contactor coils and a funny failure mode.

I have just changed a timer for an underfloor heating rig for the second time in less than 12 months. As far as  I can tell it has hardly been used - but like all things MAPJ1 there is a twist.

To save the thermostat (rated at '16A' and a very feeble looking relay) the heating of about 20A is switched by a couple of relays, and they in turn are operated by that thermostat. but the supply they switch  comes from a pair of changeover contactors that switch between Daytime and Economy 7 power, depending on the press of a 'day boost'  over-run timer.


The contactors are Garo 20A units and work seamlessly, and the thermostat and the two relays have been faultless - but the over-run timer has now failed twice, both times when the day time power has been switched off.

My unproven suspiscion is that this occurs when the contactor coils lose supply, but the heating resistance is not in parallel due to a satisifed thermostat.

Now I have heard figures of  an inductance of 20 henry in series with a few hundred ohms for single module contactors, but I have not yet measured these units to see how true this is.

If so the kick from the contactors when supply is removed may be quite noticable, (stopping ~ 40mA each though a pair of  20H inductor I think is 33mili joules. ) but is it enough, really, to break an electronic timer.?

It does not feel quite right.

Has anyone else seen commercial grade  electronics in the vicinity of a contactor fail suddenly  when the supply stops, or analysed the back EMF kick back from contactors to see if that anecdotal figure is way off beam ?

Mike.

(yes I'm going to add a transorb anyway - I'd just like a warm feeling it was perhaps going to do some good.)

Parents
  • What brand of timer did you use for your underfloor heating rig and did it have a LCD screen? I had similar problems where the display failed and went dim supplying underfloor heating at dog kennels.

  • The thing that seems to keep failing is one of these

    https://www.tlc-direct.co.uk/Products/SMTGBD4.html

     Timeguard   TGBD4

    taking it apart,  internally it has a capacitor input power supply (no transformers here) and then a 3 legged voltage regulator - which seems to be the thing that actually dies at the key moment. Once it has gone, then a rather larger voltage (about 20) that is used for the relay. is fed onwards unregulated   to the timer chip beyond, which previously had a regulated 5V.,

    I'd like to think this sort of thing is tested to withstand surges as per IEC_61000-4-5 for its CE marking, but I'm not too sure which limit would apply.

    Mike

  • It is funny that I had problems with time guard time clocks with LCD screens. As I mentioned, the display would fade or vanish completely. Several of the new ones malfunctioned intermittently. And as you suggested, the solution is to install a transient-voltage-suppression diode to protect the timer from the back EMF.

  • It is useful to know that someone else has seen problems, and the cure was as suspected to clamp the cotactor back EMF.

    Bit worrying though

    M

  • I installed the new time clocks and contactors with stats to control underfloor kennel heaters about 6-7 years ago. However, they started to malfunction one after another. I had to return and replacthem at no cost to the client. It was disappointing that they were not designed to handle the interference from the adjacent contactors. And thanks for the detailed explanation, very helpful Thumbsup

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  • I installed the new time clocks and contactors with stats to control underfloor kennel heaters about 6-7 years ago. However, they started to malfunction one after another. I had to return and replacthem at no cost to the client. It was disappointing that they were not designed to handle the interference from the adjacent contactors. And thanks for the detailed explanation, very helpful Thumbsup

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