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Overcurrent protection, what is a cheap and simple way

Catwell

I hope this message gets read....

 

 

 

In a circuit I am working on, I am powering two pins at the same time. These pins are powering a single LED.

When I power both pins I am doubling the current powering an LED.

 

What I need to do is protect the LED from excess amperage. What suggestions do you have? I know there are many ways of doing the same thing, I just want to collect them all.

 

Any help would be appreciated.

 

Cabe

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  • You really should have a specification for what the input voltage is.  Making assumptions is best kept to where you have experience with the application, otherwise it's the road to failure.

     

    There are sooo... many ways this could be accomplished, however it's also easy to pick the wrong approach without knowing enough information.  If you could find out what the input voltage was, you might be able to get away with two diodes (cathodes wired together) and a 620ohm 1/2W resistor.  Giving a solution with a nominal current of 19mA.  If the supply goes to 16.5V, the current would be 22.2mA, or 15.7mA at 11.5V.  Assuming you using a red LED.  That sort of variation isn't going to effect the brightness on a typical LED much.

     

    That's the lowest component count and footprint size.  Least complicated and least expensive.  You could even use a SOT23 with dual diode, common cathode.

     

    If the LED current needs to be more accurate, or the voltage swing on the inputs is higher, then we would need more circuitry.  This could be in the form of a zener diode to clamp the voltage.  Or it could be with a controlled current source.  But depending on the voltage, you may or may not need an additional resistor and you may also need something bigger than a SOT23 to dissipate the power.  If your circuit had a regulator somewhere, perhaps this could even be used.

     

    It really comes down to needing more information.  It would be a waste of time to go through and cover all bases, or direct you with a circuit that is otherwise overkill and may not fit etc... Otherwise I'd be here for an hour hahaa...

  • in reply to Former Member

    The easiest thing to do is add a Transient Suppression DiodeTransient Suppression Diode on each input. I plan on insisting to do so.

     

    I believe I can get these is small packages. Heat is an issue, but the whole board will be attached to a heatsink. I also have a few other components in the design that get incredibly hot, dissipating the excess power.

     

    Cabe

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