Advice on flip alarm clock project please. - Page 1
Forum section: electronics / beginners

author: Utah77
author_rank: Newbie
timestamp: November 07, 2022, 05:06:40 pm
content: Hi all, my name is Johnny from the UK. I am a Gas Engineer by trade and my electronics knowledge is rudimental to say the least so please be gentleI am keen to listen and learn. I decided to repair and restore this 22 year old "Kundo 2000 Lux" 240v 50Hz electro-mechanical clock to keep me busy on the dark winter nights.The mechanical side of things, although tricky, I can deal with. But I do have some questions regarding the somewhat simple looking PCB. I have attached photos for reference.The main problem was that the motor was gummed up and has now been freed, the motor overload had caused the 240 ohm resistor to fry. It is a fairly large resistor measuring 32mm x 7.5mm, I am stumped as what wattage this would be there are no relevant markings on the PCB or components.I know this is probably an obvious thing to resolve, but as I have said I am a beginner, any help would be greatly appreciated, so I can move onto my next project.Thanks Johnny.

author: Gyro
author_rank: Super Contributor
timestamp: November 07, 2022, 07:10:30 pm
content: Welcome to the forum Johnny,It looks like a carbon film or carbon composition type, those tend to be physically large for their wattage rating, so I would assume 3W or less.There is a possibility that the resistor is intended to be a fusible type, to protect the motor winding from overheating and causing a fire under fault conditions. Your image seems to indicate that its value has gone up to 2240 ohms, but I'm not sure if that's a typo.

author: Utah77
author_rank: Newbie
timestamp: November 07, 2022, 07:13:42 pm
content: Thank you for that, yes it was a typo whoops. Would it be best to replace with a fusible type to be on the safe side?

author: Gyro
author_rank: Super Contributor
timestamp: November 07, 2022, 07:25:31 pm
content: Probably, although I'm struggling to locate one at the moment.It depends whether the resistor is being used for protective purposes only, or as a dropper resistor for a lower voltage motor. Are there any markings on the motor to indicate that it is anything other than a 240V one?If being used as a dropper, I would probably substitute a 5W wirewound (higher wattage = lower surface temperature). Normally I would suggest a distributor but, due to shipping costs, using a UK ebay seller would be more economical, eg.https://www.ebay.co.uk/itm/264670791241As I say, it would be helpful to know if the motor has any markings.

author: Utah77
author_rank: Newbie
timestamp: November 07, 2022, 07:30:25 pm
content: Unfortunately not, checked all over the motor when I removed it but nothing at all, not much help I know.

author: Gyro
author_rank: Super Contributor
timestamp: November 07, 2022, 07:42:42 pm
content: That's a shame.In that case, a possible compromise would be to add a low current fuse - if it was being manufactured today, it would have some form of protective device anyway. Assuming a complete motor short, the 240R resistor would pass 1A, however I would be worried if the clock started pulling more than around 2.4W. Taking the opportunity to add a 100mA fuse in series with the mains lead could be a good compromise.Other opinions will probably be along shortly.

author: Utah77
author_rank: Newbie
timestamp: November 07, 2022, 07:49:15 pm
content: Thanks for all your advice and time, I checked the link and there are 240 ohm and 270 ohm options but no 250 ohm? I really like your suggestion of the fuse.

author: Gyro
author_rank: Super Contributor
timestamp: November 07, 2022, 07:58:02 pm
content: Isn't the resistor 240 ohm (Red, Yellow, Brown)?If adding a fuse, these end caps will turn a standard 20x5mm one into a wire-ended one.https://www.ebay.co.uk/itm/361817760421I don't know whether all those slots in the PCB are used for something, if not, they could be used for mechanical mounting. Otherwise some heatshrink would do the job.

author: Utah77
author_rank: Newbie
timestamp: November 07, 2022, 08:07:16 pm
content: That's another typo so yes 240sorry about that. Those fuse caps look ideal, I'm very grateful