charging and red for float). The circuit will otherwise function normally. The fix involves removing and reinstalling the LED so that the flat on the LED is opposite the marking on the circuit board. Be careful not to overheat the LED, they are easily destroyed by too much heat.

Change of value for R16: The value of R16 has been changed from 1M to 300K as of August, 2018. This sets the amount of hysteresis on the circuit activation chip, IC4, to a lower level which can improve the start-up behavior. It is possible to solder a 470K resistor across the existing 1M resistor to create a 320K resistance, which is close enough to 300K. Repairs

If you still can't get your SCC3 kit to work, please mail it back to:
PO Box 3500
Boulder, CO 80307
Please include your email address. You will receive a quote for the cost of repairs. Payment for parts/labor and return postage will be required.

Back to the SCC3 page.