EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: m3vuv on November 30, 2019, 09:34:42 pm
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Hi all ive a coffee perculator to look at for a friend,long story short,the heating element relay wont pull in,its a 12v one,from what i can see its powered from the mains via a few diodes and caps,if i remove the relay from the board the 12v feed to it switches from the control board ok,if i send the on signal to the relay pcb pads then conect the relay it pulls in ok,but it wont switch on if i connect it before sending the on signal,ie send the on comand then connect the relay and it pulls in ok,conect it then send the on signal and nothing happens,its like its not getting enough current to pull in,ive tested the caps and diodes,they seem ok,any ideas?
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I always regret it when I accept to look into a coffee machine for someone, one of the rare appliances I'll admit should be thrown away :horse:
How does the voltage at the coil measure in both scenarios?
Supply voltage at the zener / regulator, relay on / relay off?
The usual culprit is the largish MKT/MKP voltage dropping capacitor on the mains input.
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There will be a big film capacitor on that board, probably around 470n 250V to 680n 250V. Replace it, along with all the small electrolytic capacitors there, and the capacitive power supply will work again. The capacitors, especially the big film one, only has to be around 10% low before the power supply is unable to deliver the current to hold the relay in, and the small electrolytics the same.
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I get 15v on the pcb pads without the relay in the board,ie not connected,thats in the on position and zero in the off position,heres some pics,would the cap you mention be the yellow square one?
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There will be a big film capacitor on that board, probably around 470n 250V to 680n 250V. Replace it, along with all the small electrolytic capacitors there, and the capacitive power supply will work again. The capacitors, especially the big film one, only has to be around 10% low before the power supply is unable to deliver the current to hold the relay in, and the small electrolytics the same.
Yes, ten to one, it's the film cap that has self-healed to the point where it no longer has sufficient capacitance to supply enough relay coil current.
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well,icwapped the electrolytics yesterday,they were ok,today i changed the film cap,that sorted it,cheers folks,i wouldnt ushaly of bothered with it but it was a friends whos daughter brought it for him years ago,sort of sentimental value!,thats my good deed done for the day!