# LED fuel indicator instead of halogen

I have this problem which is driving me crazy: I want a LED light to indicate when my motorbike is running on emergency fueltank. The system works with a normal halogen bulb, and the system is generating 13,5V whenever is runs on the emergency tank. When the fuel level is above the critical limit, it Will generate ni voltage which is just fine. But, whenever i change the halogen light bulb to s LED with an appropiate resistor it stats on forever and generate 13,5 V ni matter what the fuel level is. It's maybe Nice to know, that whenever the system start it Will turn on the fuel indicator for 2 sec, and turn off again, it's like it's testing for something. Best regards

The LED isn't a heavy enough load to pull the voltage down. Apparently whatever was driving the halogen can "leak" a bit, and the designers thought it was okay because the halogen wouldn't show it.

For a quick solution, you could put a second resistor in parallel with the LED/resistor combination that is roughly the same or slightly higher resistance than the halogen was. Note that halogens, like incandescent bulbs in general, increase their resistance when they get hot, so you might want to account for that by a factor of 2 or 3 from the cold measurement. Also don't forget to handle the power as Watts = Volts^2/Ohms.

Or you could redesign the driving circuit to allow a lighter load.

• Thank you for yout reply. I'm at 1. Semestre at electrical engeneering and tour suggestion sounds very likely. When i get the opppotunity to test it i Will com back here Commented Apr 2, 2015 at 18:36

Something might be providing a small trickle of current, which was not enough to light the halogen but is enough for the LED. This could be the sensor for a failed lightbulb, or if it's not such a fancy bike, perhaps some current leaking through the fuel gauge.

Try connecting a 1 watt 220 Ohm resistor in parallel with your LED+resistor, that might draw enough current to prevent the LED from lighting.

It could also be that the circuit provides a low voltage, not enough to light the halogen but the LED only needs 2 volts or so. Try a 6 V Zener, reverse biased, in series with the LED, and reduce the series resistor accordingly.

Let us know what you find, there might be another way.

My guess that the low-fuel sensor is a NTC thermistor. The light bulb consumes enough current that the thermistor tries to heat up. When it is submerged in fuel, it can't get hot enough to lower its resistance to the point where the bulb glows brightly.

When the fuel level drops below the sensor, it self-heats and the resistance drops. Current to the lamp increases and it illuminates brightly.

You need to provide a similar load as the original light bulb.