Starter solenoid relay & protection diode
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  1. #1
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    Default Starter solenoid relay & protection diode

    Howdy electronics fans,

    I'm in the process of sorting out my starter issue by installing a relay to operate the solenoid. However, I've heard that the same issue that can damage the starter switch contacts can also damage the relay contacts over time - the reverse current spike from the solenoid when it switches off and the field collapses. This can be a couple of hundred volts.

    I've Googled for a while and come up with a solution called a freewheeling diode, which from the info I have is connected in parallel with the load (solenoid or other inductive load). This apparently causes the spike to circulate through the field coil/diode circuit until it dissipates via heat in the coil.

    However, my meagre electronics knowledge tells me that this would make the normal operating current go straight to earth, as the diode is installed + to - across the terminals. Should it be reversed, maybe?

    What am I missing? Any ideas?

    Stuey

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    1991 PEUGEOT 405 Mi16

  2. #2
    farmerdave
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    Quote Originally Posted by Stuey
    Howdy electronics fans,

    I'm in the process of sorting out my starter issue by installing a relay to operate the solenoid. However, I've heard that the same issue that can damage the starter switch contacts can also damage the relay contacts over time - the reverse current spike from the solenoid when it switches off and the field collapses. This can be a couple of hundred volts.

    I've Googled for a while and come up with a solution called a freewheeling diode, which from the info I have is connected in parallel with the load (solenoid or other inductive load). This apparently causes the spike to circulate through the field coil/diode circuit until it dissipates via heat in the coil.

    However, my meagre electronics knowledge tells me that this would make the normal operating current go straight to earth, as the diode is installed + to - across the terminals. Should it be reversed, maybe?

    What am I missing? Any ideas?

    Stuey
    Stuey, could you post or PM the link please. You are correct in that there will be a spike of hundreds or thousands of volts, I'm picking that a heavy duty diode like that used on AC clutchs will be heaps for a starter solenoid.

    Farmerdave

  3. #3
    Real cars have hydraulics DoubleChevron's Avatar
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    Stuey,

    it's only a relay, about 5bux. Personally I wouldn't worry about it.

    If you crack the relay open you'll probably find it already has diode protection inside.

    seeya
    Shane L.
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  4. #4
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    Default

    Yeah, good point I suppose. I wasn't worried at all about the cost, but the reliability. But I s'pose it'd warn me well in advance by the same clickiness it's got now, wouldn't it, if the contacts started karking... Guess I'm just a fuss arse. Might give it a miss - thanks for the advice!

    BTW I've had the casing open just to make sure the relay Jaycar sold me is OK quality wise. Don't want to be stuck somewhere... It's got no diode - the ones with diodes are installed across the relay coil, not the switched output, to stop EMF interference. Oh, and the relay was pretty well built.

    Dave, there wasn't one single site that I could put my finger on, but an afternoon's surfing committed to memory.

    Cheers

    Stuey

    1991 PEUGEOT 405 Mi16

  5. #5
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    Stuey,

    The diode would have to be reversed because as you said as soon as you turned it on a large current would be dumped to earth cooking the diode and fuses if not the wire. The idea of the protection diode on a relay is to give the current generated by the collapsing magnetic field of the coil when power is removed some where to go. You should be right using a 1N4004 (about 5c at Dick Smith) as you will only be switching 12V. See attatched diagram, disregard transistor, it's only there to switch the relay coil on and off. Also see reference about half way down page:

    http://www.kpsec.freeuk.com/components/diode.htm

    As Shane said, the relay is only worth about $5 and is probably already protected.


    Matt.
    Attached Thumbnails Attached Thumbnails Starter solenoid relay & protection diode-diode-protection.gif  

  6. #6
    farmerdave
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    The example confuses the relay protection with the solenoid protection- in principle they are the same but only the solenoid is important here.
    The diode which is sometimes fitted to relays when used in transistorised circuits is to protect the transistor. It isn't important here as the relay coil spike is small.
    Fitting a suitable diode in the same manner across the terminals of the solenoid will protect the relay terminals from the solenoid spike.

    It is suprising the voltage that some of these solenoids will give- for example, I have measured 590v from the transmission solenoids in an R25 gearbox when the power to the solenoids is turned off. That's a fair sort of spike to go through any sort of circuit.

    Farmerdave

  7. #7
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    Thanks Ralph.

    Actually Dave, we're ALL at cross purposes! I was taking about protecting the relay contacts that are in the solenoid circuit solely to maintain reliability - not to protect the solenoid.

    Ralph, I understand that the diode built in is to protect the relay coil. The relay contacts operate the power to the solenoid on another circuit and it's this circuit that the diode is installed into that I'm referring to. However, the diagram you posted is spot on because it confirms the polarity alignment of the diode - what they called 'backwards'.

    To protect these contacts would need at least a 10A diode. The references I read suggested that the diode should match the maximum current, and be rated at 10x the voltage. So around 10A and 400V (to round to a standard diode voltage) would do it. From what Dave's said, even a 1000V one might be needed if the spike is 590V+.

    But as I've decided, it's probably not worth it - if there are any mishaps they should come on gradually and as Shane said, a relay is only a fiver.

    Again, thanks for the input - it's all good stuff.

    Stuey
    Last edited by Stuey; 17th February 2005 at 11:47 PM.

    1991 PEUGEOT 405 Mi16

  8. #8
    Fellow Frogger! Ralph's Avatar
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    Stuey,


    The diode is generally there to protect the cicuit that is connected to the relay. The coil is a fairly robust device. What is connected to the coil may include IC's and other devices that might suffer from a rush of "back EMF" caused by the collapsing field in the coil when the supply is cut from the coil.

    A relay is primarily used to isolate one cicuit from another. I found out many years ago that you can't directly connect a set of driving lights to your high beam cicuit with out melting the contacts inside the high/low beam switch.

    Matt.

  9. #9
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    Ralph, again we're at cross purposes. I'll admit that my words were badly chosen - protect the relay coil isn't what I meant - as I said previously about the relay I have to hand, It's got no diode - the ones with diodes are installed across the relay coil, not the switched output, to stop EMF interference. Oh, and the relay was pretty well built.

    However, this would do nothing to protect the relay contacts in the operating circuit form the much larger reverse spike from the solenoid coil. This is where the larger diode would be installed.

    I browsed a local tech bookshop today and one electronics book mentioned installing the diode across the load, and the current rating must be at least equal to the current draw of the load. It didn't mention a voltage rating. And yes, it was reversed, -ve to the + of the solenoid.

    Oh, and I know what a relay does! Do I sound that dumb?

    Cheers!

    Stuey

    1991 PEUGEOT 405 Mi16

  10. #10
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    Since what you seem to want is the reduction/suppression of arcing across the relay contacts, I would generally consider an R-C snubber network (a resistor and capactor in series, connected across the contacts) to be more appropriate.

    The freewheeling diode is of primary use in protecting a semiconductor device - like in the schematic previously posted here.

    Remember though, that the most reliable system is generally the one with the fewest parts.
    Your starter relay doesn't switch all that often ( few times a day, no?) so perhaps the best solution is to just use a chunky high current relay, with good contact spacing, and be done with that.
    "You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles... Radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat." - Albert Einstein

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  11. #11
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    Yeah, thanks...as stated, I'll just stick with the modest relay and monitor any future 'clicking' problems...they're just a $6 plug-in.

    Stu

    1991 PEUGEOT 405 Mi16

  12. #12
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    Quote Originally Posted by Stuey
    Yeah, thanks...as stated, I'll just stick with the modest relay and monitor any future 'clicking' problems...they're just a $6 plug-in.

    Stu
    Stuey,

    Just been there with the CX. My choice was a good quality Bosch relay with heaps of current capacity. Let's compare notes in, say, 5 years!

    Will you be at Perry Lakes on Sunday?

    Cheers

    JohnW

  13. #13
    Real cars have hydraulics DoubleChevron's Avatar
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    My CX has 15 relays at last count. They all look like the original 30year old relays .... Seriously I think you guys have nothing to worry about. The back EMF from the starter soleniod hasn't affected anything in the last 20years, why would it now

    I'd be very suprised if you had any relay problem, if you do just swap it with one of it's neighbours, hey I can live without high beam or electric windows for a drive home.

    seeya,
    Shane L.
    'Cit' homepage:
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    '63 ID19 http://www.aussiefrogs.com/forum/showthread.php?t=90325
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    '78 GS1220 pallas
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  14. #14
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    Quote Originally Posted by JohnW
    Stuey,

    Just been there with the CX. My choice was a good quality Bosch relay with heaps of current capacity. Let's compare notes in, say, 5 years!

    Will you be at Perry Lakes on Sunday?

    Cheers

    JohnW
    John, the Jaycar one is actually pretty good. It has big contacts, a proper hinge arrangement with a good quality welded copper braided interconnect, a nylon coil former etc. and looks well put together. It's rated at 40A.

    Perry Lakes? I wish. No, I've got Driveshaft II - The Lefthand Sequel out on DVD this weekend...it's only a weekend rental.

    Cheers

    Stuey

    1991 PEUGEOT 405 Mi16

  15. #15
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    Stuey,

    Sorry, I didn't mean to sound condescending.

    Matt.

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