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Flyback Diode Implementation/Sizing w/ Relays/Switches

  1. Mar 24, 2009 #1
    I searched but had no luck. Sorry if this kind of thing has been answered a bunch.

    I'm designing a circuit for my car. If you wanna know more just ask and I will explain. I want to know when I need to use diodes to protect stuff. I have attached a wiring diagram.

    I know I need diodes D1 and D2 because those relays are being switched by a computer.

    Latching and Latching2 are single coil latching relays with a coil rating of 25mA 12VDC.

    Do I need D3 and D4? Those relays are activated by a physical switch with a rating of 5 amps but will only have 150mA going through. K1 and K2's coils are rated at 150mA 12VDC.

    Do I need another diode between the contacts of relay K2? Because of the starter solenoid?

    And now on to sizing. I have no idea where to start. I read Schottky diodes are what I want because of the whole .2V thing instead of .7V. I started to look on digikey, but realized I was in over my head. I pretty much just learned about diodes and relays within the last few days, so there is still much to learn. If you could give me some rules to follow (if there are any) when selecting diodes in case my specs change, that'd be great.

    These are the specs I know.
    Latching/Latching2 = Coil - 25mA 12VDC. Contact 10A 250VAC (won't see more than 150mA 12VDC)

    K1/K2 = Coil - 150mA 12VDC. Contact - 70A 12VDC (won't see more than 40A)

    Thanks in advance.

    Attached Files:

  2. jcsd
  3. Mar 24, 2009 #2


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    Staff: Mentor

    It's good hygene to put the catch diodes across coils that can kick back when switches or transistors are opened. Just use a jellybean medium power diode (Schottky definitely not needed), like the 1N4002:


  4. Mar 24, 2009 #3
    Alright, cool. Thanks. Can I use the 1N4002 to go across the solenoid contacts as well, or would that not be able to handle the current? The solenoid draws somewhere between 20 and 40 amps. How do I know how many amps and volts I need a diode to be rated at for a certain application? Thanks, again.
  5. Mar 24, 2009 #4
    You know, 'fly back' current is a misnomer. It's really fly forward current. Once the current is flowing around an inductor, it will tend to maintain in the same direction. The mechanical equivalent of an inductor is a flywheel. And 'kick back' should be called 'kick forward'.
  6. Mar 24, 2009 #5
    Now you're just trying to confuse me. I do understand it, though. Any input as to what diode I need for the starter solenoid?
  7. Mar 25, 2009 #6
    Sorry, your schematic doesn't display well for me. I have no idea what's on the contact side of your relays, which would determine if you needed anything across the contacts.
  8. Mar 25, 2009 #7
    It's a relay who's coil is connected to 12v and a momentary switch. The contacts close the positive line to a solenoid on the starter motor of the car. From what Berkeman said, I do need a diode across the solenoids terminals, but I don't know how large. The solenoid will draw between 20 and 40 amps at 12VDC. I assume a 1N4002 would not suffice.
    Last edited: Mar 25, 2009
  9. Mar 25, 2009 #8
    A 1N4002 does seem a little small. For something like this, there's no clear answer. There are two parameters you want to meet. Your peak nonrepetative current is about 12*40 amperes. The diode has to absorb the stored energy in the inductance.

    Manufactures will rate max. peak nonrepetative current over a half cycle of the AC line, so about 8.3 msec. If we assume a forward voltage drop of 1.2 volts for 8.3 msec, and a 60 amp. peak nonrepetative forward current, the part can handle 0.6 Joules of energy in one pulse, though shorter pulses require increased rating.
    Last edited: Mar 25, 2009
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