Author Topic: Why does not this hair straightener work?  (Read 1849 times)

0 Members and 1 Guest are viewing this topic.

Offline saalucTopic starter

  • Contributor
  • Posts: 34
  • Country: ie
Why does not this hair straightener work?
« on: December 26, 2024, 06:16:56 am »
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?
« Last Edit: December 26, 2024, 02:41:36 pm by saaluc »
 

Online inse

  • Super Contributor
  • ***
  • Posts: 1029
  • Country: de
Re: Why this hair straightener does not work?
« Reply #1 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?
 
The following users thanked this post: saaluc

Offline saalucTopic starter

  • Contributor
  • Posts: 34
  • Country: ie
Re: Why this hair straightener does not work?
« Reply #2 on: December 26, 2024, 09:10:04 am »
Did you verify the connection of SCR cathode to uC GND?
And do those solder joints look crappy?

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).
 

Offline timeandfrequency

  • Regular Contributor
  • *
  • Posts: 246
  • Country: fr
Re: Why this hair straightener does not work?
« Reply #3 on: December 26, 2024, 09:32:37 am »
Hello saaluc,

Again in the hairdressing appliance repair ?

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

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


« Last Edit: December 26, 2024, 09:44:00 am by timeandfrequency »
 
The following users thanked this post: saaluc

Offline saalucTopic starter

  • Contributor
  • Posts: 34
  • Country: ie
Re: Why this hair straightener does not work?
« Reply #4 on: December 26, 2024, 10:01:15 am »
Hello saaluc,

Again in the hairdressing appliance repair ?

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

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

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.
 

Offline Kurets

  • Contributor
  • Posts: 43
  • Country: se
Re: Why this hair straightener does not work?
« Reply #5 on: December 26, 2024, 10:36:57 am »
This does not seem like a device which should be repaired. However, if you see no voltage to the elements I would suspect blown thermal fuse, this is likely connected to the two yellow wires labelled TF1.

But, messing around in mains connected devices while not being sure how the circuit works does npt seem wise. And hair straighteners are not high value items worth repairing.
 
The following users thanked this post: saaluc

Offline saalucTopic starter

  • Contributor
  • Posts: 34
  • Country: ie
Re: Why this hair straightener does not work?
« Reply #6 on: December 26, 2024, 11:09:02 am »
This does not seem like a device which should be repaired. However, if you see no voltage to the elements I would suspect blown thermal fuse, this is likely connected to the two yellow wires labelled TF1.

But, messing around in mains connected devices while not being sure how the circuit works does npt seem wise. And hair straighteners are not high value items worth repairing.

The TF1 is OK in buzzer test.
Thank you for your advice.
Where I am, unfortunately, this device is expensive and worth repairing.
« Last Edit: December 26, 2024, 11:23:36 am by saaluc »
 

Offline timeandfrequency

  • Regular Contributor
  • *
  • Posts: 246
  • Country: fr
Re: Why this hair straightener does not work?
« Reply #7 on: December 26, 2024, 11:31:13 am »
Plug the appliance to the mains and with care, using a multimeter, mesure the voltage on the ZD1 zener diode.

At first, mesure DC voltage(*) and you should find  - I would say - between 3 and 8 VDC
Then mesure AC voltage and you should see less than 100 mV AC
Report the results.

Safety procedure to follow because of mains voltage and open device

Once unplugged from the mains, mesure the DC voltage on the big capacitors (yellow, brown) : when the voltage becomes below 20 VDC, you're safe and can touch the board.


(*) An of course, the Zener diode is polarized reversed, so :
+ (red) probe goes to the cathode (black ring on the part)
- (black) probe goes to the anode (the other side of the part)


Tell us the value and voltage of the second black electrolytic capacitor (the one close to the light blue switch, or whatever this blue part is).
« Last Edit: December 26, 2024, 11:34:47 am by timeandfrequency »
 
The following users thanked this post: saaluc

Offline saalucTopic starter

  • Contributor
  • Posts: 34
  • Country: ie
Re: Why this hair straightener does not work?
« Reply #8 on: December 26, 2024, 02:19:54 pm »
On ZD1, the DC voltage is 4.852, and the AC voltage is 0.073 V (73 mV).

