Sewing machine power board  - Page 1
Forum section: electronics / repair

author: Robert A
author_rank: Contributor
timestamp: October 01, 2024, 03:31:33 pm
content: Hi hive mind. I’m trying to fix a sewing machine power board and I’ve found a power resistor that’s blown. I have a circuit diagram (IMG_0056) and the resistor in question is R10 2.2ohm 3W. Firstly what is its function? Is it current limiting, voltage limiting or something else? IMG_0056, for illustration purposes only, shows the type of resistor in the circuit but at 47Kohm obviously I can’t use it. Can I use, nearest value wise, this resistor that I have got, in IMG_0058? What would it’s impact be on the circuit if I were to use it?

author: Bryn
author_rank: Regular Contributor
timestamp: October 01, 2024, 03:34:46 pm
content: I don't see the images attached (as your post was supposed to refer to them). Could you try reuploading them please?

author: Robert A
author_rank: Contributor
timestamp: October 01, 2024, 03:36:42 pm
content: Sorry about that, don’t know why it didn’t attach them.

author: Whales
author_rank: Super Contributor
timestamp: October 01, 2024, 03:47:42 pm
content: Is it possible to get the rest of the circuit diagram? It would be good to see what those power rails are powering.

author: Robert A
author_rank: Contributor
timestamp: October 01, 2024, 04:21:27 pm
content: Quote from: Whales on October 01, 2024, 03:47:42 pmHere’s the full circuit diagram

author: Whales
author_rank: Super Contributor
timestamp: October 01, 2024, 04:39:06 pm
content: Thankyou.At the top we have proper isolated power supplies.The bottom section is non-isolated, touching any part may lead to shock. I suspect this bottom power supply is mainly for driving the motor. Its design is a bit confusing.Q1.What is U138 (two big solid marone rectangles)? It might be a transformer (which means T110 can only be driven in pulses).Q2.Is this an original circuit diagram made by the manufacturer (should be perfect), or one put together by reverse engineering the board (might have mistakes)?EDIT: I think it's accurate. Just laid out a bit too strangely for my 2AM brain to comprehend, sorryTransistor T110 directly drives the motor as a high-side switch. The isolated power rail in the middle of the diagram lets it be used this way.This is in turn controlled through transformer U138 by transistor T113. The transformer means you can only turn the motor on in pulses, not continuously. I'm not sure why they went for this design (seems unnecessarily expensive and complicated, maybe its still cheaper than a better motor?), but it works.The only purpose I can think of for your 2.2ohm R10 is a fusible resistor. Maybe it also limits inrush currents to the motor. If R10 is blown then replacing it might only be a temporary fix, there might be a short circuit elsewhere in the bottom half of this diagram, not sure.

author: Whales
author_rank: Super Contributor
timestamp: October 01, 2024, 04:53:31 pm
content: Sidenote: T114 (mosfet at the bottom) is a curious design detail. It shorts out the motor. Which will blow things up if T110 is also turned on at the same time.I wonder if it's an intentional self destruct feature for safety compliance or an interlock? It is wired to connector P161 but the pin is labelled as 'x', so I am not sure if that pin is connected to anything else. Would be worth checking just in case this is why your resistor blew.EDIT: yeah the two 'x' pins would be useful as part of an interlock switch. If someone opens the machine whilst the motor is powered then it could self destruct the circuit to stop the motor as quickly as possible. Could perhaps this have been why it stopped working? Hopefully only R10 is blown.Replacing R10 with an actual fuse might also work, but I can only guess as to the value. I don't know the wattage or type of the 2.2ohm resistor originally used.

author: DeepLink
author_rank: Regular Contributor
timestamp: October 02, 2024, 05:36:16 am
content: R10 is likely as fusible resistor, but also helps with inrush current limitation at power upFusible resistors (https://www.elprocus.com/fusible-resistor/) are available, sometimes called flameproof resistor or safety resistorhttps://www.digikey.dk/da/products/detail/vishay-beyschlag-draloric-bc-components/PR03000202208JAC00/7350732The circuit is quite straight forwardA DC voltage is present at C52-C54T110 is the switch in a stepdown switch-mode converterCurrent pass through L128 (and motor)D86 is the current path when T110 is offT114 is a stop function (brake) for the motor (yes - not intended to be in use when T110 is on)You can replace R10 and remove D85 for testing (if T114 is shorted or circuit x-x defective, it will burn R10 again)Remember that the circuit around the motor is not isolated from mains and high voltage are presentMonitor the DC voltage at C52-C54 after power off - don't touch the circuit before R11 has discharged the bulk capacitors

author: Robert A
author_rank: Contributor
timestamp: October 02, 2024, 01:11:06 pm
content: Quote from: Whales on October 01, 2024, 04:53:31 pmTo answer your questions:-1) U138 is a transformer2) not an original circuit diagram3) C50 was a Rifa capacitor which had blown, might explain R10 blowing too4) connector P161 is for a daughter board, pin 8 goes to a LM324 quad Obama and pin 9 goes to a LM339 quad diff comparator.

author: Robert A
author_rank: Contributor
timestamp: October 02, 2024, 01:35:09 pm
content: Quote from: DeepLink on October 02, 2024, 05:36:16 amThanks for the info. C50 was a Rifa capacitor which had blown, I guess took R10 with it and hopefully nothing else. I’ve removed D85 and T114 tests good, so looks like we’re ok.

author: DeepLink
author_rank: Regular Contributor
timestamp: October 02, 2024, 03:40:56 pm
content: Quote from: Robert A on October 02, 2024, 01:35:09 pmFailure in C50 should not blow R10

author: coromonadalix
author_rank: Super Contributor
timestamp: October 02, 2024, 03:47:24 pm
content: the motor rotor should be visually checked ...

author: Whales
author_rank: Super Contributor
timestamp: October 02, 2024, 11:13:56 pm
content: Quote from: DeepLink on October 02, 2024, 03:40:56 pmAgreed.There's a chance C50 could also be a symptom of the problem that blew R10 (mains overvoltage?), but I'm not sure. Was the RIFA one of the clear types, or a more modern RIFA?Quote from: coromonadalix on October 02, 2024, 03:47:24 pmA shorted motor could have caused the resistor to blow. Opening it for visual inspection would be really good, but some motors make that difficult. Youmightalso be able to try connecting it to a bench power supply and seeing if it makes strange noises or gets really hot when running with no mechanical load. But that might not be a completely conclusive test.It would be worth running the repaired circuit with the motor unplugged, to make sure no other parts are bad. Then turn it off, plug the motor back in, turn it on again and see what happens.(Again I'll hazard that all parts of the motor driving circuitry are at mains referenced potential, dangerous to touch, unplug the whole machine from the wall before doing any of this).

author: Robert A
author_rank: Contributor
timestamp: October 03, 2024, 09:19:18 pm
content: Quote from: Whales on October 02, 2024, 11:13:56 pmIt was a clear Rifa, spilled it’s guts all over the placeI did do a quick resistance test across the poles of the motor which didn’t show any zero or very low ohms readings. Good idea using a separate power supply to test the motor.