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Picking the correct diode

Discussion in 'General Electronics Discussion' started by schmidtbag, Dec 23, 2012.

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  1. schmidtbag

    schmidtbag

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    Nov 8, 2012
    I am driving a motor at 24V 6A and want to control the speed of it using PWM (from a microcontroller) and the NTD5867NL-1G. The problem is preventing EMF from damaging the transistor. I'm aware a diode (I believe suppressor diodes to be more specific) are what I need to solve this problem.

    However, I'm not sure which one I need. Diodes seem to have maximum, continuous, and/or average current ratings. I'm not sure which I need to focus on, but I've noticed the ones with the high continuous ratings seem to be obscenely expensive or more than 2 pins.

    Does anyone have a recommendation as to what I can use?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I'd probably go a 1N5404 if the PWM frequency is in the audible range.

    If it's higher in frequency, maybe a 1N5822
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    I'd recommend a Schottky diode rated for at least as much current as the motor draws, and safely more than the maximum supply voltage. It's probably a good idea to add a small capacitor e.g. 10 nF in parallel with the diode. These diodes are all under USD 1.00 and available in one-off quantities from Digikey.

    SR1045 10A 45V cylindrical USD 0.79: http://www.digikey.com/product-detail/en/SR1045-TP/SR1045-TPMSCT-ND/2334476

    MBR735 (Vishay) 7.5A 35V TO-220 USD 0.83 http://www.digikey.com/product-detail/en/MBR735-E3/45/MBR735-E3/45GI-ND/2153276

    80SQ045 (ON Semi) 8A 45V cylindrical USD 0.83 http://www.digikey.com/product-detail/en/80SQ045NG/80SQ045NGOS-ND/1475457
     
  4. schmidtbag

    schmidtbag

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    Nov 8, 2012
    hmm the SR1045 seems nice, thanks. I've heard about the capacitor thing but isn't that more for reducing noise rather than protect a current backflow? Also, I noticed the diodes steve mentioned used considerably lower average amperages. This is exactly one of the reasons why I posted this question, because I don't know if the super cheap models he posted would work just as effectively, if at all. Kris' models seem to operate at a range I feel much more comfortable about but I'm not sure if the average rectified current really matters when the motors are drifting. I guess the problem is if the motors continue to drift for a few seconds.
     
    Last edited: Dec 23, 2012
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Kris' suggestions are better. Those he suggests can handle he peak current with ease. The choices I suggested were based on the average current over a cycle.

    The type of diode selected is not going to change the current.
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    I suggested the capacitor in parallel with the diode to protect the diode from voltage spikes from parasitic inductances in the wiring. Schottky diodes can't handle overvoltage and can be damaged by voltage spikes. If you use one, use a good quality capacitor and connect it directly across the diode.
     
  7. schmidtbag

    schmidtbag

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    Nov 8, 2012
    Ok I'll keep that in mind. Thanks for all the input, I really appreciate it.
     
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