The second black electrolytic capacitor (beside the blue multiturn) is a 220uF 10V capacitor. The voltage on it, while the device is plugged, is 4.852 VDC.
« Last Edit: December 26, 2024, 02:26:44 pm by saaluc »
 

Offline timeandfrequency

  • Regular Contributor
  • *
  • Posts: 246
  • Country: fr
Re: Why does not this hair straightener work?
« Reply #9 on: December 26, 2024, 03:10:44 pm »
All this sounds rather good.
Can you now locate the GND pin of the µC ?
And as @inse suggested earlier, can you confirm that the cathode of the SCR is also connected to this GND ?

 

Offline saalucTopic starter

  • Contributor
  • Posts: 34
  • Country: ie
Re: Why does not this hair straightener work?
« Reply #10 on: December 26, 2024, 03:39:38 pm »
All this sounds rather good.
Can you now locate the GND pin of the µC ?
And as @inse suggested earlier, can you confirm that the cathode of the SCR is also connected to this GND ?

By "µC", do you mean "microcontroller"?
 

Offline timeandfrequency

  • Regular Contributor
  • *
  • Posts: 246
  • Country: fr
Re: Why does not this hair straightener work?
« Reply #11 on: December 26, 2024, 03:47:13 pm »
By "µC", do you mean "microcontroller"?
yes
 
The following users thanked this post: saaluc

Offline saalucTopic starter

  • Contributor
  • Posts: 34
  • Country: ie
Re: Why does not this hair straightener work?
« Reply #12 on: December 26, 2024, 03:57:13 pm »
By "µC", do you mean "microcontroller"?
yes

Is the GND pin of the microcontroller, the pin that is connected to the negative pin of the black electrolytic capacitor beside the blue multiturn?
 

Online inse

  • Super Contributor
  • ***
  • Posts: 1029
  • Country: de
Re: Why does not this hair straightener work?
« Reply #13 on: December 26, 2024, 04:02:18 pm »
Power supply GND needs to be connected to the SCR and the micro controller.
Just do a continuity check from the electrolytic capacitor.
Does any of the LEDs light up?
 
The following users thanked this post: saaluc

Offline saalucTopic starter

  • Contributor
  • Posts: 34
  • Country: ie
Re: Why does not this hair straightener work?
« Reply #14 on: December 26, 2024, 04:31:30 pm »
Power supply GND needs to be connected to the SCR and the micro controller.
Just do a continuity check from the electrolytic capacitor.
Does any of the LEDs light up?

You said "Just do a continuity check from the electrolytic capacitor."
1. When I put one probe of the multimeter on the negative pin of the electrolytic capacitor, and the other probe on the cathode of SCR, there is no continuity beep but 91.4 ohm.
2. Is it not also necessary to locate the microcontroller GND (the datasheet cannot be found on the internet) and check its continuity to the cathode of SCR? If it is, can you guide me to locate it?

You asked "Does any of the LEDs light up?"
3. When I received this device, pushing the buttons could light up all of the LEDs BUT the device did not work (elements were cold).
After I disconnected the yellow X2 capacitor, one pin of the VDR, one pin of the polyester capacitor, and one pin of the black capacitor, to check them out of the circuit, and then reconnected them and plugged the device, the LEDs did not light up any more.
But when I did voltage test on the push-buttons, the LEDs lighted up again. Probably they will light up again if I do voltage test on the push-buttons.

If you need more pictures, please tell me.
« Last Edit: December 27, 2024, 08:25:46 am by saaluc »
 

Online inse

  • Super Contributor
  • ***
  • Posts: 1029
  • Country: de
Re: Why does not this hair straightener work?
« Reply #15 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.
« Last Edit: December 27, 2024, 09:43:26 am by inse »
 
The following users thanked this post: saaluc

Offline timeandfrequency

  • Regular Contributor
  • *
  • Posts: 246
  • Country: fr
Re: Why does not this hair straightener work?
« Reply #16 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 :

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).
... 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
 
The following users thanked this post: saaluc

Offline saalucTopic starter

  • Contributor
  • Posts: 34
  • Country: ie
Re: Why does not this hair straightener work?
« Reply #17 on: December 27, 2024, 11:14:27 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.

