Protection Circuit for 12V LED Lamp

Discussion in 'The Projects Forum' started by Kamy_King, Dec 20, 2011.

  1. Kamy_King

    Thread Starter New Member

    May 4, 2011
    I have a circuit that uses a power MOSFET to control the state of a water valve. The valve is hit with an initial 50 VDC pulse (1.5 ms) to ensure that it opens, and then the voltage is dropped to 10 VDC to maintain the valve open.

    I would like to use the same MOSFET output that controls the valve to control a 12 VDC, 41 mA LED lamp (that would show the status of the valve). I am looking for the best way to protect the LED lamp from the initial 50 VDC pulse. Would it be okay to simply use a resistor and a zener diode?

    Any suggestions would be greatly appreciated.
  2. wayneh


    Sep 9, 2010
    You could put a capacitor in parallel with the LED, so that a pulse gets shorted to ground around the LED rather than through it. The LED wouldn't take any current until the cap has absorbed a bit of the initial charge. The cap will need a discharge path - not just the LED - so that it is fully discharged when the valve is off. You don't want repeated "on" pulses before the cap has a chance to discharge.

    Or why not just send the LED current through the MOSFET also?
  3. t06afre

    AAC Fanatic!

    May 11, 2009
    Please tell us more about the LED. 12 volt sound strange if it is a discrete LED. Zener and resistor might work. Perhaps also an inductor can block the 50 volt pulse.
  4. crutschow


    Mar 14, 2008
    A resistor and a zener would also work.

    What type of LED uses 41mA? That's a rather odd value. Most use 20mA.
  5. Kamy_King

    Thread Starter New Member

    May 4, 2011
    Thanks for sharing your ideas. I would like to send the LED current through the MOSFET. However, the 50 VDC pulse is wired to the same output. I have attached a schematic of the relevant parts of the circuit.

    I am looking to use one of those LED light stacks. A single lamp at 12 VDC uses around 40 mA.

    Thanks again for your help.

  6. Wendy


    Mar 24, 2008
    Why not put the LED/resistor/zener circuit across the solenoid? It would be much simpler than making a separate MOSFET circuit, and work as well. Most low power LEDs max out at 20ma, and if you really want long life design for 10ma. They will still be plenty bright.