UK Monster Owners Club Forum - View Single Post - Warning light is dim after fitting led indicators.

spuggy · 01-08-2020, 06:42 PM

Incandescent bulbs are just a coil of wire. A little heater coil that glows white-hot. They don't care which direction the current is flowing, they work either way round. They have a fairly high resistance - which is converted into heat as well as light.

Many old indicator circuits are designed around using bulbs; as these aren't polarity-sensitive at all, as soon as you provide current with a path to ground, it's "job done"...

An LED is just a diode that happens to emit light (they actually generate small amount of electricity if you expose them to light as well, but usually not enough to be useful)

The voltage the LED itself wants is determined by the doping of the substrate - and thus the colour they produce. White LEDs only work with 3V, red ones want 1.9V, UV ones 4.1V. They either have a current-limiting (aka "ballast") resistor built-in to the package to allow it to be directly connected to a 12V circuit, or sometimes multiple - eg 4 - LEDs are wired internally in series in the package.

LEDs only allow current to flow in one direction. Even with the ballast resistor, they have a very low resistance to current flow in this direction - which is why they draw so little power.

(A 2W bulb, at 12V, draws 160ma, and a 3W bulb 250ma. A typical white miniature LED uses 10ma - and probably brighter than a 10W bulb. It'll generate a little warmth in the ballast resistor - but still a lot less power overall than a bulb.)

However, current flow in the other, "wrong", direction, with an LED has a very, very, high resistance - think of it as infinite. Much, much more resistance than a bulb. That's the "diode" part of "light emitting diode".

With non-LED flasher relays and no load resistors, the relay hyper-flashes because the the forward resistance of the LED - even with the ballast resistor - is nothing compared to a bulb. So it behaves as though a bulb is out.

If you resort to load resistors, you pretty much throw away the "low current draw" advantage of LEDs (the load resistor just turns the extra current into heat), but you do get to keep the "I don't blow bulbs", "they're brighter" and "they respond almost instantly" benefits.

01-08-2020, 06:42 PM	#8
spuggy Registered User Join Date: Apr 2016 Location: Farnborough Bike: M900sie Posts: 171	Incandescent bulbs are just a coil of wire. A little heater coil that glows white-hot. They don't care which direction the current is flowing, they work either way round. They have a fairly high resistance - which is converted into heat as well as light. Many old indicator circuits are designed around using bulbs; as these aren't polarity-sensitive at all, as soon as you provide current with a path to ground, it's "job done"... An LED is just a diode that happens to emit light (they actually generate small amount of electricity if you expose them to light as well, but usually not enough to be useful) The voltage the LED itself wants is determined by the doping of the substrate - and thus the colour they produce. White LEDs only work with 3V, red ones want 1.9V, UV ones 4.1V. They either have a current-limiting (aka "ballast") resistor built-in to the package to allow it to be directly connected to a 12V circuit, or sometimes multiple - eg 4 - LEDs are wired internally in series in the package. LEDs only allow current to flow in one direction. Even with the ballast resistor, they have a very low resistance to current flow in this direction - which is why they draw so little power. (A 2W bulb, at 12V, draws 160ma, and a 3W bulb 250ma. A typical white miniature LED uses 10ma - and probably brighter than a 10W bulb. It'll generate a little warmth in the ballast resistor - but still a lot less power overall than a bulb.) However, current flow in the other, "wrong", direction, with an LED has a very, very, high resistance - think of it as infinite. Much, much more resistance than a bulb. That's the "diode" part of "light emitting diode". With non-LED flasher relays and no load resistors, the relay hyper-flashes because the the forward resistance of the LED - even with the ballast resistor - is nothing compared to a bulb. So it behaves as though a bulb is out. If you resort to load resistors, you pretty much throw away the "low current draw" advantage of LEDs (the load resistor just turns the extra current into heat), but you do get to keep the "I don't blow bulbs", "they're brighter" and "they respond almost instantly" benefits.