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need help on diode selection — Parallax Forums

need help on diode selection

allieallie Posts: 109
edited 2009-04-04 20:14 in General Discussion
·I have a circuit where there is a coil energized switch drawing 6 amps at 12 volts.·There is also a 5 watt light wired·from the·switch to ground for visual confirmation when switch is turned on.
·Question;·I will need·a diode for reverse voltage spike when switch is turned off·but how many amps above the 6 should I use.

allie
·

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2009-04-02 13:31
    The diode doesn't conduct the load current. It only conducts the reverse voltage produced when the coil's magnetic field collapses. I would probably use something like a 1N4001 diode which is rated for 1A average current.
  • Carl HayesCarl Hayes Posts: 841
    edited 2009-04-02 15:06
    Mike's suggestion of a 1n4001 is an excellent choice.· After the coil circuit is opened, the reverse diode, or "kick" diode, or "back" diode as it is sometimes called, briefly conducts whatever current (not voltage) was flowing through the relay coil, enabling that current to die down gracefully because of the resistance in the coil.· The 1n4001 has a peak momentary current rating of 30 amperes, so if the relay coil current is only several amperes the 1n4001 will serve well.

    I think you quoted the (irrelevant) current through the relay contacts rather than through the relay coil anyway.· There are relays that require a coil current of six amperes, but that's a mighty big relay.· Mighty big.

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    · -- Carl, nn5i@arrl.net
  • PhilldapillPhilldapill Posts: 1,283
    edited 2009-04-02 18:18
    Just FYI, the 1N400X series of diodes have a maximum SURGE current of 30A. I think this is for 200uS, which should be well within your "die-down" time for the current.

    This depends on how many turn on's/off's your switch is doing. Is this a manual switch, or are you switching a transistor or something at a high frequency?
  • allieallie Posts: 109
    edited 2009-04-03 14:05
    ·Thanks for the answers.

    Carl you are right about, it is a big current but it is the coil current.
    ·I don't have any 1n400x series diodes but I do have some 1n914 switching diodes,will these work also.

    allie
    ·
  • Carl HayesCarl Hayes Posts: 841
    edited 2009-04-03 15:22
    allie said...
    ·Thanks for the answers.

    Carl you are right about, it is a big current but it is the coil current.
    ·I don't have any 1n400x series diodes but I do have some 1n914 switching diodes,will these work also.

    allie
    Not reliably.· When the circuit is opened, the diode will conduct the full current that was flowing through the inductance (the relay coil) -- no more, no less (though it will die down).· That, you say, is six amperes, but the maximum momentary current rating of a 1n914 is only four amperes.· Poof!

    Almost any diode that is intended for service as a power rectifier will serve you, but a small-signal diode like a 1n914 would be a very poor choice.· When it fails, it will probably fail open.· Then you lose its protection, and you burn up whatever was trying to shut off the coil current.

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    · -- Carl, nn5i@arrl.net

    Post Edited (Carl Hayes) : 4/3/2009 3:27:58 PM GMT
  • allieallie Posts: 109
    edited 2009-04-03 22:12
    Thanks Carl for the answer with the quick response.
    I found 1 1n4001 diode which I'll use. But I have another circuit which draws 3 amps and I desoldered a 1n4002 which I'll use. As I understand the 1 stands for 1 amp and the 2 stands for 2 amps and 3,4,---so on.
    ·So the 1n4002 should do the job.

    allie
    ·
  • Carl HayesCarl Hayes Posts: 841
    edited 2009-04-03 22:31
    No, the type number is just an assignment from a series, and no part of the type number is derived from any rating. 1N4001 through 1N4007 all have the same current ratings (1A average, 30A surge), but differ in their reverse breakdown voltage ratings. Any of these would serve well for your kick diode application.

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    · -- Carl, nn5i@arrl.net
  • SRLMSRLM Posts: 5,045
    edited 2009-04-04 03:39
    Is it safe to connect several diodes in parallel to increase the maximum current?
  • Mike2545Mike2545 Posts: 433
    edited 2009-04-04 03:53
    If one goes then you have problems, if you know what I mean.

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    Mike2545

    This message sent to you on 100% recycled electrons.
  • mctriviamctrivia Posts: 3,772
    edited 2009-04-04 04:09
    it is not good to put diodes in parallel. try puting several leds in paralel and you will see why. 1 will be bright the others dim or not lit at all. Every diode has a slightly different voltage drop acrose it. This results in 1 diode taking most of the current while the others take a small part.

    If you must put diodes in parallel. place a small resister in series with each diode.

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  • Carl HayesCarl Hayes Posts: 841
    edited 2009-04-04 20:14
    SRLM said...
    Is it safe to connect several diodes in parallel to increase the maximum current?
    It is safe as long as you enclose them in armor plate so that when they explode one at a time, the particles won't get you.

    They will explode because one of them (the one with the least forward drop) will conduct all the current meant for several, and it will overheat.· After that one explodes, the one with the next lowest drop will conduct all the current until it explodes.· And so forth.

    Sometimes they don't explode but only burn out, if the total current is small enough.· In that case you can omit the armor plate.

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    · -- Carl, nn5i@arrl.net

    Post Edited (Carl Hayes) : 4/4/2009 8:22:42 PM GMT
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