Central heating control operates with bench Pw Sply but not mains?

[TheBrick]

Hello all,

Perhaps you all might be able to help me out here. I have a control box for a central heating system that should operate a relay under x/y/z conditions. It stopped working so I stated to investigate.

First of I checked the relay by operating it off of a bench power supply and it operates ok. The relay is rated as a 24v relay.

Then I replaced a 0.68 mu F polypropylene capacitor as this regularly fails apparently. No luck.

Then I checked the voltage directly after the bridge rectifier. It reads 18V a little low I thought. Occasionally the relay will buzz trying switch and if I press on the relay is will switch over.

This led me to think the relay is not quite getting enough voltage to operate. I thought I would disconnect the mains and connect my bench power supply at the bridge rectifier (same point I measured from) and see at what voltage the relay would operate. The result of this experiment was that if I fed in 17.2 of greater the relay operated fine. Below 17.2 it stated to become flaky. This reading was taken off of the power supply as I had to use my test leads to power the circuit.

So could anyone give me some ideas of where to go from here? and why when powered from the bench top at 17.2 v everything operates fine but at 18v fed from the mains the relay struggles to operate. Something to do with ripple?

[ptricks]

It could be ripple, or a failing transformer. Try measuring the amp load between the mains powered and the bench powered and see if there is a difference .

[SeanB]

Capacitive power supply and the 470-1000uF capacitor after the bridge rectifier is low in capacitance or open circuit. Pic will enable us to see which one it is and the value.

[TheBrick]

Thanks for the replies.

@ptricks I'll give that a go. I don't have a full compliment of leads at the moment so will have to run to maplin in the morning a pick up some bit to make up another set of leads to do this. Thanks for the suggestion. One thing I did notice on the voltage over the coil of the relay was that when initially switched on there was spike and a the meter read much higher for a moment. I would have though that that as the coil is charged the voltage would take time  build up and only have a spike as the relay is switched off and the magnetic field collapses.


@SeanB. This is a pic of the power side of the board.



I've replaced the big yellow cap (0.68 micro F). The [strike]blue cap[/strike] varistor is between the phase and neutral before the resisters and rectifier. EDIT: Its a varistor S10K275, I'm not sure I understand them to be honest. The two large black resisters both read 10 Ohms and the smaller brown one reads 0.990 M Ohm (990 K Ohm if you prefer!).

This is the other side of the board.



I measured the voltage after the bridge rectifier if the bottom left hand corner of the board. This is also where I input the bench power supply. I can't figure out what the silver caped thing near the diode that make up the rectifier is (9724750b). I guess this could be the smoothing cap after the rectifier? Its very hard to follow the tracks after the rectifier.

[SeanB]

That is it, a 47uF 50V surface mount electrolytic capacitor. Probably will be best if replaced with a 220uF 50v unit, though it probably will not fit in the space, and will most likely have to be mounted via short wires to the pads. A bigger pic where the black markings would be better to read the value.

[TheBrick]

Thanks SeanB. I was just  about it I think that it is a cap as I think I can see one side attached to the grd plane and a trace from the +ve side of the B.R.

Sorry where do you mean by  "A bigger pic where the black markings would be better to read the value."

Do you mean ontop of the cap? This reads

972   // Something to do with the order of magnitude?
47    // the 47 micro F as you say
50B // the voltage I guess as you say 50v?

The smaller cap reads

931
10
35W.

[SeanB]

That is right, it is a 47uF unit. probably open circuit and needs to be replaced. If you can get a 100-220uF 63V unit it will last longer, though it will not fit in the space but will need to be insulated and connected with leads to allow it to be fitted in the housing.

[TheBrick]

That is it, a 47uF 50V surface mount electrolytic capacitor. Probably will be best if replaced with a 220uF 50v unit,

I've found a 50V 100 Micro F electrolytic cap. I guess the capacitance being greater would not be a problem? Just a slower initial charge and discharge time. I'll probably have to mod the case anyway but it's in a cupboard so aesthetics are not important.


EDIT: Ok you've just answered the question! Being as I've got to head into town tomorrow I'll pick up 63V one and post up the results.

As far as sticking the cap the the board (mechanically) are there any glue types I should be cautious of using due to conductive properties? I seen some white stuff used that looks similar to silicone bathroom sealant.

Thanks again for your help.

[SeanB]

Any silicone that smells of vinegar will eat the board to pieces in no time. Use a drop of thick superglue to hold it down. It will leave a white bloom on the board, but will do no damage otherwise. Silicones are bad near copper, despite plumbers loving the stuff.

[ptricks]

Something else you could do to solve the power issue is make a small board and place it outside the unit in its own little box. You would only need a bridge rectifier and cap and then wire that to the same point as you did the bench supply, this way wouldn't require altering the original product.

[TheBrick]

@ptricks. (I keep on wanting to call you pthread, work creeping into life) Good idea. That can be a back-up plan. Its a simple obvious solution but because I'm only just getting into electronics it completely passed me by.

@ SeanB. The cap replacement worked. Just got to make a minor mod to the case. Thanks for the guidance.