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Drayton Digistat SRC wireless heating control.

Discussion in 'Troubleshooting and Repair' started by TheBrick, Dec 23, 2012.

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  1. TheBrick


    Dec 23, 2012
    Hello all,

    I'm looking for some advice and guidance on where to go with regard to a repair I am trying to undertake on a central heating controller. The control receives a RF signal process by at ATmega microcontroller and then switches on. There is also an override button that I use to activate the realy without the need for the RF unit.

    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. 20 V AC goes in and 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?

    I'm not sure of what to test next. I could always swap out a resister on the AC side of the bridge rectifier to that a higher voltage passes though but this seems poor idea as the circuit operates fine when powered by a nice linear bench power supply.
  2. TheBrick


    Dec 23, 2012
    For reference there was a 47 micro F electrolytic cap after the b.r. I did not identify that as I'm not failure with SMD component yet. Replacing this cap solved the problem. Which was nice.
  3. davenn

    davenn Moderator

    Sep 5, 2009
    hi there
    welcome to the forums
    and a Merry Xmas :)

    Well done !
    without that cap working properly there would have been a significant lack of smoothing
    meaning your supposed DC voltage would have been pulsing wildly at 120Hz
    and anything looking for relatively smooth Dc to operate would have been groaning under
    the stress

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