What diode for EMF protection?

Discussion in 'General Electronics Chat' started by Fenris, Sep 14, 2009.

  1. Fenris

    Thread Starter Active Member

    Oct 21, 2007
    288
    2
    Hi all

    I need to pick a diode for a friend to fit in the power line of his relay. This is not the coil supply it is the power line that the relay switches to turn a pair of bulbs on and off.

    They are controlled by audio input and are dynamic in operation. The draw on the line is 4A and the supply voltage is 12VDC. What diode would suit this situation?

    The reason I think we need this is because the supply line is used by 2 other circuits. One is the relay driver circuit which has diode protection against back emf built in to the back end of course but due to the way the supply is configured there is a good chance that back emf from the bulbs operation is shooting through the supply lines in to the front end of both circuits.

    I probably know the answer but I just want to make sure I steer him right.

    regards

    Fenris
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    I may be wrong, but I think a 1N4001 (or one of their series up to the 1N4007). The reason for the qualifier is you may need a higher amperage, but it is the way I would go. Any power diode will work.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Why don't you post a schematic?

    If you want the diodes to suppress EMF from the bulbs themselves, you will need to connect the diodes across the bulbs, cathode towards V+

    A 1N5404 would be good for up to 3A continuous, quite a bit more (up to 200A non-repetitive) peak.

    If you are talking connecting the diodes in series with the bulbs, then you'd need something more like a MUR1635. Those are rated for 16A continuous. The advantage is a low Vf and really high surge current.
     
  4. Fenris

    Thread Starter Active Member

    Oct 21, 2007
    288
    2
    Hi Guys.

    Here is the arrangement of the bits. As I say the sound to light circuit has diodes to deal with the relay induced emf. I hope the pic is good enough to follow :D

    The good news is that the reason I am looking at this is the guys voice mod 'died' and the thought then occured to me about the bulb induced emf. But I have just received a message that it's not dead it is in fact the external power amplifier that has died.

    But this still warrants looking at I reckon. Cheers as ever gentlemen

    regards

    Fenris

    PS it's not my voice mod circuit. It is another variation, and a good one, on a theme.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    A light bulb doesn't induce EMF, but any coil will. You're safe from light bulbs, basically electronically they are a funky resistor.
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello Fenris,

    The relays must have the EMF diodes accross the coils.
    The lightbulbs do not need EMF diodes.
    A lighbulb draws most current when just switched on,
    the current will decrease very soon as the filament heats up.

    Greetings,
    Bertus
     
  7. Fenris

    Thread Starter Active Member

    Oct 21, 2007
    288
    2
    Hi Chaps

    Thanks for putting me right............again :D . I have let my friend know that there is nothing to worry about and he is now hunting for a new amplifier.

    I think that may have been killed by having the volume of the on board amplifier (less than 1W it's an LM386) to high?

    I believe line level is around 1V measured as an AC voltage?

    regards

    Fenris
     
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