Pump Controller For Solar Hot Water System LM335Z

This circuit optimises the operation of a solar hot water system. When the water in the solar collector is hotter than the storage tank, the pump runs. The circuit comprises two LM335Z temperature sensors, a comparator and Mosfet. Sensor 1 connects to the solar collector panel while Sensor 2 connects to the hot water panel. Each sensor includes a trimpot to allow adjustment of the output level. In practice, VR1 and VR2 are adjusted so that both Sensor 1 and Sensor 2 have the same output voltage when they are at the same temperature. The Sensor outputs are monitored using comparator IC1.

When Sensor 1 produces a higher voltage than Sensor 2, which means that sensor 1 is at a higher temperature, pin 1 of IC1 goes high and drives the gate of Mosfet Q1. This in turn drives the pump motor. IC1 includes hysteresis so that the output does not oscillate when both sensors are producing a similar voltage. Hysteresis comprises the 1MO feedback resistor between output pin 1 and non-inverting input pin 3 and the input 1kO resistor. This provides a nominal 12mV hysteresis so that voltage at Sensor 1 or Sensor 2 must differ by 12mV for changes in the comparator output to occur.

Since the outputs of Sensor 1 and Sensor 2 change by about 10mV/°C, we could say that there is a degree of hysteresis in the comparator. Note that IC1 is a dual comparator with the second unit unused. Its inputs are tied to ground and pin 2 of IC1 respectively. This sets the pin 7 output high. Since the output is an open collector, it will be at a high impedance. Mosfet Q1 is rated at 60A and 60V and is suitable for driving inductive loads due to its avalanche suppression capability. This clamps any inductively induced voltages exceeding the voltage rating of the Mosfet.

The sensors are adjusted initially with both measuring the same temperature. This can be done at room temperature; adjust the trimpots so that the voltage between ground and the positive terminal reads the same for both sensors. If you wish, the sensors can be set to 10mV/°C change with the output referred to the Kelvin scale which is 273K at 0°C. So at 25°C, the sensor output should be set to (273 + 25 = 298) x 10mV or 2.98V.



Note:

The sensors will produce incorrect outputs if their leads are exposed to moisture and they should be protected with some neutral cure silicone sealant. The sensors can be mounted by clamping them directly to the outside surface of the solar collector and on an uninsulated section of the storage tank. The thermostat housing is usually a good position on the storage tank.


LM335Z circuit

H-Bridge Motor Driver Using Bipolar Transistors 2N2907A

The classic beginner’s DC motor driver circuit that appears in every electronics textbook is the bipolar transistor H-bridge.

An H-bridge is an arrangement of transistors that allows a circuit full control over a standard electric DC motor. That is, an H-bridge allows a microcontroller, logic chip, or remote control to electronically command the motor to go forward, reverse, brake, and coast.

For the purposes of this article, I’m focusing on a basic H-bridge that is a good choice for most robots (including BEAM robots) and portable gadgets. This H-bridge can operate from a power source as low as two nearly-exhausted 'AAA' batteries (2.2V) all the way up to a fresh 9V battery (9.6V).

In later pages, I'll compare the performance of three different part numbers of popular transistors (2N3904/2N3906 vs 2N2222A/2N2907A vs Zetex ZTX1049A/ZTX968) using a common robot motor from Solarbotics.

The H-bridge circuit (below) looks complicated at first glance, but it is really just four copies of a resistor + transistor + diode.


Schematic of a bipolar transistor hbridge circuit to drive a DC motor. Can you see the letter 'H'?
There are many different ways to draw the circuitry, but the above wiring diagram matches the model of most h-bridges.

Q1, Q3: These are NPN transistors. They connect the motor to ground (negative terminal of the battery).
Q2, Q4: These are PNP transistors. They connect the motor to +2.2V to +9.6V (positive terminal of the battery).
R1-R4: These resistors prevent too much current from passing through the base (labeled B) control pin of the transistor. The resistor value of 1 kilohm (1000 ohms) was chosen to provide enough current to fully turn on (saturate) the transistor. A higher resistance would waste less power, but might cause the motor to receive less power. A lower resistance would waste more power, but wouldn’t likely provide better performance for motors running on consumer batteries.
D1-D4: Diodes provide a safe path for the motor energy to be dispersed or returned to the battery when the motor is commanded to coast or stop. I notice many H-bridge circuits on the web lack these diodes. I suppose that’s safe enough for light loads at low voltages, but without diodes, a motor voltage spike can force its way through the unprotected transistors, damaging or destroying them.
M1: This is a direct-current (DC) motor. These are very common. You can find them in surplus stores online or in salvaged toys. The motor should have only two wires. Measure the resistance of the two motor wires using a multimeter. If the motor resistance is less than 5 ohms, then the transistor parts listed in this article are too weak to power the motor.