Schematics and electronic stuff
Warning! All circuits described here are designed to be used with 230V AC power.
If you don't know what you're doing, keep your hands off it! Remember this is high voltage,
you are under life danger!
Phase controlled modulation
In case you don't know yet how phase controlled modulation works, here is a very short description.
There is lots of documentation avaliable, so you'll find a better and more detailed
one if you need it.
On electricity networks, the phase crosses zero several times a second. In Europe,
it's 100 times per second which is what we call a 50 Hz AC voltage.
Triacs (two bipolary connected thyristors) have the character of getting conductible
after their ignition until they have same potential of electric tension on both sides.
This is - in the circuit used here - the zero crossing point. So, all we have to do
is switch it on after each phase cycle, it switches itself off automatically.
Thus, by alternating the time called T in the graphic above, we can
influence the amount of power (grey area) a consumer load can use. For example, this can
be the luminance of a light bulb.
And this is the whole concept already:
- measure the zero crossing points and cause interrupts on a microcontroller every time
- start counting after each zero crossing and ignite a triac after a certain amount of time (trigger time)
- receive update values via USB so an application can modify that trigger time for each channel individually
Zero crossing detection
The detection of zero crossings is done by two antiparallel optocouplers:
| TR2 | a transformator that generates 6-8V on secondary side |
| R1 | depends on the type of optocoupler you use, typically around 400 Ω |
| R2 | roughly 10 kΩ |
| Vcc | +5V |
It should generate an output like this:
The rising edge of this signal is used to give interrupts to a microcontroller.
The pulse width can be modulated by changing the values of R2 or R1.
Output stage
Once the microcontroller decided the time has come to switch a triac, it sends an
output signal to the following output stage circuit:
| R2 | about 60-100 Ω |
| L1 | 50-100 µH, current limit depeding on your consumer load |
| F1 | depends on your consumer load, in my project 1A |
| C1 | roughly 100 nF, 400V |
In my project, the sum of all consumer loads was limitated by the main input plug anyway which has a limit of 16A (around 3500W on 230V electric networks). So there was no need to make each of the 20 channels extremly powerful.