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Thread: Headlights and side lights on - wont go off.

  1. #11
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    Quote Originally Posted by pinger View Post
    Ehhh???????
    Soz, was out on a bike ride.

    Every electromechanical relay has a coil and a plunger which eventually makes and/or breaks the contact pairs.
    That coil has an inductance, as you drop the power there is a back EMF (a voltage) induced in that inductance, a diode needs fitting across the coil to limit that reverse voltage.
    If youíve fitted a relay that doesnít have a diode already fitted, youíll cause all sorts of problems for the driving electronics/electricals.

    See here for pictorial info.. https://progeny.co.uk/back-emf-suppression/

    A fellow smarty that I never actually met fitted some non-smart foglamps to a 453, he initially fitted a relay (sans diode) and he got into all sorts of bother with weird symptoms, after I explained why he replaced the relay with a dioded version and everything went back to normal

    Some relays have them fitted internally and will specifically say so in the datasheet/info
    Last edited by reinaldo_is_spam; 19-06-2019 at 07:54 PM.
    You're so money supermarket and you don't even know it!

  2. #12
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    If the relay coil is being powered from the existing headlight circuit then any back EMF will be clamped to very tiny levels by the resistance of the headlight bulbs.

  3. #13
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    Quote Originally Posted by mikerj View Post
    If the relay coil is being powered from the existing headlight circuit then any back EMF will be clamped to very tiny levels by the resistance of the headlight bulbs.


    Heís replaced high beam bulbs for 100w bulbs....so the original bulbs arenít there.
    So Iím not sure where you think the bulbs will help dumping the back EMF, theyíre not there. The new bulbs on the contacts side of the relay, not on the coil side.

    Iíll wager a 50 pence that the relay has no protection diode and is causing the problem, whatís the part number on the side of it ?
    You're so money supermarket and you don't even know it!

  4. #14
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    Quote Originally Posted by reinaldo_is_spam View Post
    Soz, was out on a bike ride.

    Every electromechanical relay has a coil and a plunger which eventually makes and/or breaks the contact pairs.
    That coil has an inductance, as you drop the power there is a back EMF (a voltage) induced in that inductance, a diode needs fitting across the coil to limit that reverse voltage.
    If you’ve fitted a relay that doesn’t have a diode already fitted, you’ll cause all sorts of problems for the driving electronics/electricals.

    See here for pictorial info.. https://progeny.co.uk/back-emf-suppression/

    A fellow smarty that I never actually met fitted some non-smart foglamps to a 453, he initially fitted a relay (sans diode) and he got into all sorts of bother with weird symptoms, after I explained why he replaced the relay with a dioded version and everything went back to normal

    Some relays have them fitted internally and will specifically say so in the datasheet/info
    Thanks RIS - info there I was unaware of. If I understand correctly, in essence, when a relay is switched off, momentarily (as the plunger travels back within the coil) it acts as a mini (linear) generator and creates a voltage that electronics don't like.

    However, not in this case is that the problem. Here's my thinking as to why.
    1) I don't think the SAM actually 'controls' the headlight circuit, merely houses its fuses (if I'm wrong, I'll happily be corrected).
    2) It did cross my mind that a specific resistance (ie 55W bulbs) would be sought (in the same way MEG checks that sensors are present) but conclude that that wasn't the case as there is no facility to report a blown bulb. And, for years, the circuit saw only the relay and there were no problems.

    It is fixed now. Drove 120 miles today and no problems. The old relay was definitely faulty as with a meter across the relevant terminals closed circuit occurred when one of the terminals was wiggled. A mechanical failure then.

    The dioded relay though is something I'll bear in mind for future if ever I'm employing one where the main control is via electronics rather than just mechanical switching. Good info - cheers.
    I'm just relieved it's sorted with a £6 relay - one hundredth of the cost if it had been the SAM that was goosed. A bullet well swerved!

  5. #15
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    Quote Originally Posted by pinger View Post
    Thanks RIS - info there I was unaware of. If I understand correctly, in essence, when a relay is switched off, momentarily (as the plunger travels back within the coil) it acts as a mini (linear) generator and creates a voltage that electronics don't like.

    However, not in this case is that the problem. Here's my thinking as to why.
    1) I don't think the SAM actually 'controls' the headlight circuit, merely houses its fuses (if I'm wrong, I'll happily be corrected).
    2) It did cross my mind that a specific resistance (ie 55W bulbs) would be sought (in the same way MEG checks that sensors are present) but conclude that that wasn't the case as there is no facility to report a blown bulb. And, for years, the circuit saw only the relay and there were no problems.

