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Diode Type

Discussion in 'General Electronics Discussion' started by Electricflow, Dec 5, 2011.

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  1. Electricflow


    Dec 5, 2011
    I have two different power supplies that will power a fan (call one manual and one auto). The manual power can be turned on via a switch. I am using two Normally Closed relays on the Auto power supply so that when the switch is flipped for the manual power it opens the relay disconnecting the auto power

    However in this set up when the auto power is on it runs through the fan to the switch power wiring and closes the relays!

    I can either fix this by adding two more relays of the normally open kind on the manual power side. or isolate the manual power side using diodes.

    My question is what type of diodes would I need? The Voltage that would pass through on the manual side is about 28 volts running at about 4 amps with possible 7 amps on start up. The voltage is we need to block would be the same.

    Any other set up ideas would be appreciated !
  2. davelectronic


    Dec 13, 2010
    Hi Electricflow.
    Welcome to you.
    I would think adding more relays is a bit much especially two more.
    Not that i know every diode in existence, but blocking a reverse voltage of 28 volts at a possible 7 amps is a lot, not saying it cant be done, it probably can, you can get some very heavy duty diodes out there, but if you take that approach read the data sheet clearly first, and as far as i know some diodes can pass a tiny bit, even reveres biased. :)
  3. Electricflow


    Dec 5, 2011
    Thanks for the welcome.

    Finding Normally closed SSR relays that could handle 7 amps was hard enough. Finding the diode might be equally as hard.

    Anyone know if they make a COTS Smart power Switch that could do this for me?
  4. davenn

    davenn Moderator

    Sep 5, 2009
    can you draw up a schematic of the circuit you are playing with so we can get a better idea of what you are trying to achieve

    that way we may be able to offer other suitable suggestions :)

  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    There are a bazzilion diodes that have ratings like that. Here is just the first which matched those specs.

    Note that I haven't read exactly why you want it, I've just pointed you to a diode that can handle up to 7.5A and block 28V (it's a 12A 45V part)
  6. Electricflow


    Dec 5, 2011
    Here is a crud drawing of what I am attempting to do


    The SSR are 3.5 - 40 VDC input and are Normally Closed


    The diode you linked too will it handle 30Vdc at 7 Amps passing through with the Max Wattage going through it of 210 Watts?
    Last edited: Dec 6, 2011
  7. jackorocko


    Apr 4, 2010
    Are the diodes gonna be forward or reverse biased?
    .550V * 7A = ~4W will be dissipated in the diode AFAIK. Remember current that doesn't flow is not dissipating any power if you are using the diode to block current flow.
    Last edited: Dec 6, 2011
  8. Electricflow


    Dec 5, 2011
    That makes sense, So about 4 watts of power dissipating. Can the diode handle this? I assume so because of the spec sheet saying .55V at 12 amps meaning it can handle at least 6.6 watts? I did not see any Power dissipation figures on the spec sheet.

    Maybe I should try something that I can heat sink easier? --->

    Thanks for the replies
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