Hi everyone,

First - I am only an enthusiastic amateur, but I wonder if anyone can help. I have a small timed relay that has gone wrong. Basically it is mains (UK 240Vac) powered and the relay is 24Vdc. When I try and switch it on I can hear the relay click on, then most of the time it almost immediately it clicks off again. When I checked the power to the coil it is down to 6.6v, so I think the problem lies here somewhere. The small circuit board has a DB107 bridge rectifier and when I check the voltages to/from it I seem to get about 26Vac in and 24Vdc out when it is switched off, but as soon as I switch it on the voltages drop to just over 6Vac in and 6Vdc out!

To achieve the correct voltage at the DB107 there are 2 resistors, one on each side of the AC input. However , I am having trouble identifying the resistance of them as I can’t determine if the colour rings are red or brown! On one side there is a large grey resistor which seems to measure about 100 ohms which would fit as the rings could be brown/black/brown/gold and on the other side there is a much smaller one that measures 1M ohm (or at least 998k ohm) and the rings could be brown/black/green/gold and a capacitor which as far as I can determine is 0.47uf.

However, I am not cleaver enough to work out if this combination of resistors would produce the required 26 or so ac volts at the DB107 rectifier. I would change the resistors if I was confident that the values were correct. Does anyone have an idea if these values are right and perhaps an idea as to why the voltage is dropping to 6v when switched on? Perhaps I should just change the DB107?

Any help gratefully accepted.

 

Pictures would be better, sounds like a transformerless psu, mains droped down via two resistors to the relay coil.

Usually these use a Capacitor around 330nF and 1meg resistor to bleed the cap off...

 

Pictures would be better, sounds like a transformerless psu, mains droped down via two resistors to the relay coil.

Usually these use a Capacitor around 330nF and 1meg resistor to bleed the cap off...

And the problem described is what happens when that capacitor has reduced in value - which they commonly do. You need an 'X' rated capacitor to replace it.

 

Not sure I understand "bleed the cap off..." but anyway, here is a picture of the board. I don't know if it makes sense to you, but R1 is on one side and R2 the other side of the mains input. The big yellow thing is the capacitor and it is labeled 40/085/21/C and has .47 above that. All the other stuff on it seems to just be make and votage rating. Like I said, I am OK at soldering and understand basic electronics and components, but I don't really know much about electronics as such!

 

And the problem described is what happens when that capacitor has reduced in value - which they commonly do. You need an 'X' rated capacitor to replace it.

Thanks AlberHall, that sounds promising. Here are the details on the capacitor. Am I right that it is a 0.47uf or have I got that wrong? and what about those resistance values - do you think 100 ohm and 1M ohm would give me 26 or so volts at the bridge rectifier or am I reading the colours wrong. Mind you, if you think it is the capacitor then no reason to change the resistors I suppose...

 

If your meter can measure capacitance, check that the yellow capacitor is somewhere near 0.47μF.
The original is a 0.47μF 250VAC/275VAC X2 rated capacitor.

 

If your meter can measure capacitance, check that the yellow capacitor is somewhere near 0.47μF.
The original is a 0.47μF 250VAC/275VAC X2 rated capacitor.

Sadly I have no meter for capacitance, but what you say makes sense, so I'll get one ordered and see if that fixes it. I'd buy a whole new timer, but the timer goes down to the second, which is hard to get and all the ones I can find are quite expensive. All the timer functions and LCD display bits are fine, just not switching on and off any more...
Thanks for the advice.

 

If your meter can measure capacitance, check that the yellow capacitor is somewhere near 0.47μF.
The original is a 0.47μF 250VAC/275VAC X2 rated capacitor.

Blimy - not easy to find the exact same thing - do you think these might be OK?

https://www.ebay.co.uk/i/1823253173...=9046195&device=c&campaignid=856841495&crdt=0

Only difference I can see is that they are 310v rated, but I don't know if that would make a difference...

 

They would be fine electrically. Will they fit your board?

 

They would be fine electrically. Will they fit your board?

Yes, it was one of the few that was the same size! There seem to be lots that are 25mm wide with 22mm lead spacing, but the one on my board is only 17mm long. Have ordered them.

Thanks for all the advice - fingers crossed it fixes it!

 

They would be fine electrically. Will they fit your board?

Got the capacitor (actually two as the only place selling them sold them in pairs for £3) and soldered one in and the timer is working perfectly again!

Thanks for the advice - brilliant cheap fix! And I learned a lot too - very happy!

 

Excellent! Keep the spare, you may need it in the future.