Diode value

Discussion in 'The Projects Forum' started by John Locke, Sep 19, 2014.

  1. John Locke

    Thread Starter New Member

    Sep 19, 2014
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    I intend to use a communicator in conjunction with a burglar alarm. This is, obviously, not integral to the alarm system, otherwise an issue would not arise. The alarm manual says "It is recommended that a back EMF protection diode is used in parallel with the relay coil.". Question is, how do I determine the value of the diode? The communicator spec says the supply is "11 VDC – 14 VDC (12 VDCnominal) @ 70 mA standby, 200 mA maximum." You would think that the supplier of this device would know the answer, but they do not.
     
  2. wayneh

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    Sep 9, 2010
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    So you are using a relay to turn on the communicator, and that relay is to be activated by the existing alarm system?

    If that's right, you need to know about the current spec for the relay coil. (The communicator specs are irrelevant to this issue.) I believe the diode should be rated to at least 2X more current than the relay coil current, and to at least 2X the nominal voltage. So for instance if the relay draws 50mA at 12V, I'd want a diode rated to 1A and 50V. Grossly over-rated, but these things cost just pennies. Look at 1N4007, for instance.
     
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  3. SgtWookie

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    Sounds to me like you are leaving out some details. Since the alarm manual talks about the back-EMF diode, the relay must be a part of the alarm? Any of the 1N400x series diodes could be used, but you need to show some more details before we can be sure we're advising you correctly. The diode goes directly across the relay coil, with the cathode end (the end with the band) goes towards the more positive supply voltage.

    (Edited: typo 2N400x -> 1N400x )
     
    Last edited: Sep 23, 2014
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  4. faley

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    Aug 30, 2014
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    Sounds like a clamp. If that's the case, A 1N914 will be OK.

    (Sorry wayneh, I missed part of your post- yep a 1N4007 would probably work fine. The 914's tend to be my "goto" in these cases.)
     
    Last edited: Sep 19, 2014
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  5. #12

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    It depends on what you picked up at the surplus store. 1N914 and 1N4148 are good for 150 ma. The 1N4000 series is good for 1 amp. They all work pretty well on small relays.
     
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  6. SgtWookie

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    I've had 1N914's/1N4148's fail when used as reverse-EMF diodes, even when used within their rating. I'd rather have some overkill here and have it not break.
     
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  7. MrChips

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    I would never have thought of using 1N914 or 1N4148 as snubbers.
    I would go with 1N4007 to be safe.
     
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  8. faley

    Member

    Aug 30, 2014
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    Absolutely. I agree with both you and wayneh. Bigger is better. As for myself, I've had good luck with Fairchild 1N914's. I used them on relays driven by PLC's and robot controllers. In these cases the relays are typically of the Idec ice cube variety- SPDT and DPDT for the most part. Contact ratings run between 3 to 5 amps. Hence, the coils aren't substantial. Inrush is pretty low and when the field collapses the amplitude of the corresponding spike is minimal and short-lived. I believe-if memory serves me- when I worked with larger relays the 1N4002's worked well. But, don't quote me on that one. It's been awhile.
     
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  9. MaxHeadRoom

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    Jul 18, 2013
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    When using a Picmicro driven DIP reed relay, I use the smaller 1N4148, anything bigger I have a stock of 1N4007's.
    Max.
     
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  10. John Locke

    Thread Starter New Member

    Sep 19, 2014
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    Many thanks for your reply. To clarify further, here is an excerpt from the alarm manual:

    "OUTPUTS (J3): These outputs are programmed as active low output. They are held at 13V and fall to 0V
    when active, it can source or sink 10mA.

    These outputs would normally be connected directly to the input channels of wire in type communicators.
    Alternatively each output can be used to drive a relay (coil resistance > 1200 Ohms ) connected between the
    output terminal and the 13V supply terminal. The relay will energise when the output port operates. It is
    recommended that a back EMF protection diode is used in parallel with the relay coil."

    Reading this more carefully, it looks as if the communicator can simply be attached to the output, and that a relay is only needed if it is used for some other purpose. Would you agree with that? In any case. the clarification on the relay question is helpful, since I might conceivably use the output for something else. I shall have to study what you have said in conjunction with the other replies below, but if you have any further information, it would be welcomed.

    Just a further point of clarification, the reason for using a non-integrated communicator is current constraints, since the communicator is separately powered.
     
  11. #12

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    200 ma (post #1) exceeds the limits of the small signal diodes 1N914 and 1N4148. Use the 1N4000 series diodes if you attach a relay. Anything from 1N4001 to 1N4007 will be reliable. They all cost a few pennies each and overkill is a good thing in this case.

    ps, typo in post #3. Wookie called them, "2N400x series". They are 1N devices.
     
    Last edited: Sep 20, 2014
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  12. MaxHeadRoom

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    1200 ohms is only 10ma at 13v, the DIP style reed relays I use the 1n4148 with are typically 10ma-12ma coil, maximum.
    http://www.mantech.co.za/datasheets/products/EDRXXX.pdf
    Max.
     
    Last edited: Sep 20, 2014
  13. faley

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    Aug 30, 2014
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    If someone already answered this point- my apologies folks. (I'm finding that my eyes are once again changing, for the worse of course.)

    You'll need a relay if the load exceeds the rating of the output or if, for some reason, the circuitry between the two is incompatible.

    Of course, if you go with a relay, the clamp diode is sized according to the relay coil. 10 mA isn't much to work with, especially when it's the peak.
     
  14. wayneh

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    Sep 9, 2010
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    I can't really say. Do you have more details on the communicator? Your earlier post implied that it takes quite a bit more than the 10mA available, and thus needs power from somewhere else. Using a relay could control that other power source. But maybe you leave the communicator powered all the time and use the pull-down from the controller as an input signal? Sorry, I just don't even know what a "communicator" is or how they work.
     
  15. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Sgt. Wookie....good to see you again! Have you been TDY?
     
  16. SgtWookie

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    Thanks, nice to "see" you too :) Actually, I DO post occasionally; just not very much - I've been very busy lately - and after not doing anything with radio for many years, I took/passed the FCC exam for the Technician level today ... took the General with practically no studying at all (one practice test) and missed that by a couple ... no biggie (there were some really odd questions which came up on test1-1 ... so, going to dally a while on hamstudy.org and re-take it in a month. I would've re-taken it right then and there, but didn't think to bring another $15 cash with me; have gotten too used to plastic. :rolleyes:

    1chance has gone full-on crazy cat lady; we have our own kitties and then a dozen kittens and several semi-ferals - it can be pretty entertaining, but lots of clean-up needed constantly, as they leave a trail of rubble behind them... take care!
     
  17. KL7AJ

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    Congratulations on your ham ticket! I trust you're aware that I'm a co-author of the W5YI/Gordon West study materials. Gordo's coming up to visit in December. :)
    Eric
     
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  18. John Locke

    Thread Starter New Member

    Sep 19, 2014
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    Thanks, will do that if I actually get around to fitting a relay.
     
  19. John Locke

    Thread Starter New Member

    Sep 19, 2014
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    Thanks for the info...
     
  20. John Locke

    Thread Starter New Member

    Sep 19, 2014
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    A communicator (in this context) is an auto-dialler used with a burglar alarm system to dial pre-programmed telephone numbers. Actually, I've realised that I can use a different output channel for that which does not need the relay. I might still use the relay-dependent output channel for something else, but that is in the future; some of the other posts have given pointers for that. In any case, I seem to have managed to stimulate a useful discussion.

    Thanks again everybody. What would we do without the internet? Can't help remembering how about fifty years ago I bought ham radio magazines.
     
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