Relay Buzzing

[rTx]

About 20 years ago, a friend (who I know longer have contact with) designed some circuitry to control my heating system. It involved the use of relays etc to switch things on and off. One of the relays buzzes like mad (suddenly). When I replace it with a new identical relay, the same thing happens to the new one. The coil side of the relay is 24V DC.

Opening up the box, I can see a circuit board with what looks like 3 diodes and a bridge rectifier. I can see a supply for 23V AC from a transformer. I think the diodes and the bridge rectifier are some sort of power supply circuitry to make the AC into DC. I am no expert though. Does this make sense? Is it likely that one of these components has failed and that AC is getting to the relay to cause the buzzing?

There is no obvious sign of failure or problems with the existing components.

I am up for replacing all the components but I am struggling to identify their specification. I can take a photo later and post if that helps.

 

[shrtrnd]

The components probably have identifying markings on them. You sound like you might know how to solder. If you (WITH THE POWER OFF/DISCONNECTED, OF COURSE), un-solder one leg of each diode, you should be
able to read the numbers on the parts. Yeah, your box sounds like a rectifier to turn AC into DC. If you have a bridge there already, those other three components might not be diodes. The part numbers will tell us.
And yeah, sounds like your relay buzzing is from a bad component or two in you rectifier box.

 

[KrisBlueNZ]

Yes, post several photos.

 

[rTx]

Thank you for the helpful responses. Here is a picture of the four components:

 

[davenn]

and a photo of the underside of the board so we can see how the diodes are connected
and another of the top side zoomed out a little more so we can see where the termination wires are going to

 

[shrtrnd]

Nice picture so far. Yep, those three components are diodes. 1N400X- type, (kind of looks like 1N4003, but we'll know with more pictures).
KrisBlueNZ and davenn are waiting on additional pictures, you'll get his fixed with those two on the case.
Looks like you might have some heating at the end of that brown wire to the board(?). Maybe your bridge rectifier is shot. But that's just me speculating
right now. KrisBlueNZ and davenn will fix you up with more pictures.

 

[rTx]

Thank you for the time you have spent responding. Here are more pictures as requested:

 

[KrisBlueNZ]

Wow, that's the proverbial can of worms!

Rather than try to follow all those wires, can I suggest a simple experiment. Add a 2200 µF, 35V electrolytic capacitor (http://www.maplin.co.uk/p/2200f-35v-85c-radial-electrolytic-capacitor-vh55k) across the + and - terminals of the bridge rectifier (the four-wire cylindrical black component), to smooth the DC supply rail. This MAY fix the problem.

The capacitor is polarised and must be connected to match the polarity of the bridge rectifier. Also, keep well away from it when you first apply power, because if the bridge rectifier is actually faulty, the capacitor may turn inside out with a bang! Don't be concerned; this is just a quick way to potentially fix the problem and test for another problem at the same time.

 

[rTx]

I'll certainly try that. I'll have to mail order the capacitor. From the photos, is it possible to identify the specification of the bridge rectifier and the diodes? If I am to buy a capacitor, I might as well order the other components at the same time and save on the p&p. I'm getting very inclined to doing more electronics work and have been looking at Arduino projects etc. and so, I do not think getting a couple of extra components would be a waste.

In the meantime, I think I may try to sketch which wires go where. Those wires in the bottom-most photo dropping from the box go to 24V wax valves (rated at about 2W each, I think) which open by slow heating when activated by a thermostat via this circuit. There are a number of other control boxes which do various bits and pieces, but it's this one that is causing the problem.

Again, very many thanks for taking the time to respond.

 

[KrisBlueNZ]

The bridge rectifier looks like a W01 which is rated for 100V, 1.5A. Suitable replacement is http://www.maplin.co.uk/p/kbpc104-2a-bridge-rectifier-aq99h

The three diodes look like 1N4001 but to be safe you can use 1N4004: http://www.maplin.co.uk/p/1n4004s-1a-silicon-rectifier-ql76h

 

[rTx]

Great. Would this work for the bridge rectifier: bridge-rectifier-2a-800v or even this: bridge-rectifier-2a-200v and for the diodes this: diode-1a-400v I want to get some other stuff from CPC, so may order at the same time.

 

[KrisBlueNZ]

That diode is exactly right, but those bridge rectifiers are quite a bit bigger and their wire leads may not fit easily. Here's an exact match to the original: http://cpc.farnell.com/_/am151/bridge-rectifier-1-5a-100v/dp/SC07472

 

[rTx]

I'm really grateful for the help you have offered. Thank you very much. I'll update later on the progress of the repair. Thanks again.

 

[(*steve*)]

I would look closely at these two soldered joints.

I am suspicious that one or both of them is bad.

 

[cjdelphi]

Buzzing to me sounds like a bridge rectifier issue...

 

[(*steve*)]

Buzzing to me sounds like a bridge rectifier issue...

And you will note that the possibly poor solder joints include one of the bridge rectifier leads

 

[Scotophor]

If the bridge rectifier might be supplying a large inductive load (like a motor), then a 100 V rated part on a 24 VAC circuit might be overstressed. If it were my choice, I would go up a size or so; either of the parts mentioned in post #11 would be a good choice. If the leads don't fit the board, drill it out just until they do.

But as Steve wrote, those suspect solder joints would be the place to start (additionally, the one marked below looks like it currently conducts but is mechanically sub-optimal, in that the copper is not wetted on one side of the lead). Maybe you don't need any new parts at all.

 

[rTx]

I have already ordered the replacement parts, but will take a look at the solder issue, too. There are no motors attached to these components. I have to work out what is doing what, but I think these are just driving a relay. Thanks for the help.

 

[rTx]

Thank you all for your help. I replaced the bridge rectifier and all seems to be working properly now.

 

[davenn]

Thank you all for your help. I replaced the bridge rectifier and all seems to be working properly now.

That's great, well done