led indication of 12vdc heater element state

Discussion in 'The Projects Forum' started by herrmnnn, Aug 1, 2012.

  1. herrmnnn

    Thread Starter New Member

    Aug 1, 2012
    I am putting grip heaters on my motorbike, and want to add an led indicator to show if they are on high or low or off.

    The heaters have a resistance of 7.5 ohms each, running at 12 volts DC from the bikes charging system/battery. I will wire it into an on-off-on DPDT rocker switch so that on low the heaters are connected in series, and on high in parallel. Simple enough.

    What I can't work out is how to wire leds into the circuit to show the high/low state. Ideally I'd like a single tricolour led (eg red for high, green for low) but if it's much simpler using two leds then so be it.

    It seems I can't wire the leds in series with the heaters as the necessary resistors will limit the current and therefore the heat produced, which defeats the purpose.

    Is there a simple way to do this? Keeping in mind that it will be on a bike and exposed to moisture and vibration, so ideally any components could be potted into epoxy or something.
  2. herrmnnn

    Thread Starter New Member

    Aug 1, 2012
    Will this circuit work?


    Will the heater element still get most of the power, less the small amount of current to drive the led?

    Do I need the diode to protect the led from reverse voltage?
    Last edited: Aug 2, 2012
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    No diode needed..
    make sure you use a 1/2W resistor
    A 3 leg bi-color LED can be used for your high/low indication. One 1 resistor is needed if you go that way on the common leg.
  4. wayneh


    Sep 9, 2010
    Yes, but you don't need a diode (the LED is a diode, anyway), as mcgyvr has noted. He recommended a 1/2W resistor because, at the LED current of 20mA, power dissipation is I^2•R = 0.02A^2•560Ω = 0.224W. You don't want to run anywhere near the resistor's rating on a continuous basis, hence the factor of 2 increase in rating, to get headroom.

    Another (better, IMHO) approach would be to use 1K resistors. This will drop the current - allowing the LED to last longer - but the LED will still be plenty bright as an indicator, and you could use a 1/4W. Try it to see if you like the results

    Of course you still need to work out the logic of your switch.
  5. herrmnnn

    Thread Starter New Member

    Aug 1, 2012
    I found a few circuits on the net with reverse voltage protection diodes (the leds I looked at had a reverse voltage threshold of 5v), but I couldn't see how this circuit would need it unless something external to it failed (reversing the polarity of the entire circuit). In which case I would have bigger problems than a burnt out led...

    Thanks for the advice on resistors - I don't want it too bright so I'll order 1k resistors as well. The bike runs around 13.5 volts anyway (should have put that in the diagram sorry), so a bit bigger will be better.

    Switch wiring, relays etc is easy just for some reason I couldn't my head around this led part - all clear now.

    Thanks again.