Electronics > Repair

Why does not this hair straightener work?

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saaluc:
This is a hair straightener. It does not work, that is, there is no heat on the heating elements. When the main switch is on, there is almost no output voltage (either DC or AC) for the heating elements. AC voltage across the heating element connected to the board No. 1 is 0.075.

In the pictures attached, the board with AC input is board No. 1; the other board is board No. 2.

The resistance of each heating element is 92 ohm.

There is mains voltage (220) at entry. The fuse is OK. On the board No. 2, anode and cathode of the SCR have 220VAC.

I desoldered and checked the “yellow X2 100nF capacitor”, the green VDR (connected in parallel between Line and Neutral), The polyester capacitor on the board No. 1, and the electrolytic capacitor on the board No. All of them are OK.

The supply voltage of MCU (the voltage across the ceramic capacitor C5), is 4.85 VDC.

I removed the SCR from the board and tested it according the second method in https://www.electricalvolt.com/how-to-test-scr-using-multimeter; It was OK according to that test.

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).

Why the device does not work?

inse:
Did you verify the connection of SCR cathode to uC GND?
And do those solder joints look crappy?

saaluc:

--- Quote from: inse on December 26, 2024, 07:10:36 am ---Did you verify the connection of SCR cathode to uC GND?
And do those solder joints look crappy?

--- End quote ---

1. Where is the "uC" GND? Can you mark it on the picture?

2. Do you mean the solder joints of SCR? If yes, they are strong and well-connected (but not tidy).

timeandfrequency:
Hello saaluc,

Again in the hairdressing appliance repair ?


--- Quote from: saaluc on December 26, 2024, 09:10:04 am ---
--- Quote from: inse on December 26, 2024, 07:10:36 am ---Did you verify the connection of SCR cathode to uC GND?
And do those solder joints look crappy?

--- End quote ---

1. Where is the "uC" GND? Can you mark it on the picture?

--- End quote ---
Probably the same as the '-' pin of the black electrolytic cap. Tell us it's value and voltage to confirm.

I see another interesting part to check : the one inside the white (fiberglass) braid.
Fuse resistor ? Thermal fuse ?


saaluc:

--- Quote from: timeandfrequency on December 26, 2024, 09:32:37 am ---Hello saaluc,

Again in the hairdressing appliance repair ?


--- Quote from: saaluc on December 26, 2024, 09:10:04 am ---
--- Quote from: inse on December 26, 2024, 07:10:36 am ---Did you verify the connection of SCR cathode to uC GND?
And do those solder joints look crappy?

--- End quote ---

1. Where is the "uC" GND? Can you mark it on the picture?

--- End quote ---
Probably the same as the '-' pin of the black electrolytic cap. Tell us it's value and voltage to confirm.

I see another interesting part to check : the one inside the white (fiberglass) braid.
Fuse resistor ? Thermal fuse ?

--- End quote ---

Hi timeandfrequency. Welcome.
Honestly, it is an absolute accident that this topic too is about a device related to what you said.

The black electrolytic cap is 470uF 10V.   The connection of SCR cathode to uC GND should be verified by buzzer of the DMM?

The part inside the white (fiberglass) braid is a resistor about 46 ohms.

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