While this app note is intended to replace mechanical thermostats, we’re not really interested in that part of it. What does interest us is how to build a controller that interfaces with loads on the mains power line. The neat thing about this design is that it is powered from mains, using a capacitor, zener diode, and two 5W resistors to drop the voltage to 5V.
The app note describes how to detect zero crossing and control the power applied to a load through a triac. As a side note, if you replace the potentiometer part of the circuit with a IR receiver you could easily make a light dimming control circuit that would fit inside lighting fixtures.
This circuit uses live mains voltage, so be sure you know what you are doing before building it.
Great post! Thank you! I am interested in interfacing a controller that interfaces with loads on mains power. It is a timely post in my case – had to fix one of my two ovens right before Thanksgiving so I am still thinking about that … ;-)
I don’t know how good of a thermostat this would be, since there is no temperature feedback loop onto which the controller would keep stable temp. but, if you add a thermistor to the GP0 insted of a diode you could built it
Yes, it is not a thermostat. It is a variable power drive. However, this is intended for electric skillets and ranges, not home heating and cooling. For that, power control is typical and usually good enough. In any case, implementing useful temperature feedback on a range mostly requires effort on the mechanical side: you need to find a way to get a temperature sensor with the appropriate range to measure a relevant location. Most thermistors don’t work at such high temperature. A thermocouple or a non-contact IR thermometer is a better option.
Very risky, if you see that VCC is actually the live wire that comes from grid.
It is not very effective schematic as it requires 2 5W resistors and those are big, not really something you can slam into a wall socket easily :(
Way easier schematic is something like this: http://elco.crsndoo.com/files/220vac-5vdc.png
What Arup said. Be very careful about a power supply design like this as there is no isolation from the mains and you can get a lethal shock from this. This design is usually used in double insulated devices which have no exposed metal contacts or parts. Ground potential on it is not going to be 0V.
Highly dangerous design, normally you would put the resistors in the “L” feed.
They have used Two resistors for safety…. in case one goes defective, TUV has this requirement for Touch controls where there is an isolating capacitor or resistors in this type of design.
A far ‘better'(better being relative,) design is to have a wire wound inrush resistor feeding a 400v 047/.68uf capacitor followed by a Single diode/Zener arrangement.
However circuits of this design are not suitable for non-polarised mains plugs (Europe/USA), generally they should never be used as part of a non-permenent connection method, since I have seen these circuits fail catastrophically regularly.
Also be very aware that the voltages with reference to ‘N’ or ‘E’ are going to be :
Mains voltage minus operating voltage.