led indication of 12vdc heater element state

Thread Starter

herrmnnn

Joined Aug 1, 2012
3
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.
 

Thread Starter

herrmnnn

Joined Aug 1, 2012
3
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:

mcgyvr

Joined Oct 15, 2009
5,394
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.
 

wayneh

Joined Sep 9, 2010
17,496
Will this circuit work?
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.
 

Thread Starter

herrmnnn

Joined Aug 1, 2012
3
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.
 
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