Couple questions about a Coffe Maker - No Power

[fzabkar]

Re: Couple questions about a Coffe Maker - No Power

No, it reroutes to "A", the active terminal of the AC supply.

I suspect that JP2-3 provides mains pulses for a clock.

BTW, when you "grounded" the base of transistor Q1, which ground did you use, and was the controller PCB still connected to the PSU when you did this?

[caphair]

Re: Couple questions about a Coffe Maker - No Power

Yes it was and I grounded the red wire to the white ground

I'm a bit stumped why its configured to reroute to A, excuse my brain deadness lol

[fzabkar]

Re: Couple questions about a Coffe Maker - No Power

Your grounding was probably OK. I was concerned that you may have used the AGND, in which case you would have driven the MCU's output pin about 20V below DGND.

The rerouting is not intentional. It just happens that way when the resistor goes open. The "horizontal" signal diode (D) clamps the mains clock pulse (?) to 0.6V below DGND. It was not intended to do anything else.

[caphair]

Re: Couple questions about a Coffe Maker - No Power

"Clamps the mains clock pulse to 0.6v below DGDN"

I think I'm confused about this function in the circuit

[fzabkar]

Re: Couple questions about a Coffe Maker - No Power

A clamping diode just prevents a signal from going outside the microcontroller's allowable input range. You don't want -300V appearing at the input to a device that can only handle 0V - 5V.

[caphair]

Re: Couple questions about a Coffe Maker - No Power

A clamping diode just prevents a signal from going outside the microcontroller's allowable input range. You don't want -300V appearing at the input to a device that can only handle 0V - 5V.

I think that's why I'm confused, why's it connected to the ac line then using clamping diodes. Can't that part be omitted? Or am I missing something?

[fzabkar]

Re: Couple questions about a Coffe Maker - No Power

The micro needs to detect the mains frequency for some reason. Therefore it needs a safe way to connect to the mains supply. The 220K (?) resistor limits the current while the diodes limit the voltage range.

[caphair]

Re: Couple questions about a Coffe Maker - No Power

Ooh ok gotcha. Thanks a lot. Is this typical for microprocessors? Also I only have a 2k resistor in my scrap boards. Would that work in replacement of the 1k?

[fzabkar]

Re: Couple questions about a Coffe Maker - No Power

Some appliances need to monitor the AC waveform so that they can switch an inductive load at the zero-crossing point, usually via a TRIAC or SCR. Without seeing the rest of your circuit, and without knowing what features/functions your coffee maker has, I cannot say what your controller is doing with these pulses.

As for the resistor, if you choose a larger value, then you limit the supply current to around 10mA rather than 20mA. I don't think this is a good idea. Also be aware that the dissipation in the resistor is ...

P = (28V - 5V) x (28V - 5V) / 1K = 529mW

Therefore I would choose a 1W resistor or better.

[caphair]

Re: Couple questions about a Coffe Maker - No Power

Yeah I noticed that about the 1W resistor. I didn't have one on hand but I did find a 1k 1/4w resistor that I soldered in. And it's been working but I'm thinking that resistor won't last many cycles as its a lower wattage. At least I'll know where to look next time

About the circuit that monitors the AC waveform, if those two diodes clamp the voltage how is it monitoring any voltage at that point?

[caphair]

Re: Couple questions about a Coffe Maker - No Power

I found 3 3k 2W resistors in a junk board. Could I connect them in parellel to get 1k and have the circuit function normally or would that change the current draw?

[fzabkar]

Re: Couple questions about a Coffe Maker - No Power

The 3 parallel 3K resistors should work fine.

As for your conceptual difficulties in regard to voltage clamping, try to visualise what would appear at the Vin input of the MCU as the AC voltage varies from -100V to 0V, from 0V to +2.5V, from +2.5V to +5V, and from +5V to +100V. You should convince yourself that Vin varies from -0.6V to +5V. Moreover, since the MCU's input pin is most likely a digital Schmitt trigger input, then Vin will most likely register as either a digital high or low. In other words, the sinusoidal AC waveform will be detected as a 5V square wave.

               VDD (+5V)
                ___
                 |
           .----------+
           |     |
           .-.     |
           | |     |
           | |     |
           '-'   .------.
           |   |   |
       .-------+----->|Vin  |
      D |    |   |   |
       V   ---   | MCU |
       -   ---   '------'
    ___  |    |     |
AC o---|___|--+--|<---+----------+
                 |
         D       ===

               DGND (0V)

[caphair]

Re: Couple questions about a Coffe Maker - No Power

Ok I think I'm having trouble understanding the two diodes being connected at the cathodes and producing a voltage. As to me it looks like they're just preventing any voltage in

[fzabkar]

Re: Couple questions about a Coffe Maker - No Power

Ok I think I'm having trouble understanding the two diodes being connected at the cathodes and producing a voltage. As to me it looks like they're just preventing any voltage in

During the positive AC half cycle, both diodes are reverse biased, ie no current flows through them.

               VDD (+5V)
                ___
                 |
           .----------+ +5V
           |     |
           .-.     |
           | |     |
           | |     |
           '-'   .------.
           | +5V |   |
       .-------+----->|Vin  |
      D |    |   |   |
       V   ---   | MCU |
       -   ---   '------'
    ___  |    |     |
AC o---|___|--+--|<---+----------+ 0V
      +100V        |
+100V       D       ===

               DGND (0V)

During the negative AC half cycle, both diodes are now forward biased.

               VDD (+5V)
                ___
                 |
           .----------+ +5V
           |     |
           .-.     |
           | |     |
           | |     |
           '-'   .------.
           |  0V |   |
       .-------+----->|Vin  |
      D |    |   |   |
       V   ---   | MCU |
       -   ---   '------'
    ___  |    |     |
AC o---|___|--+--|<---+----------+ 0V
      -0.6V        |
-100V       D       ===

               DGND (0V)

[caphair]

Re: Couple questions about a Coffe Maker - No Power

Ok but where's the -0.6v from? Is because the remaining voltages in the negative half are going straight to ground and only forward voltage of diode is left thus Vin sees 0v?

[fzabkar]

Re: Couple questions about a Coffe Maker - No Power

Sorry, I don't know how to help you get over your mental block. Perhaps someone else could better explain to you how clamping works.

Just to confuse you further, the following example is a case where Vin falls within the 0V - 5V range. One diode is forward biased while the other is reverse biased.

               VDD (+5V)
                ___
                 |
           .----------+ +5V
           |     |
           .-.     |
        22K | |     |
           | |     |
           '-'   .------.
           | 2.5V |   |
       .-------+----->|Vin  |
      D |    |   |   |
       V   ---   | MCU |
    220K  -   ---   '------'
    ___  |    |     |
AC o---|___|--+--|<---+----------+ 0V
      +1.9V        |
-23.1V      D       ===

               DGND (0V)