Couple questions about a Coffe Maker - No Power

[caphair]

Hi guys I have this digital coffee maker that uses a em78p163n microcontroller.

Nothing happens when on button pressed, traced it to 1v on the base of the transistor that switches the relay and powering the heating element.

Noticed if I ground this pin the heating element turns on.

What I'm wondering is, does the microcontroller ground this pin enabling it on as I'm simulating manually?

Another thing there's a power on indicator led in series with this pin that has 1.52v on its anode. It powers on with a multimeter on diode scale but if I try grounding the anode nothing happens. Maybe it needs more than 1.52v to turn on which makes me question if the microcontroller isn't outputting enough voltage? Am I wrong to assume that?

If anyone can help me better understand exactly what's happening (during normal operation) when power switch is pressed I'd greatly appreciate it. Thanks!

[budm]

Re: Couple questions about a Coffe Maker - No Power

So the transistor that drive the relay has 1V feeding the Base of the transistor, so you do have 0.6~0.7VDC between the Base and Emitter legs of the transistor? How is the relay coil connected to the Transistor and the Voltage source (VCC) for the relay coil?
Pictures of the board and transistor/relay section?

[caphair]

Re: Couple questions about a Coffe Maker - No Power

I'll get some pics when I can. I was looking at the Datasheet for the microprocessor and it says it needs an operating voltage of 2.1-5.5v but I only measure 1.52v at the VDD pin.

[fzabkar]

Re: Couple questions about a Coffe Maker - No Power

It definitely sounds like a problem with the supply. These appliances often derive a transformerless supply directly from the AC input via an 0.47uF 600V film capacitor or similar. Is there a zener diode at the VDD pin?

What is the voltage spec for the relay?

[caphair]

Re: Couple questions about a Coffe Maker - No Power

Here's a pic of the power section. Unfortunately don't have access to a computer and my phone only uploads one pic at a time so excuse my posts just for pics

[caphair]

Re: Couple questions about a Coffe Maker - No Power

The top side

[stj]

Re: Couple questions about a Coffe Maker - No Power

resolder that 4pin connector.

[momaka]

Re: Couple questions about a Coffe Maker - No Power

resolder that 4pin connector.

Not just that. Look at the solder joints for the two zener diodes (ZD1 and ZD2) too - they look quite bad. And that's not surprising, because you can see they darkened the board a little. It's possible that the film cap could have gone leaky, letting more current in that necessary. Therefore, I would suggest to check all diodes for short-circuit and check all resistors for open-circuit. Those (probably no-name) electrolytic caps could be letting too much ripple too, if they have gone high-ESR or O/C.

[caphair]

Re: Couple questions about a Coffe Maker - No Power

Yes those two zener diodes did darken the board around them. They tested fine in circuit, but one of the mains diodes outputs the low voltage 1.52v leading to the microcontroller VDD. I'm guessing this board uses zeners as voltage regulators?

[fzabkar]

Re: Couple questions about a Coffe Maker - No Power

ZD2 regulates the supply for the microcontroller. I believe you should measure 5V across it.

ZD1 is the 24V supply for the relay.

I will upload a circuit diagram if you can first measure the resistors.

[caphair]

Re: Couple questions about a Coffe Maker - No Power

All of the resistors? Btw I measure 28v at ZD1 and 1.52 at ZD2

[stj]

Re: Couple questions about a Coffe Maker - No Power

desolder zd2 and see what's printed on it.

[caphair]

Re: Couple questions about a Coffe Maker - No Power

Actually the resistor above ZD2 with color band of 1k measures open in circuit

[fzabkar]

Re: Couple questions about a Coffe Maker - No Power

That 1K resistor must be the culprit.

                        .-------+------> JP2-3
                       D |    |    brown
                        V   ---
                        -   ---
                     ___  |    |
    .---------------------------------|___|--+--|<---+-----.
    |                           |
    |                      D     |
    |  film                        |
    | capacitor      bridge              |
    |  ||    ___  .-----.  C2  ZD1    ZD2  |  black
 A o--+-+--||---+---|___|---|~  +|------+---+--+--+---+--+--)-> JP2-1
    | | ||  |    .--|~  -|--.  |+ | | |+ | | |  VDD
    | | ___ |    | '-----' | === z | === z --- |
    | '-|___|-'    |  D x 4  | --- A | --- A --- |
    |         |      |  |  | | |  | | |
    |         |      |  |  | | '---+--+--+-> JP2-2
    |         |      |  |  | |   |     white
 N o--)------------------+      |  |  | |   .-.    DGND
    |         |      |  |  | |   | |
    |         |      |  |  | |   | | 1K
    |         |      |  |  | |   '-'
    |         |      |  |  | |   |
    |         .-.   .----+---+---+--)------'
    |     Heating | |   |  |    | |
    |     Element | |   |  |   === |
    |         '-'   |  |   AGND |
    |         |   |  |     |
    |         |   |  |     |       relay
    |  12A 250VAC  o  __|  | D5    |       control
    |        \  ||(    V      >|  ___
    |     Relay \. ||(    -    Q1  |--|___|---> JP2-4
    |         o ||(__   |  2SA733 /|      red
    |  24V 15mA coil |   |  |     |
    |         |   '----+----------'
    '------------------'

https://cdn.badcaps-static.com/pdfs/...ba95df4f6d.pdf

[caphair]

Re: Couple questions about a Coffe Maker - No Power

Thanks for that fzabkar! I'm curious how that resistor being open causes low voltage? Bc it doesn't complete the zener's path to ground?

[fzabkar]

Re: Couple questions about a Coffe Maker - No Power

I'm curious how that resistor being open causes low voltage? Bc it doesn't complete the zener's path to ground?

There is an alternative path back to the active terminal.

BTW, I would replace ZD1. ZD2 normally carries around 20mA, so when the 1K resistor went O/C, ZD1 would have been called on to carry this additional load. At 28V, that's an additional 0.5W. Normally ZD1 would only be carrying the relay coil current (when the relay is off) plus a little extra.

[caphair]

Re: Couple questions about a Coffe Maker - No Power

Can you explain what you mean by there is an alternative path? Just trying to understand exactly how that resistor caused the issue, was it helping limit voltage?

[fzabkar]

Re: Couple questions about a Coffe Maker - No Power

From JP2-2 go upwards and left. There is a diode and resistor going back toward A.

ZD1 preregulates the supply to 28V (analogue). Then the digital supply is derived from the analogue supply by a traditional resistor/zener combination. The resistor limits the current to about 20mA before ZD2 begins to lose regulation.

[stj]

Re: Couple questions about a Coffe Maker - No Power

if you replace the zeners or that resistor, put them a few mm above the board to improve cooling

[caphair]

Re: Couple questions about a Coffe Maker - No Power

From JP2-2 go upwards and left. There is a diode and resistor going back toward A.

ZD1 preregulates the supply to 28V (analogue). Then the digital supply is derived from the analogue supply by a traditional resistor/zener combination. The resistor limits the current to about 20mA before ZD2 begins to lose regulation.

So with that resistor open it reroutes to JP2-3? What's this line for?