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.
« Last Edit: December 27, 2024, 03:03:48 pm by saaluc »
 

Offline saalucTopic starter

  • Contributor
  • Posts: 34
  • Country: ie
Re: Why does not this hair straightener work?
« Reply #18 on: December 27, 2024, 11:27:50 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 :

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).
... 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

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).
 

Online inse

  • Super Contributor
  • ***
  • Posts: 1029
  • Country: de
Re: Why does not this hair straightener work?
« Reply #19 on: December 27, 2024, 03:08:54 pm »
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.
 
The following users thanked this post: saaluc

Offline saalucTopic starter

  • Contributor
  • Posts: 34
  • Country: ie
Re: Why does not this hair straightener work?
« Reply #20 on: December 27, 2024, 03:35:38 pm »
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.

To solve this problem, how can I help? Is it helpful to mark the paths between microcontroller and gate on the picture and upload it for you?
« Last Edit: December 27, 2024, 03:53:57 pm by saaluc »
 

Online inse

  • Super Contributor
  • ***
  • Posts: 1029
  • Country: de
Re: Why does not this hair straightener work?
« Reply #21 on: December 27, 2024, 03:58:50 pm »
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?
 
The following users thanked this post: saaluc

Offline timeandfrequency

  • Regular Contributor
  • *
  • Posts: 246
  • Country: fr
Re: Why does not this hair straightener work?
« Reply #22 on: December 27, 2024, 04:08:47 pm »
According to the parts we see on the board, and what you explaned, the gate drive should be rather simple : there's no optocoupler, no ZCD, no pulse transformer. The additionnal diodes need more investigation.
But clearly, there's  no isolation: the µC is directely connected to live voltage. A very common principle used in many appliances.


I agree with @inse : we do not understand how you can have 4.8 VDC when you say :   pin No. 7 of the MCU is connected (through a resistor and a diode) to the Gate of the SCR;
and at the same time you have no voltage at the gate, because you wrote :
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).

Between MCU and SCR, there must be a direct path and a return path : the curent flows into the gate only if there's a circuit/loop : if there's no circuit, no SCR firing can occur.

Also check the diode and the resistor in the gate path (coming from the MCU).


Yes the TNY612 is an SCR (also named 'Thyristor') and it's legs are named K, A & G.
It's a standard part and to be fired, it needs VGT = VGK  > 1,3 VDC  and at least 2 mA of current injected.
« Last Edit: December 27, 2024, 04:19:14 pm by timeandfrequency »
 
The following users thanked this post: saaluc

Offline saalucTopic starter

  • Contributor
  • Posts: 34
  • Country: ie
Re: Why does not this hair straightener work?
« Reply #23 on: December 27, 2024, 04:26:58 pm »
The diode and the resistor in the gate path (coming from the MCU), are OK.
 

Offline saalucTopic starter

  • Contributor
  • Posts: 34
  • Country: ie
Re: Why does not this hair straightener work?
« Reply #24 on: December 27, 2024, 04:36:09 pm »
According to the parts we see on the board, and what you explaned, the gate drive should be rather simple : there's no optocoupler, no ZCD, no pulse transformer. The additionnal diodes need more investigation.
But clearly, there's  no isolation: the µC is directely connected to live voltage. A very common principle used in many appliances.


I agree with @inse : we do not understand how you can have 4.8 VDC when you say :   pin No. 7 of the MCU is connected (through a resistor and a diode) to the Gate of the SCR;
and at the same time you have no voltage at the gate, because you wrote :
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).

Between MCU and SCR, there must be a direct path and a return path : the curent flows into the gate only if there's a circuit/loop : if there's no circuit, no SCR firing can occur.

Also check the diode and the resistor in the gate path (coming from the MCU).


Yes the TNY612 is an SCR (also named 'Thyristor') and it's legs are named K, A & G.
It's a standard part and to be fired, it needs VGT = VGK  > 1,3 VDC  and at least 2 mA of current injected.

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?
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf