Why does not this hair straightener work?

[saaluc]

Get a continuity tester and draw the schematic of the SCR drive, there seem to be more components involved.
For example what are the diodes around the SCR for?

1. I have a continuity tester but am not able to draw the schematic. Can you guide me by asking a series of specific questions?

2. I do not know what are the diodes around the SCR for, but I can explain what is before and after them, etc.

3. Can you set aside or ignore those two test that I have done (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)) and independently guide me diagnose the fault(s) of this device?

[saaluc]

_And the cathode-GND path?_

I will mark it on the picture for you.

[timeandfrequency]

OK, let's keep these early measurements aside.
But after that, we do not own any magic wand or sorcery trick to repair a defective board. Unfortunately.
If a simple solution existed, I would have already shared it here.


There are just parts and those are connected by traces that provide electrical connexions.
If no obvious burnt part can be seen, no defective parts can be measured, no wire disconnected, no weak solder joint, the path to success is much steeper : it's all about understanding how the circuit works.
And this means reverse enigineering it.
But we don't have the schematic, so you need to draw it.

Other members asking for help on any electronic repair forum have to do exactly the same work : you can check some examples here and here.

You may draw the schematic by hand or use a CAD tool :
https://easyeda.com/en
https://www.digikey.com/en/schemeit/project
https://www.circuit-diagram.org/

[saaluc]

_And the cathode-GND path?_

Please download the attached picture and open it 100% size.
Pay attention to the light blue circle, red line, dark blue line, purple rectangle.

1. The light blue circle marks the "via" which is connected to negative pin of the black capacitor. I think there can be no path from cathode to this "via" because of the diode next to the cathode.
2. The red line is from pin 6 of microcontroller to cathode.
3. The purple rectangle marks the resistor and the capacitor connecting cathode and gate.
4. The blue line marks the path from cathode to the heating element on the other board and finally to ACN (AC Neutral).

I hope these informations help you.

Will it help to mark also the path which goes through the heating element of board No. 2?

[saaluc]

OK, let's keep these early measurements aside.
But after that, we do not own any magic wand or sorcery trick to repair a defective board. Unfortunately.
If a simple solution existed, I would have already shared it here.


There are just parts and those are connected by traces that provide electrical connexions.
If no obvious burnt part can be seen, no defective parts can be measured, no wire disconnected, no weak solder joint, the path to success is much steeper : it's all about understanding how the circuit works.
And this means reverse enigineering it.
But we don't have the schematic, so you need to draw it.

Other members asking for help on any electronic repair forum have to do exactly the same work : you can check some examples here and here.

You may draw the schematic by hand or use a CAD tool :
https://easyeda.com/en
https://www.digikey.com/en/schemeit/project
https://www.circuit-diagram.org/

What I really mean by asking "Can you set aside or ignore those two test that I have done and independently guide me diagnose the fault(s) of this device?", is:
Are my tests and checks necessary and sufficient?
If I had done no tests and checks, what tests and checks would you recommend? would you recommend the same tests and checks that I did?

[inse]

Are the LEDs back on again?
Are the wire connections between the two boards OK?
Is there any noticeable change in Gate-Cathode voltage of the SCR if the heater is switched on?

[saaluc]

...

Are the LEDs back on again?
Are the wire connections between the two boards OK?
Is there any noticeable change in Gate-Cathode voltage of the SCR if the heater is switched on?

The wire connections between the two boards are OK.

Once again I did continuity test on the On/Off switch. It failed, that is, while pushing the button there was no continuity beep. (The other two switches, Up switch and Down switch, passed the test.)
I also did voltage test on the On/Off switch. It failed, that is,  on its input side there was 4.85 VDC but while pushing the button, on its output side there was only 0.7 VDC. (The other two switches, Up switch and Down switch, while pushing the button had 4.85 VDC  on both sides.)

Other than replacing the On/Off switch or using a piece of wire to connect the input and output of the switch, what else can be done to the switch for testing the circuit?

[timeandfrequency]

Hi saaluc,

If the on/off switch is defective, replacement is the best solution.
Using a wire does not correspond to the original behavior: it is a momentary switch. A wire would close the circuit permanently.
And holding the wire by hand to imitate the switch is not safe at all : the whole board is connected to the mains, so handling it is a bad choice.
Check if this one matches with the defective part. The height of the button has the same importance than the other dimensions.

[saaluc]

Hi saaluc,

If the on/off switch is defective, replacement is the best solution.
Using a wire does not correspond to the original behavior: it is a momentary switch. A wire would close the circuit permanently.
And holding the wire by hand to imitate the switch is not safe at all : the whole board is connected to the mains, so handling it is a bad choice.
Check if this one matches with the defective part. The height of the button has the same importance than the other dimensions.

What I meant by "using a piece of wire to connect the input and output of the switch", was connecting the input and output momentarily.

These switches operate with 4.85 VDC. Are they too have 220VAC? If yes, how? If no, how are they dangerous?

[inse]

Please, Saaluc for the safety of yours and anyone using the hair straightener, throw it away.

[timeandfrequency]

Hi saaluc,

If the on/off switch is defective, replacement is the best solution.
Using a wire does not correspond to the original behavior: it is a momentary switch. A wire would close the circuit permanently.
And holding the wire by hand to imitate the switch is not safe at all : the whole board is connected to the mains, so handling it is a bad choice.
Check if this one matches with the defective part. The height of the button has the same importance than the other dimensions.

What I meant by "using a piece of wire to connect the input and output of the switch", was connecting the input and output momentarily.

These switches operate with 4.85 VDC. Are they too have 220VAC? If yes, how? If no, how are they dangerous?

By reading your answer, I see that, unfortunately, you don't understand how this kind of circuit works and have no knowlegde about the risks that are associated by cobbling it.
So I can only agree with @inse's statement and give you the same piece of advice :

Please, Saaluc for the safety of yours and anyone using the hair straightener, throw it away.

No additional infomation on how to repair this appliance will be provided.

[saaluc]

Hi saaluc,

If the on/off switch is defective, replacement is the best solution.
Using a wire does not correspond to the original behavior: it is a momentary switch. A wire would close the circuit permanently.
And holding the wire by hand to imitate the switch is not safe at all : the whole board is connected to the mains, so handling it is a bad choice.
Check if this one matches with the defective part. The height of the button has the same importance than the other dimensions.

What I meant by "using a piece of wire to connect the input and output of the switch", was connecting the input and output momentarily.

These switches operate with 4.85 VDC. Are they too have 220VAC? If yes, how? If no, how are they dangerous?

By reading your answer, I see that, unfortunately, you don't understand how this kind of circuit works and have no knowlegde about the risks that are associated by cobbling it.
So I can only agree with @inse's statement and give you the same piece of advice :

Please, Saaluc for the safety of yours and anyone using the hair straightener, throw it away.

No additional infomation on how to repair this appliance will be provided.

Every one, including me and you, works on circuits at his own risk and responsibility.
In my repairs I never touch any circuit even 5 V dc ones, because I am completely aware that electricity is dangerous.
So you, instead of stopping discussion, can safely continue discussion. Of course, I do not want to bother or disturb you.

[saaluc]

I invite other users to participate in this thread. Their insights are welcome and appreciated.