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 ?


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

  • 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

     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.


  • 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's well known in the small electronics world that the back EMF from a relay turning off can fry a transistor.  The usual solution on DC is a reverse-biased diode to short out the back EMF.

    On AC, a conventional silicon diode won't work, but there are other kinds of snubbers to absorb the energy.

  • Indeed - which is why I began to wonder about it.

    And it has occurred to me later (the joy of a beer and a bathtub) that the VA rating and the wattage of the contactor coil conspire to give me a 50Hz equivalent circuit,

    8VA implies 8/230 35mA and something like 6500j ohms or 21 Henries.

    and the 1W dissipation implies 28V of resistive drop at that current so about 800 ohms of DC resistance in the series model .

    So at the crest of the 35mA we have 50mA. in 21H so I2L = 25 millijoules. But there are two of them, so 50mJ max  stored energy.

    Now, as we have no idea what that dinky regulator chip can take we could attempt a wild order of magnitude guess and start looking at the Harry Diamond EMP report where we can take a small transistor of comparable die area to that voltage reg, say a 2n2222 (page52 )and read off, 40 watts for 10usec (400uJ) or 135 watts for 1usec (135uJ), or 2.8Kw for 100nsec (28uJ). All of which are massively less than the energy available from the contactor coil by a factor of 100 or more.

    Of course most of the spike gets spread about the wiring,  and does not end up there but even so it seems feeble, especially as it is also  massively less than even the weakest test limit in set  IEC_61000-4-5 .

    Perhaps they have not tested it...

    So, a MOV style  transorb it is. And perhaps an RC trap as well for good measure.


  • Hoping it may inform  or at least entertain,  I have decided, as I sometimes do, that modelling in Spice is my friend

    And err yes it  predicts a noticeable spike on the switched mains to ~ 500V, the  polarity and exact shape varying with instant of switching.
    The 24V DC for the relay and electronics internals, already a little high when the coil relay in the timer is off, (and sensibly about 24V when it is on) goes up to about 45- 50V.. Remember this feeds the 5V regulator. Ouch.
    The various different coloured sweeps are moving the instant of switching through the mains cycle, but generally the effect is fairly similar
    The resistor that is the in model to represent the electronics when the relay is off  is not quite right, hence the slope on the DC prior to the transient, but the over-voltage  effect is pretty clear.

    Well, my gast has been flabbered.... 

  • Thats very interesting Mike.  I had some similar problems with a timeswitch failure a few years ago which on reflection might well have had the same cause.  All said though, we might reasonably expect modern electronics to withstand the sort of overvoltage your model is showing.   500V peak transients are probably quite common on UK supplies anyway especially with modern drives.

  • Interesting to have the anecdotal evidence that others have seen problems too.

    Suggests contactor coil  kick back is an effect worth considering and trying to design out around kit like this.

    yes - it is not really a massive over-voltage is it  ?- not even really enough to get a type 2 surge arrestor out of bed, so one of those would not  cut it down very much (though in this case it is on the wrong side of the interruption anyway). I'm pertty sure the mains is often quite furry at the 500V level. Interestingly the capacitor in the transformerless supply that gets the mains is X2 type rated for 330V so not a huge amount of of slack.

     In that case R-C snubbing accross the contactor coils may actually be better. Just need the resonance of the L of the contactor and the snubber C to be not too far from 50Hz and the sine wave will carry on and die out gracefully.

    I find myself  thinking that if I'd made something that easily upset as part of the day job, I'd be told to redesign it pretty sharpish ;-)

    Consumer electronics is a different world.


  • 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


  • 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