Can someone explain how the master interface works?
Posted: Wed Dec 14, 2016 3:51 pm
Hi, I've been pondering the design here: http://otgw.tclcode.com/schematic.html#schematic
Just out of curiosity I'm trying to understand how exactly it works. In particular the master interface (the part communicating with the boiler) isn't entirely clear to me.
What I think I understand is the following: the gateway communicates to the boiler by setting voltage. This is done by zeners D10 (or D11 if optocoupler OK1B is active).
There is some additional voltage over R1 and Q1 but that (for reasons unknown to me) is guaranteed to be low enough for the signal to not exceed the 7V and 18V maximum voltages for transmitting a 0/1 respectively.
One of the things that confuse me is the presence of a resistor (R1) between the emitter and base of Q1. The PN junction should work as a diode and no current should ever flow though R1. At least that is how far my theoretical knowledge goes.
Another point of confusion is diode D9. I think it is there to protect the input of the optocoupler from over-voltage conditions. (i.e. it is clamped to 4.7V at most and the datasheet specifies 5V as the max so this is safe).
What I don't get is why D9 does not also clamp the output signal. Some of the time Q1 will be conducting between it's emitter and collector and the output will be clamped to 4.7V as well?
Finally I'd like to know how R1/R2/R3 have been determined.
Can someone explain if my understandings so far are correct and how Q1/R1/R2/R3/D9 fit into this story?
Many Thanks,
Just out of curiosity I'm trying to understand how exactly it works. In particular the master interface (the part communicating with the boiler) isn't entirely clear to me.
What I think I understand is the following: the gateway communicates to the boiler by setting voltage. This is done by zeners D10 (or D11 if optocoupler OK1B is active).
There is some additional voltage over R1 and Q1 but that (for reasons unknown to me) is guaranteed to be low enough for the signal to not exceed the 7V and 18V maximum voltages for transmitting a 0/1 respectively.
One of the things that confuse me is the presence of a resistor (R1) between the emitter and base of Q1. The PN junction should work as a diode and no current should ever flow though R1. At least that is how far my theoretical knowledge goes.
Another point of confusion is diode D9. I think it is there to protect the input of the optocoupler from over-voltage conditions. (i.e. it is clamped to 4.7V at most and the datasheet specifies 5V as the max so this is safe).
What I don't get is why D9 does not also clamp the output signal. Some of the time Q1 will be conducting between it's emitter and collector and the output will be clamped to 4.7V as well?
Finally I'd like to know how R1/R2/R3 have been determined.
Can someone explain if my understandings so far are correct and how Q1/R1/R2/R3/D9 fit into this story?
Many Thanks,