The main control system is now done and mostly setup and tested. The Mains outputs are fully tested and work very well. For each channel, with no load or with a load of less that 0.5W the red LED comes on with a slow transition to the green one between 0.5W and 1W load. I've also tested emulating mains filters, and that shows only the red LED. I haven't fully tested the DC outputs, and it's possible I may need to make adjustments.
The schematic for these is here:
www.musically.me.uk/Switcher/Controls-Logic.pdf
Looking at one of the mains channels (left hand side):
With Sx1 open, a 5V signal from the Arduino lights LEDx3. However, close the switch and the Qx1 transistor turns on, and with a b-e voltage of only 0.6V starves the LED of power. This in turn drives RLx1 and (if there is no load) the diode chain along with LEDx2. The reason for the two diodes is so that if there is power drawn ICx1 turns on, and we want to make sure the voltage across this and LEDx1 is low enough to rob the red one of power.
The upper (mains) section uses BRx1 and MRx1 simply to provide a fairly stable AC voltage drop of between 1.8V and 2.5V when power is drawn. Rx4 acts to drop this much lower for leakage currents. The Optocoupler (ICx1) requires 1.3V to start conducting, with Rx3 limiting its current to a safe level. Cx1 provides a degree of spike and RF suppression. This system is
much simpler (and cheaper) than using the usual current transformers or hall-effect devices and their associated electronics.
Purists would want the fuse on the input side, but it's more convenient where it is, and protects the
whole of the switcher from a faulty external load.
DC channels (right hand side):
The initial control circuit is identical to the mains ones, so uses the same legends. For the upper part we use a simple transistor as the switch with MRx2 and MRx3 providing the current sensitive voltage drop, with Rx6 controlling the no-load point. We have a different problem here, in that the DC-DC converter will draw about 100mA when there is no load attached, so I might have to make adjustments to Rx6 to find the sweet spot. The current sense has to be this side, otherwise the output voltage would vary with loading, whereas the DC-DC converter can handle a wide range of input voltage.
No fuses are needed on the outputs, as both the converter and the 9V regulator are current limited at 1A. Cx2 and Rx8 are static and RF suppression.
While testing all the mains channels I had a fault on just one. This was not on the mains side, but the control itself. Neither red nor green LED would come on - yet early tests had shown no problems. After much head-scratching I discovered there was a dead short between the positive side of the green LED and the front panel - which now it was fitted was fixed at 0V, so there was an uncontrolled path between +12V, LEDx2, Dx1, Dx2, LEDx1, 0V. This took out everything except Dx2
The way these LEDs are fitted in the panel is a method I've used without any problems countless times - until now. They are simply pushed into a hole exactly the right size, with a dab of glue. A close examination revealed that this LED was slightly malformed, and the positive leg was exposed, so was touching the panel. This didn't show up when testing the panel by itself as it wasn't connected to anything.
