Build a 10 Amp Solar Charge Controller SCC2 Circuit

The SCC2 is a solar charge controller, it’s function is to regulate the power flowing from a photovoltaic panel into a rechargeable battery. It features easy setup with one potentiometer for the float voltage adjustment, an equalize function for periodic overcharging, and automatic temperature compensation for better charging over a range of temperatures.

10 Amp Solar Charge Controller Circuit Diagram With Parts List

The goal of the circuit design was to make a charge controller with analog simplicity, high efficiency, and reliability. A medium power solar system can be built with a 12V solar panel up to 10 amps, the SCC2, and a lead acid or other rechargeable up to a few hundred amp hour capacity. The SCC2 can be operated at battery voltages other than 12V, it can work at 6V and 24V by changing a few parts. Operation at voltages between 6V and 24V is also possible.

Specifications

Maximum solar panel current: 10 Amps
Night time battery drain current: approximately 1ma
Nominal battery voltage: 6V, 12V or 24V.

Theory

The SCC2 acts as a medium power DC current switch between the + terminals of the PV and battery. Diode D1 prevents reverse night time current flow from the battery back to the PV panel.

When the PV voltage is high enough to charge the battery, zener diode D2 conducts and turns on transistor Q2. Q2 switches the power for the rest of the circuit on. The circuit is switched off at night. IC2 provides a 5 volt regulated voltage to power the comparator circuits, it also provides a reference voltage for comparator IC1a.

When the battery voltage is below the desired full voltage and needs charging, comparator IC1a turns on and activates Q1 and Q3, this allows the solar charging current to flow into the battery. Note that Q3 is a P-channel mosfet, this allows the circuit to be wired with a common ground for the solar panel and battery. The solar current loop is drawn in heavy lines on the schematic.

When the battery reaches the full charge point, IC1a operates as a comparator based schmidt trigger oscillator, it switches the solar current off and on. The switching causes the battery voltage to oscillate a few tens of millivolts above and below the desired set point. A rail-to-rail op-amp is required for proper operation, 741 style op-amps will not work in this circuit.

The red/green charging/full LED is driven between the output of IC1a and IC1b. IC1b has an inverted version of the IC1a signal. Pin 5 of IC1b only needs an approximate center point to work as an on-off comparator, it is connected to the varying IC1a pin 2 so that it does not require another reference divider circuit.

The resistors and thermistor on the input side of IC1a form a resistive bridge circuit that is used to compare the battery voltage to a reference voltage coming from IC2/R8/R9. The potentiometer adjusts the voltage point around which the circuit will oscillate on full charge. Resistor R7 adds positive feedback to IC1a for a schmidt trigger characteristic. The thermistor provides thermal compensation, as the temperature goes down, the full voltage goes up.

The equalize switch, S1a, forces the circuit on for intentional overcharging. Switch S1b and R1 can be used to select a different float voltage range, you can experiment with this by using different values of R1, typically R1 should be greater than 1M.

Alignment

    Start with a charged battery, connect the solar panel directly to the
    battery until the battery voltage is at or above the desired full setting,
    this also that the panel is capable of charging the battery.
    While measuring the battery voltage, adjust VR1 clockwise to align the
    float voltage set point.  If the LED turns red before it reaches the
    desired float voltage, the battery will need to charge for a while.
    When the battery is fully charged, it should be at the float voltage and
    the led should show alternating colors.

    The float voltage should be set when the board and battery are at room
    temperature. Typical 12V set points are 13.8V for a gell cell and 14.5V
    for a wet cell.  For 6V, divide those by two, for 24V, multiply by 2.
    Follow your battery manufacturers recommendations for the best settings.
    Readjust the float voltage after the battery has reached a full charge.The float voltage should be set when the circuit is at room temperature.

Use

Connect the solar panel to the SCC2 solar panel input connectors, connect the battery to the SCC2 output connectors. Put the solar panel in the sun, and watch the battery charge up. Systems where the battery is frequently discharged way down should occasionally be run in equalize mode for a few hours or a full day. It is best to monitor the battery voltage during this operation, disable equalization if the battery voltage goes above 16V (12V version).

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Build LED Light Pen Schematic

Physicians and repair engineers often use small light pens for visual examination purposes. Rugged and expensive as these pens may be, their weak point is the bulb, which is a ‘serviceable’ part. In practice, that nearly always equates to ‘expensive’ and / or ‘impossible to find’ when you need one.

LEDs have a much longer life than bulbs and the latest ultra bright white ones also offer higher energy-to-light conversion efficiency. On the down side, LEDs require a small electronic helper circuit called ‘constant-current source’ to get the most out of them.
 
LED Light Pen Circuit Diagram


Here, T1 and R1 switch on the LED. R2 acts as a current sensor with T2 shunting off (most of) T1’s base bias current when the voltage developed across R2 exceeds about 0.65 V. The constant current through the white LED is calculated from

R2 = 0.65 / ILED
With some skill the complete circuit can be built such that its size is equal to an AA battery. The four button cells take the place of the other AA battery that used to be inside the light pen. Link

Author: Myo Min – Copyright: Elektor

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Build an op amp with three Discrete Transistors

You can use three discrete transistors to build an operational amplifier with an open-loop gain greater than 1 million (Figure 1). You bias the output at approximately one-half the supply voltage using the combined voltage drops across zener diode D₁, the emitter-base voltage of input transistor Q₁, and the 1V drop across 1-MΩ feed-back resistor R₂.

Figure 1.	This ac-coupled inverting op amp has an open-loop gain of 1 million. R1 and R2 set a closed-loop gain of −10.

Resistor R₃ and capacitor C₁ form a compensation network that prevents the circuit from oscillating. The values in the figure still provide a good square-wave response. The ratio of R₂ to R₁ determines the inverting gain, which is −10 in this example.

You can configure this op amp as an active filter or as an oscillator. It drives a load of 1 kΩ. The square-wave response is good at 10 kHz, and the output reduces by 3 dB at 50 kHz. Set the 50-Hz low-frequency response with the values of the input and the output capacitors. You can raise the high-frequency response by using faster transistors and doing careful layout. Link

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How to build Personal alarm

Small, portable, anti-bag-snatching unit
Also suitable for doors and windows control

Circuit diagram

• R1 330K 1/4W Resistor
• R2 100R 1/4W Resistor
• C1 10nF 63V Polyester or Ceramic Capacitor
• C2 100µF 25V Electrolytic Capacitor
• Q1 BC547 45V 100mA NPN Transistor
• Q2 BC327 45V 800mA PNP Transistor
• SW1 Reed Switch and small magnet (See Notes)
• SPKR 8 Ohm Loudspeaker (See Notes)
• B1 3V Battery (two A or AA cells wired in series etc.)

Device purpose:

This circuit, enclosed in a small plastic box, can be placed into a bag or handbag. A small magnet is placed close to the reed switch and connected to the hand or the clothes of the person carrying the bag by means of a tiny cord. If the bag is snatched abruptly, the magnet looses its contact with the reed switch, SW1 opens, the circuit starts oscillating and the loudspeaker emits a loud alarm sound. The device can be reverse connected, i.e. the box can be placed in a pocket and the cord connected to the bag. This device can be very useful in signalling the opening of a door or window: place the box on the frame and the magnet on the movable part in a way that magnet and reed switch are very close when the door or window is closed.

Circuit operation:

A complementary transistor-pair is wired as a high efficiency oscillator, directly driving a small loudspeaker. Low part-count and 3V battery supply enable a very compact construction.

Notes:

• The loudspeaker can be any type, its dimensions are limited only by the box that will contain it.
• An on-off switch is unnecessary because the stand-by current drawing is less than 20µA.
• Current consumption when the alarm is sounding is about 100mA.
• If the circuit is used as anti-bag-snatching, SW1 can be replaced by a 3.5mm mono Jack socket and the magnet by a 3.5mm. mono Jack plug with its internal leads shorted. The Jack plug will be connected with the tiny cord etc.
• Do not supply this circuit with voltages exceeding 4.5V: it will not work and Q2 could be damaged. In any case a 3V supply is the best compromise

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