    It is fixed now. Drove 120 miles today and no problems. The old relay was definitely faulty as with a meter across the relevant terminals closed circuit occurred when one of the terminals was wiggled. A mechanical failure then.

    The dioded relay though is something I'll bear in mind for future if ever I'm employing one where the main control is via electronics rather than just mechanical switching. Good info - cheers.
    I'm just relieved it's sorted with a £6 relay - one hundredth of the cost if it had been the SAM that was goosed. A bullet well swerved!
    It's not the action of the plunger moving back (It's just a block of metal not a magnet), it's the actual effect of suddenly removing the power from the coil (the inductor)- see wiki link later in this post.

    Whilst the diode across the coil is a crucial component to keep that 'off' spike clamped to a diode drop, unchecked it can spike to a couple of hundred volts for a moment until it decays, the other obvious key spec you need to take into account is the contact rating, you're switching ~17amps with that 200W load.... so I'd hope the relay chosen is at least 20A(DC) rated.

    Here's a couple of example 'auto' relays , one with diode, the other without.
    When you buy one without, you're supposed to be aware it's not present and make alternative external arrangements to fit one.

    With:
    https://www.ebay.co.uk/itm/Micro-Rel...kAAOSw~gRV6WOi
    Without:
    https://www.ebay.co.uk/itm/Micro-Rel...sAAOSwoRpcUbAJ

    If it's not explicitly mentioned (or shown on the casing print as in the examples) assume it doesn't have one, and add one.


    More on it here: https://en.wikipedia.org/wiki/Flyback_diode
    Last edited by reinaldo_is_spam; 20-06-2019 at 10:30 PM.
    You're so money supermarket and you don't even know it!

  6. #16
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    Quote Originally Posted by reinaldo_is_spam View Post
    It's not the action of the plunger moving back (It's just a block of metal not a magnet), it's the actual effect of suddenly removing the power from the coil (the inductor)- see wiki link later in this post.
    OK, got it now. I'm more used to the presence of a second coil when there is an induced voltage (eg, ignition coil) but there it is acting as a step-up transformer.

    Quote Originally Posted by reinaldo_is_spam View Post
    Whilst the diode across the coil is a crucial component to keep that 'off' spike clamped to a diode drop, unchecked it can spike to a couple of hundred volts for a moment until it decays, the other obvious key spec you need to take into account is the contact rating, you're switching ~17amps with that 200W load.... so I'd hope the relay chosen is at least 20A(DC) rated.
    30 Amp installed - as before.

    Quote Originally Posted by reinaldo_is_spam View Post
    Here's a couple of example 'auto' relays , one with diode, the other without.
    When you buy one without, you're supposed to be aware it's not present and make alternative external arrangements to fit one.

    With:
    https://www.ebay.co.uk/itm/Micro-Rel...kAAOSw~gRV6WOi
    Without:
    https://www.ebay.co.uk/itm/Micro-Rel...sAAOSwoRpcUbAJ

    If it's not explicitly mentioned (or shown on the casing print as in the examples) assume it doesn't have one, and add one.


    More on it here: https://en.wikipedia.org/wiki/Flyback_diode
    In this instance - as previously the installation was troublefree until the relay itself failed (mechanically) and the manual switching (rather than transistor) I'l run with the non-diode relay. Good info though - cheers.

  7. #17
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    Quote Originally Posted by reinaldo_is_spam View Post


    Heís replaced high beam bulbs for 100w bulbs....so the original bulbs arenít there.
    So Iím not sure where you think the bulbs will help dumping the back EMF, theyíre not there. The new bulbs on the contacts side of the relay, not on the coil side.

    Iíll wager a 50 pence that the relay has no protection diode and is causing the problem, whatís the part number on the side of it ?

    The original problem is that the (100W) main beam and sidelights were staying on until the battery is disconnected, and the dipped beams going mad once the system voltage fell low enough. Since back emf occurs when a relay is powered off, and the 100W lights are remaining on, why do you suspect this is the problem? Couple this with the main beam light not being illuminated (i.e. the SAM is not commanding the relay closed) and the same setup working fine for 10 years and it makes back emf as a cause vanishingly unlikely.

  8. #18
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    On the day, as it happened, couldn't have been the switching side of the relay because the main beams came on without being switched on via the stalk (so relay coil not energised).
    It was just a relay gone bad - it happens.

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