1N4001 vs 1N4148 for EMF suppression

Discussion in 'General Electronics Chat' started by tracecom, Apr 1, 2013.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    I recently looked at a commercially available circuit that uses a 555 to operate a 12V relay. The relay draws about 40 mA. The circuit included a 1N4148 across the coil of the relay, but I have always used a 1N4001. Is a 1N4148 suitable, and what specifications does a diode need for this purpose?

    Thanks.
     
  2. takao21203

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    Apr 28, 2012
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    you can use a LED if you wanted to.
    The energy from a small relay coil is quite limited.
     
  3. ErnieM

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    Apr 24, 2011
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    Don't use a LED. LEDs have a very limited reverse voltage rating, and 12V is greater then most.

    Either of those diodes should be fine, though the 1N4001 is a better choice as it is the "beefier" device: the 4001 is a 1 amp rectifier and the 4148 is a 100mA small signal device.
     
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  4. Ron H

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    Apr 14, 2005
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    If your coil only draws 40mA, then that will be the peak current through the diode. Reverse voltage will be the supply voltage for your relay.
     
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  5. takao21203

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    Apr 28, 2012
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    It is OK for small relays. Or 1n4148.

    The critical voltage can never be reached, since the LED starts to conduct much earlier.

    With a large relay, eventually you would be able to burn out a LED.

    But that way, you really know the coil is energized.
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    You mean the LED will prevent the relay from turning on? It will just rapidly heat up because the reverse current will rise very high and then it will release the magic smoke.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    The diode or LED is in connected in reverse bias across the coil (cathode to coil plus side) and thus must withstand the full relay voltage, which it typically cannot do. An LED is in forward bias when it is ON.
     
  8. JohnInTX

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    Jun 26, 2012
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    ..is a rectifier and zener as described here.

    Agreed. An LED would be a bad choice for this application.
     
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  9. takao21203

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    It is OK for small 3V relays.

    They don't break down from 3V in reverse and not from 4V, but I would not really use it much for 5V.

    More kind of a curiosity, the LED was old + 20mA type.

    You don't need to worry about a small pulse from a relay coil damaging 1n4148. These even can be used for some dc/dc converters.

    A really large relay maybe is not well suited for 1n4148.
     
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    DING DING DING

    I've actually worked on the suppressors used by this, erm, I mean "some unnamed major relay manufacturer" and they spec'ed in a 36V zener.

    Plus a 1,000 volt diode. Oh and a fuse too. Did I mention the two fly leads?

    Sits on a board .09 by .22 inches.


    One thing to note is when using a zener to increase the discharge voltage this voltage ADDS to the power supply voltage. So if you have a 28V relay and a 36V zener your switch sees 64 volts. Actually a bit more.
     
  11. WBahn

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    Mar 31, 2012
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    Then why did you tell the OP that he could use one for a 12V coil? :confused::rolleyes:
     
  12. ramancini8

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    Jul 18, 2012
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    1N4148 is faster than a 1N400x, so it might cut down on noise better.
     
  13. timescope

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    Dec 14, 2011
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    I prefer to use 1N4001. If this diode short circuits, the transistor will be destroyed attempting to switch the supply to ground.

    Timescope
     
  14. Ron H

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    What do you think would happen if the 1N4148 shorted?
     
  15. timescope

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    Dec 14, 2011
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    Sorry, poor choice of words. I was referring to 1N4148. I do not feel comfortable using a 100mA diode at 40mA for reliability reasons.

    Timescope
     
  16. panic mode

    Senior Member

    Oct 10, 2011
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    :confused:

    why not use alternator diodes, they are rated for few hundred amps and should be able to handle peak current of 40mA :rolleyes:

    1N4148 is a good match for 40mA relay with plenty of margin. it is rated for 100mA continuous, rain or shine. and several times more in pulse operation (few hundred mA). the transient when relay turns off is very short.
     
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  17. timescope

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    Dec 14, 2011
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    Noritsu and Samsung also use alternator diodes :)

    Timescope
     
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