Electronics > Repair

Why does not this hair straightener work?

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inse:
What still puzzles me is your statement of the voltage levels on the control pin of the micro controller and the SCR/TRIAC.
The connections should in principle look like the schematic taken from a ST app note.
Please verify again on your PCB as in the photo snip.

timeandfrequency:
The schematic provided by @inse is very common, and we would expect to see something similar on this board.
But at the same time you wrote :


--- Quote from: saaluc on December 26, 2024, 06:16:56 am ---The pin No. 7 of the MCU is connected (through a resistor and a diode) to the Gate of the SCR; this pin and the Gate have 4.8 volts, even if I have not pushed the on/off button of the device.

I put the negative probe of the multimeter on the Gate of SCR, and the positive probe on the Cathode of the SCR, there is no voltage (0.000).

--- End quote ---
... which might - at first sight - come from a weak solder joint or cut trace.

Where did you put the black (-) probe when you measured 4.8 V ?

Of course design creativity is not prohibited, and it's not impossible that something significantly different was devised for this appliance. In the affirmative, you will probably need need to redraw the schematic by following the tracks.
 
What we are sure is that firing an SCR requires a voltage at its gate and some current injection (*) is required. If this does not happen the heater is not fed and keeps cold.

(*) source or sink, if it's a TRIAC

saaluc:

--- Quote from: inse on December 27, 2024, 09:08:24 am ---What still puzzles me is your statement of the voltage levels on the control pin of the micro controller and the SCR/TRIAC.
The connections should in principle look like the schematic taken from a ST app note.
Please verify again on your PCB as in the photo snip.

--- End quote ---

It is possible that I have not fully understood your post.

Do you agree to proceed little by little, step by step?

Let me point out that it is a "TYN612"; so it is a SCR (not a TRIAC). According to the datasheet of TYN612, its Gate is what you marked as G; Anode is what you marked as "A2"; Cathode is what you marked as "A1".

Is your main question this: Which side of the C5 capacitor is "GND"?
If yes, let me point out that in your photo snip, the right side of the C5 (close to "RA") is GND because it is connected to negative pin of the electrolytic capacitor.

Exactly what should I verify? Do I show (mark) the GND path between negative pin of the electrolytic capacitor, C5 capacitor, and the cathode?

Exactly what is your question in the photo snip?

If I misunderstood what you were looking for, please let me know.

saaluc:

--- Quote from: timeandfrequency on December 27, 2024, 09:56:49 am ---The schematic provided by @inse is very common, and we would expect to see something similar on this board.
But at the same time you wrote :


--- Quote from: saaluc on December 26, 2024, 06:16:56 am ---The pin No. 7 of the MCU is connected (through a resistor and a diode) to the Gate of the SCR; this pin and the Gate have 4.8 volts, even if I have not pushed the on/off button of the device.

I put the negative probe of the multimeter on the Gate of SCR, and the positive probe on the Cathode of the SCR, there is no voltage (0.000).

--- End quote ---
... which might - at first sight - come from a weak solder joint or cut trace.

Where did you put the black (-) probe when you measured 4.8 V ?

Of course design creativity is not prohibited, and it's not impossible that something significantly different was devised for this appliance. In the affirmative, you will probably need need to redraw the schematic by following the tracks.
 
What we are sure is that firing an SCR requires a voltage at its gate and some current injection (*) is required. If this does not happen the heater is not fed and keeps cold.

(*) source or sink, if it's a TRIAC

--- End quote ---

You asked: "Where did you put the black (-) probe when you measured 4.8 V ?"

In measuring 4.8 volts on pin 7 of microcontroller and the Gate of SCR, I put the black (-) probe on the right side of the C5 capacitor (as I pointed out in the previous post, the right side of C5 capacitor, close to "RA", is GND because it is connected to negative pin of the electrolytic capacitor).

inse:
I think timeandfrequency and me are trying to get an answer to the same question: where is the return path of the gate current to the microcontroller?
If you have 4.8V both at the uC and the gate vs. GND but nothing between gate and cathode, the problem must be in this loop.

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