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sizing a freewheeling diode for a coil

Discussion in 'Electronic Design' started by Matt, Jun 2, 2007.

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  1. Matt

    Matt Guest

    I am controlling the coil of a relay using a switching transistor. To
    protect the transistor I intend to use a freewheeling diode in parallel
    with the coil. I'm not sure of the issues in specifying the diode.

    My impulse is to simply use a 1N4007
    http://www.fairchildsemi.com/pf/1N/1N4007.html
    (1000V peak repetitive reverse voltage, 1.0A average rectified forward
    current) because it is common, cheap, and seemingly the most heavy-duty
    of the 1N400x line. Would that be a good choice for about any
    PCB-mounted relay?
     
  2. Tam/WB2TT

    Tam/WB2TT Guest

    I just finished a project that uses 12V, 30 ma relays. Used 1N914 s.

    Tam
     
  3. Phil Allison

    Phil Allison Guest

    "Matt"

    ** Normally called a "kick back" diode - IME.

    Freewheeling diodes are associated with DC motor drives and swiching
    regulators.


    ** Long as the diode can pass the same average current need to drive the
    coil and sustain the DC voltage across it - it should be OK.

    The cheapest possible diode are commonly used - like 1N4148s or 1N4001s.




    ......... Phil
     
  4. Eeyore

    Eeyore Guest

    I've known quite small relays to 'take out' 1N4148s, 914s etc.

    Graham
     
  5. The absolute minimum repetitive surge current rating for the
    diode is the steady state current for the coil, under the
    highest supply voltage. If the diode is rated for a
    continuous current equal to or greater than the coil
    current, the brief inductive quench can't possibly overheat
    the die.

    The minimum reverse voltage rating for the diode is the
    highest possible supply voltage. Some extra voltage
    capability seldom costs much. But a lot of extra voltage
    capability may have a down side. High voltage diodes
    generally turn on and off slower than lower voltage devices
    and may recover with a vicious snap that generates high
    frequencies. So a 1000 volt diode is probably not quite as
    good as a 100 volt diode for a 24 volt coil.
     
  6. I hear people say this, but I have never seen a case of it.
    I have used 1N4148 diodes for 200 mA coils for years.
     
  7. I've had it. 1N4148s gone to a short circuit.
    Never found out why.
     
  8. whit3rd

    whit3rd Guest

    It's the energy storage, proportional to inductance and square of
    the current, that matters. A 200 mA coil on a reed switch
    has much lower stored energy than a 50 mA coil on a frame
    relay. When you're using more current than the (average)
    rating on your diode, it only succeeds if the temperature
    spike doesn't melt anything.
     
  9. ESD? Were the contacts connected to "external" signals?
     
  10. But the diode will still only see the 50mA current, even if the stored
    energy is *enormous*. It will just take a bit longer for that current
    to decay when the driver is switched off (and this will be independent
    of the diode rating).
     
  11. [/QUOTE]
    They were multiplexing resistor values to simulate
    a thermistor, into a unit that was about 3ft away.

    The relay coils were 24V/1k. I still don't see
    any reason why 3 1N4148s in a bank of 8 should
    all go s/c, but they did. The only reasonable
    explanation was possibly a bad batch of diodes.
     
  12. Fred Bloggs

    Fred Bloggs Guest

    If the switch transistor recovers very quickly and the current is large
    enough, it may be possible to fail these ss diodes with hot spot
    development. I have observed over 2V forward voltage development lasting
    for upwards of 75ns at moderate currents this way.
     
  13. Jamie

    Jamie Guest

    maybe you should be using faster switching diodes?
    also, I do know if the relay chatters it can heat up
    diodes.
     
  14. Jamie

    Jamie Guest

    If you're not worried about noise that maybe fine how ever, my self,
    I like to use low voltage diodes which tend to switch faster.
     
  15. But the temperature rise in the diode is not proportional to
    the stored energy, but is related to how fast that energy is
    dumped into the diode. A very large inductance energized at
    50 mA may take a long time to discharge, but that gives a
    long time for heat energy to escape the die, also.

    By the way, 200 mA is not higher than the average current
    rating of the 1N4148. See "IF" on page 3:
    http://ronja.twibright.com/datasheets/1N4148_1N4448_5.pdf
     
  16. Jamie wrote:
    (snip)
    I think this can be a problem if the coil is driven by
    another relay contact, rather than a solid state switch. If
    a dry contact re closes while the diode is conducting (as
    during contact bounce), there can be a rather large reverse
    current spike.
     
  17. Fred Bartoli

    Fred Bartoli Guest

    John Popelish a écrit :
    But the relatively low frequency involved by the driving relay will keep
    that average power really low. Also this will suppose that the coil
    current hasn't decayed down to zero which is unlikely a normal working
    condition as well for an all relay system.
     
  18. My point was that the most stressful moment n the life of a
    coil diode might occur with a dry contact driving the coil.
    At contact release, if there is not a clean break, the
    first opening diverts full coil current through the diode,
    but as the contacts slide apart, there can be a very abrupt
    re application of supply voltage, in reverse, across the
    conducting diode. This can cause a very strong reverse
    recovery current that dumps, not well defined coil current,
    but essentially unlimited supply current, through the die.
     
  19. ekrubmeg

    ekrubmeg Guest

    Matt. a high voltage diode is unnecessary because your high voltage
    spike from the coil is in the forward direction so it only needs to
    cover your working voltage. Like some have suggested, I don't think
    you need a 1A average but again I would be afraid to use a 914, I
    always considered that a signal diode, I didn't even know it was still
    available.
     
  20. ian field

    ian field Guest

    An application note from a relay manufacturer that I read a while ago
    suggests a small signal diode in series with a zener.

    Apparently if just a diode is used, the emf due to collapsing field can pass
    enough current round the diode/coil circuit to cause faltering contact
    separation and contact burn. There are other serious issues but I'd have to
    search out the appnote to remind me what they are.

    The zener should be wired so it would forward conduct with the transistor on
    and the small signal diode in series pointing the other way to prevent that
    happening, a guesstimate of Vz might be about 60% of the transistor's
    breakdown voltage.
     
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