Why does not this hair straightener work? - Page 1
Forum section: electronics / repair

author: saaluc
author_rank: Contributor
timestamp: December 26, 2024, 06:16:56 am
content: 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 inhttps://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?

author: inse
author_rank: Super Contributor
timestamp: December 26, 2024, 07:10:36 am
content: Did you verify the connection of SCR cathode to uC GND?And do those solder joints look crappy?

author: saaluc
author_rank: Contributor
timestamp: December 26, 2024, 09:10:04 am
content: Quote from: inse on December 26, 2024, 07:10:36 am1. 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).

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: December 26, 2024, 09:32:37 am
content: Hello saaluc,Again in thehairdressing appliance repair?Quote from: saaluc on December 26, 2024, 09:10:04 amProbably 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 ?

author: saaluc
author_rank: Contributor
timestamp: December 26, 2024, 10:01:15 am
content: Quote from: timeandfrequency on December 26, 2024, 09:32:37 amHi 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.

author: Kurets
author_rank: Regular Contributor
timestamp: December 26, 2024, 10:36:57 am
content: 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.

author: saaluc
author_rank: Contributor
timestamp: December 26, 2024, 11:09:02 am
content: Quote from: Kurets on December 26, 2024, 10:36:57 amThe TF1 is OK in buzzer test.Thank you for your advice.Where I am, unfortunately, this device is expensive and worth repairing.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: December 26, 2024, 11:31:13 am
content: 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 VDCThen mesure AC voltage and you should see less than 100 mV ACReport the results.Safety procedure to follow because of mains voltage and open deviceOnce 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).

author: saaluc
author_rank: Contributor
timestamp: December 26, 2024, 02:19:54 pm
content: 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.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: December 26, 2024, 03:10:44 pm
content: 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 ?

author: saaluc
author_rank: Contributor
timestamp: December 26, 2024, 03:39:38 pm
content: Quote from: timeandfrequency on December 26, 2024, 03:10:44 pmBy "µC", do you mean "microcontroller"?

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: December 26, 2024, 03:47:13 pm
content: Quote from: saaluc on December 26, 2024, 03:39:38 pmyes

author: saaluc
author_rank: Contributor
timestamp: December 26, 2024, 03:57:13 pm
content: Quote from: timeandfrequency on December 26, 2024, 03:47:13 pmIs the GND pin of the microcontroller, the pin that is connected to the negative pin of the black electrolytic capacitor beside the blue multiturn?

author: inse
author_rank: Super Contributor
timestamp: December 26, 2024, 04:02:18 pm
content: 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?

author: saaluc
author_rank: Contributor
timestamp: December 26, 2024, 04:31:30 pm
content: Quote from: inse on December 26, 2024, 04:02:18 pmYou 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 LEDsBUTthe 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.

author: inse
author_rank: Super Contributor
timestamp: December 27, 2024, 09:08:24 am
content: 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.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: December 27, 2024, 09:56:49 am
content: 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... 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

author: saaluc
author_rank: Contributor
timestamp: December 27, 2024, 11:14:27 am
content: Quote from: inse on December 27, 2024, 09:08:24 amIt 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.

author: saaluc
author_rank: Contributor
timestamp: December 27, 2024, 11:27:50 am
content: Quote from: timeandfrequency on December 27, 2024, 09:56:49 amYou 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).

author: inse
author_rank: Super Contributor
timestamp: December 27, 2024, 03:08:54 pm
content: 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.

author: saaluc
author_rank: Contributor
timestamp: December 27, 2024, 03:35:38 pm
content: Quote from: inse on December 27, 2024, 03:08:54 pmTo 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?

author: inse
author_rank: Super Contributor
timestamp: December 27, 2024, 03:58:50 pm
content: 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?

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: December 27, 2024, 04:08:47 pm
content: 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 theTNY612is 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.

author: saaluc
author_rank: Contributor
timestamp: December 27, 2024, 04:26:58 pm
content: The diode and the resistor in the gate path (coming from the MCU), are OK.

author: saaluc
author_rank: Contributor
timestamp: December 27, 2024, 04:36:09 pm
content: Quote from: timeandfrequency on December 27, 2024, 04:08:47 pmCan 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?

author: saaluc
author_rank: Contributor
timestamp: December 27, 2024, 04:44:30 pm
content: Quote from: inse on December 27, 2024, 03:58:50 pm1. 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?

author: saaluc
author_rank: Contributor
timestamp: December 27, 2024, 04:47:23 pm
content: Quote from: inse on December 27, 2024, 04:36:21 pmI will mark it on the picture for you.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: December 27, 2024, 05:49:38 pm
content: 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 exampleshereandhere.You may draw the schematic by hand or use a CAD tool :https://easyeda.com/enhttps://www.digikey.com/en/schemeit/projecthttps://www.circuit-diagram.org/

author: saaluc
author_rank: Contributor
timestamp: December 28, 2024, 04:23:09 pm
content: Quote from: inse on December 27, 2024, 03:58:50 pmPlease 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?

author: saaluc
author_rank: Contributor
timestamp: December 29, 2024, 02:58:32 pm
content: Quote from: timeandfrequency on December 27, 2024, 05:49:38 pmWhat 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?

author: inse
author_rank: Super Contributor
timestamp: December 29, 2024, 03:03:12 pm
content: 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?

author: saaluc
author_rank: Contributor
timestamp: January 10, 2025, 09:44:12 am
content: Quote from: timeandfrequency on December 27, 2024, 05:49:38 pmQuote from: inse on December 29, 2024, 03:03:12 pmThe 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?

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: January 10, 2025, 01:37:52 pm
content: 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 ifthis onematches with the defective part. The height of the button has the same importance than the other dimensions.

author: saaluc
author_rank: Contributor
timestamp: January 10, 2025, 03:05:22 pm
content: Quote from: timeandfrequency on January 10, 2025, 01:37:52 pmWhat 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?

author: inse
author_rank: Super Contributor
timestamp: January 10, 2025, 03:09:31 pm
content: Please, Saaluc for the safety of yours and anyone using the hair straightener, throw it away.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: January 10, 2025, 03:26:17 pm
content: Quote from: saaluc on January 10, 2025, 03:05:22 pmBy 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 :Quote from: inse on January 10, 2025, 03:09:31 pmNo additional infomation on how to repair this appliance will be provided.

author: saaluc
author_rank: Contributor
timestamp: January 10, 2025, 04:33:45 pm
content: Quote from: timeandfrequency on January 10, 2025, 03:26:17 pmEvery one, including me and you, works on circuits at his own risk and responsibility.In my repairs Inevertouch any circuit even 5 V dc ones, because I amcompletelyaware 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.

author: saaluc
author_rank: Contributor
timestamp: January 13, 2025, 01:08:04 pm
content: I invite other users to participate in this thread. Their insights are welcome and appreciated.