12V 4 AA Cell Differential Temperature Charger

This project includes a number of improvements over my older Temperature Controlled NICD Charger circuit. This circuit runs on 12VDC, allowing it to be used in a car or from a 12V solar power system. Additionally, a current sensor LED verifies that the cells are receiving charging current. Note that the current sensor circuitry is not shown in the circuit board photo above, it was added to the side of the main board via a small perfboard.

12V, 4-AA Cell Differential Temperature Charger Circuit diagram

The current is adjustable in three steps from 100 to 300mA, allowing fast charging of AA, AAA or other small cells. Battery packs from 1 to 6 cells can be charged with this circuit. NiMH and older NiCD cells are supported. The circuit is protected from reverse input voltage and reversed cells.

Connections:

• 12VDC (nominal) power input
• Connections for the Battery Under Charge

Controls:

• Power On/Off switch
• Charge Start switch
• 3 step Charge Current Select jumper
• Calibrate/[Latch] mode jumper
• Temperature sensor calibrate trimmer
• Red/Green Charging/Done light
• Amber Current Flow light

Theory:
12VDC power is supplied to the circuit from an external source such as a car battery system, a 12V solar power system or a regulated "wall wart" supply. If DC power is applied backwards, the 6A05 crowbar diode causes the 3A fuse to blow, protecting the circuit from reverse voltage. The power switch routes power to the 78L09 voltage regulator and the battery. The 78L09 regulator provides regulated power to the rest of the circuit.

The battery current loop starts with the +12V supply, then runs through the battery and through a 1N5819 reverse voltage protection diode. Current continues through the LM317 1 amp adjustable voltage regulator which is wired as a constant current source, through the IRFD110 power MOSFET transistor, which switches charging current on and off, through the 0.1 ohm current sensing resistor to ground (12VDC negative). The charge current is selected by jumpering one of three current-set resistors on the negative side of the LM317 regulator. The 120 ohm trickle charge resistor always allows 10mA of current to flow through the battery.

The temperature control part of the circuit starts with a matched pair of 10K NTC thermistors. One thermistor is epoxied to a small metal reference temperature plate. The other thermistor is epoxied to a metal battery holder. The temperature sensors are balanced by the calibrate potentiometer. The 100nF and 50nF capacitors across the thermistors cause different start-up time delays to insure that the following circuitry powers up in the off state.

The upper half of the TLC2272CP rail-to-rail op-amp is wired as a latching comparator when the Cal/Latch jumper is present (operate mode), the circuit becomes a regular comparator with hysteresis when the jumper is off (calibrate mode). Assuming the battery is cold and the circuit is in operate mode, the start switch turns the op-amp on for a charging cycle. When the battery temperature exceeds the reference temperature, the op-amp turns off and the diode in the feedback loop latches the op-amp off. The op-amp output also drives the IRFD110 current switch MOSFET.

The lower half of the TLC2272CP simply inverts the output from the upper part of the op-amp, this gives a bipolar drive signal for running the Red/Green Charging/Done light.

An optional current flow lamp was added to the circuit. Rechargeable batteries tend to get corroded contacts which can prevent the charging current from flowing. The lamp provides an indication that the charging current is really making its way through the batteries. The current Flow lamp circuit consists of an LM358 op-amp wired as a current measurement amplifier that monitors the voltage drop across a 0.1 ohm resistor. The LM358 is specially suited for this type of circuit. The output of the first LM358 stage is further boosted and offset by the second LM358 stage. This produces a digital signal that drives the indicator LED through a current limiting resistor. If you dont want to add the Current Flow circuit, replace the 0.1 ohm resistor with a wire jumper.

Construction:
The circuit was built on a custom home-built circuit board, a hand-wired perf board would also make a good platform for this project. The LM317 regulator is mounted on an aluminum heat sink under the main board, the heat sink should be kept away from the two temperature sensors. The reference temperature sensor is mounted to a small piece of aluminum that is thermally isolated from the battery holder and the rest of the circuitry. All of the sub-components are mounted on a piece of plexiglass or another non conducting material.

Calibration:
Put the circuit in one location and allow the temperature to stabilize for an hour or so. Remove the Cal/[Latch] jumper. Adjust the 20 turn Calibrate trimmer a little bit past the point where the Charge/Done light turns red. Put the Cal/[Latch] jumper back.

Use:
The charger should only be used in a cool location with a fairly constant temperature. Install the batteries to charge in the battery holder, start with the negative side of the socket and connect the alligator clip to the + side of the last cell. Put a piece of insulating foam over the battery holder to keep the warmth in the battery. If the battery is already hot, allow it to cool down before starting the charge cycle. The Charging/Done light should now be green.

Push the Start switch and the Charging/Done light should turn red. The Current Flow light should turn on, if it doesnt, try reseating the cells in the holder. After some amount of charging, the battery will warm up, the Charging/Done light will turn green and the battery charge cycle be finished. If you want to equalize the weaker cells in the battery, allow the pack to cool down then run another charging cycle, the second time should not take very long.

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Solar Hot Water Panel Differential Pump Controller

This circuit optimises the circulation of heated water from solar hot water panels to a storage cylinder. It achieves this by controlling a 12V DC pump, which is switched on at a preset temperature differential of 8°C and off at about 4°C. This method of control has distinct advantages over some systems that run the pump until the differential approaches 0°C. In such systems, the pump typically runs whenever the sun shines. A small 10W solar panel charging a 12V SLA battery is sufficient to run the controller. Most commercial designs use 230VAC pumps, which of course don’t work when there is a power outage or there is no AC power at the site.

 

Operation:

Temperature sensors TS1 & TS2 are positioned to measure the highest and lowest water temperatures, with one at the panel outlet and the other at the base of the storage cylinder. The difference between the sensor outputs is amplified by op amp IC1d, which is configured for a voltage gain of about 47. As the sensors produce 10mV/°C, a difference of 8°C will produce about 3.76V at the op amp’s output (pin 14). The output from IC1d is fed into the non-inverting input (pin 10) of a second op amp stage (IC1c), which is wired as a voltage comparator. The op amp’s inverting input (pin 9) is tied to a reference voltage, which can be varied by trimpot VR3. When the voltage from IC1d exceeds the reference voltage, the output of the comparator (pin 8) swings towards the positive rail.

A 10MW resistor feeds a small portion of the output signal back to the non-inverting input, adding some hysteresis to the circuit to ensure positive switching action. A third op amp stage (IC1b) acts as a unity-gain buffer. When the comparator’s output goes high, the buffer stage switches the Mosfet (Q1) on, which in turn energises the pump motor. Mosfet Q1’s low drain-source on-state resistance means that in most cases, it won’t need to be mounted on a heatsink. The prototype uses a Davies Craig EBP 12V magnetic drive pump, which draws about 1A when running and is suitable for low-pressure hot water systems only (don’t use it for mains-pressure systems as it may burst!). For mains-pressure systems, the author suggests the SID 10 range of brass-body magnetic drive pumps from Ivan Labs USA.

Circuit diagram:

Solar Hot Water Panel Differential Pump Controller Circuit Diagram

Setup:

Each LM335 temperature sensor and its associated trimpot is glued to a small copper strip using high-temperature epoxy. It is then waterproofed with silicon sealant and encapsulated in heatshrink tubing. Standard twin-core shielded microphone cable can be used for the connection to the circuit board. Before sealing the two units, adjust their trimpots to get 2.98V at 25°C [(ambient temperature x .01) + 2.73V] between the "+" and "-" terminals. When both have been adjusted, clamp them together and allow their temperatures to stabilise for a few minutes. Next, measure the output voltage from the differential amplifier (IC1d), which should be close to 0V. If not, tweak one of the pots until it is.

Separate the two and warm the panel sensor (TS1), monitoring the output of IC1d. You should see a marked increase in voltage, remembering that an 8°C difference between the sensors should give an output of about 3.76V. The pump switch-on point is set by VR3 and can be adjusted over a practical range of about 4-10°C differential (1.88-4.70V). Adjust VR3 to get about 3.8V on pin 9 of IC1c as a starting point. If set too low and the panels are located far from the cylinder, much of the heat will be lost in the copper connecting pipes. On the other hand, if set too high and the weather is mostly cloudy, then the pump will not switch on very often, as the panels will not get hot enough. For best results, use copper pipes for the panel plumbing and insulate them with tubes of closed-cell foam.

As the pipes cool down between pump operations, small diameter pipes of 15mm are more efficient than larger sizes as they contain less static water. In practice, the pump in the author’s setup switches on for about 30 seconds every 4-5 minutes. As the Davies pump shifts 13 litres/minute, it displaces the heated water from a single panel in about 14 seconds. There is a thermal lag in the sensor readings, so after the pump stops, the temperature difference will keep decreasing for 40 seconds or so as the panel sensor cools down and the cylinder sensor heats up.

 

 

 

Source by : Streampowers

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