The sensor current is selected so that the current value times the nominal sensor resistance at room temperature results in a voltage that is higher than temperatures results in a voltage that is higher than the full wave voltage at zero crossing for a finite time period preceding the zero value and following the zero value thereby providing a window for sensing.
This timing relationship at the zero crossing with a clock pulse, which is generated at each zero crossing of the supply voltage, is subsequently superimposed in the sensing window. Having thus described principle embodiments of the present invention, it is a principle object hereof to provide controlling of an electrical heater by using the resistance versus temperature characteristic of the heater as the heater's own temperature sensor.
One object of the present invention is to construct a heater using wire materials which have an apprciable, positive temperature coefficient of resistivity. Another object of the present invention is to excite the heater by a low level D. This provides a sensing window with respect to the clock pulse. The supply voltage of the power source is full wave rectified A. The full wave unfiltered voltage is also the source for the heater power through a controller transistor The full wave voltage also is used to power the control circuitry by charging a filter capacitor 4, through a rectifier 3.
Resistor 5 and zener diode 6 form a voltage regulator that powers the control circuit. The sub-circuit shown in dashed lines of box 7 generates a clock pulse at each zero crossing of the supply voltage. This waveform is illustrated in FIG. The clock signal clocks form-D flip flops 26A and 26B, and transfers the logic levels present on the "D" inputs to the respect "Q" outputs. The heater is always excited by a low level D. This current typically is about 10 millamperes, and does not substantially contribute to heating the sensor.
The D. In this microsecond time interval, a control amplifier 21A amplifies the sensor voltage. The output of amplifier 21A is then compared too the set point voltage which is set by a potentiometer 23, and appears as a D. The actual comparison is made by an operational amplifier comparator 21C. If the output of amplifier 21A is higher than the output of the amplifier 21B, the output of comparator 21C will be low. This low will be clocked to the "Q" output of the flip flop 26A on the falling edge of the clock pulse.
The "Q" output will be a logic low. This indicates the heater is hotter than the selected set point. This will then turn off a transistor When the transistor 30 is nonconducting, the output of power transistor 15 is rendered non-conducting, removing the full wave rectified voltage from the heater. When the output of the amplifier 21A decreases and is less than the set point amplifier 21B output, the comparator 21C will switch to a high logic level.
When the next clock pulse occurs, the output of the flip flop 26A will switch high. This turns on the transistor 30, which turns on transistor 15, and again applies power to the heater. Another enhancement of the control circuit is that the circuit protects itself in the event that the heater short circuits. Typically when this happens, excessive current flows through the power transistor 15, and can destroy the device.
A common prior art protection method might be to fuse the circuit with inherent cost and nuisance, but in this instance, comparator 21D and the flip flop 26B provide the necessary protection. The plus input is referenced to a low D. If the heater is short circuited, the output from the amplifier 21A drops to zero. When this occurs, the positive reference voltage on the plus input of comparator 21D switches the output of an amplifier 21D high.
When the next clock pulse occurs, the "Q" output of the flip flop 26B will switch HI. This signal is connected to the reset input "R" of the flip flop 26A. Since reset has priority, the reset causes the "Q" output of the flip flop 26A to swtich LO turning off the transistor 30, and in turn the heater power by turning off the transistor An LED 31 indicates the status of the controller and heater power.
When power is first applied and the heater is cold, the LED 31 will be on steady. When the heater is being cyclially controlled, the LED 31 cycles an and off with the heater power. Resistor 17 and Zener diode 18 protect the amplifier 21A from possibly damaging voltage on the inputs during the major portion of the power cycle. Dependent upon the type of wire material which is utilized, the sensor voltage and current may very well be of the values previously set forth above by way of example for purposes of illustration only.
The description of operation of the electrical circuit has been previously described in describing the electrical control circuit schematic diagram, and is not repeated here for the sake of brevity. It is important to note that any type of heater wire can be utilized so long as the heater wire has an appreciable, positive temperature coefficient of resistivity.
Of course, other widths of clock signals maybe utilized. Zero voltage level is below the D. Various modifications can be made to the present invention without departing from the apparent scope thereof. I claim: 1. And like many systems of that era, it consisted of only a two-wire configuration - red and white wires. It probably also saved on building construction costs. However, it was a challenge for thermostats with electronics that required a power source.
Two techniques have been used to circumvent this limitation. If the resistive load of the electronics is placed in parallel with the thermostatic switch left , it could provide enough current to power the electronics as long as it wasn't sufficient to trigger the heating relay. The other possibility is if the resistive load of a battery charging circuit is in series with the thermostatic switch right then during the heating cycle enough power could be stored to run the electronics while not heating.
Neither are great solutions as the former solution is challenging given that the relay's current draw is not known. And the latter relies on enough heating cycles to charge a large enough battery to sustain the electronics in between heating cycles. I imagine that manufacturers decided to avoid this kind of challenge and focus on home automation in newer construction where common wires are more often available.
For older constructions, the recommendation is to supplement the two-wire system with an external 24V AC power supply. The one exception is the Nest thermostat; it is based on the in-series battery charging design with a micro-usb charging port as a backup in case the battery dies between heating cycles.
And with its simple user interface, I've always been a fan of the Nest thermostat so I was happy to install it. Other systems also offered a second heating or cooling stage. For common setups, check out this extensive guide. A single breadboard is great when prototyping with a microcontroller and a couple of small DIP packages. And with breadboards that snap together, expanding is definitely possible.
But needing to move the project from one workspace to another is a recipe for breadboards separating and countless connections needing to be mended. Coupled with a growing number of projects that also need multiple voltages or multiple power supplies for servos and stepper motors, decided to "design a better mousetrap".
More information on building can be found on thingiverse. With the encoder more or less figured out and the basic circuitry understood, I set out to build a fully working breadboard prototype. The green boxes each represent the 3-bit encoders needed for the thermostat knob and the mode selection knob. Two DRV stepper drivers are in blue. Since the pycom has an onboard voltage regulator, it likes to run above the 3. The dc-to-dc step downs that are on the prototyping board are lousy at maintaining a constant voltage I tried several manufacturers too!
To avoid an issues of having the IO pins being driven above their safe limit, pycom's input voltage is 4. Vmot for the stepper motor drivers are regulated separately at To verify that the encoder disc design would work, I started with a single-bit of the eventual 3-bit encoder and just the one stepper motor. In this first pass, I didn't have any LMs in my parts bin. So while I awaited delivery, I used an op-amp LM and a 2N transistor to get the digital input needed for the microcontroller.
Above the pycom microcontroller is a DRV for the stepper motor. The photodiode in reverse bias acts as the upper half of a voltage divider. The other leg of the LMN comparator input is another voltage divider with a variable potentiometer. Usually used to adjust the sensitivity of the circuit and change reflected distance, it is less needed in this case; the distance between the two diodes is fixed and the diodes face each other thereby removing any IR loss due to the not-perfect-reflective surface.
Since this is this board schematic, it doesn't include the IR LEDs and photodiodes as they'll be off-board. Three of the dual comparator LMs will take a fair amount of real estate on the final board; but there isn't a size constraint and the low-volume there are only three HVAC units in the apartment won't make the the extra PCB board real estate cost prohibitive. About Us Contact Hackaday.
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I'm assuming you want to set the thermostat between 20°C and 30°C, with thermistor values of 63kΩ and 40kΩ resp. This microcircuit provides control of various electronic devices. The PIC16FA microcontroller contains 2 analog comparators, an internal oscillator. The device is an automotive grade comparator incorporated with high gain and 2 or 4 circuits on a single chip. Furthermore, the device has a.