...and why they might not work as you expect. I am here referring to after-market LED indicators.
Recently, I had a conversation with a member about the operation of after-market indicators, particularly when their bike was stopped (ie engine not running).
The scenario was that the LED indicators worked fine with the engine running but were "misbehaving" when the engine was stopped. The important thing here is to remember that the voltage of a bike's electrical system will be higher with the engine running than with it stopped providing, of course, that everything else it working as it should (alternator, reg/rec and battery).
Now, LEDs are curious things and not quite so simple as they first appear. To start with LEDs require the current through them to be regulated. If the current is too high there is the very real danger of blowing the LED completely or seriously shortening its working life. The other oddity is that LEDs have a fixed voltage drop across them that is dependent on their colour - I kid you not. (and for readers of Terry Pratchett the reason is "quantum" - honestly).
So, how does this affect our LED indicators? To start with the voltage drop across the LED - for Orange LEDs this is 2 volts. What this means in practice is that we can put a number of LEDs in series across a supply. If we put 6 LEDs in series, then the total voltage drop across them all will be 12 volts - nice and handy that figure, don't you think?
Now the other thing we need to do is to restrict the current through the LEDs and we can do this with a resistor of a suitable size. But hang on a minute, if we have 6 LEDs in series we already have a voltage drop of 12 volts so putting a resistor in series with the LEDs will cause problems, no?
Not really no (well, for now anyway). Since we know that the bike's "12 volts" is actually higher than a true 12 volts, then the "spare" voltage will be across the resistor. Let's take a realistic running voltage of 14.6 volts and work with this. So, we have 12 volts across our 6 LEDs which leaves us with a "spare" 2.6 volts across the limiting resistor. If we want to limit the current to, say, 20 mA (which is a very common current for a high-brightness LED) we will need (using Ohms Law) 2.6 Volts divided by 0.02 Amps (or 20 mA - same thing) which will give us 130 Ohms.
So, we wire our 6 LEDs in series with our 130 Ohm resistor and we have our LED indicator. Job done!
Not quite. We have assumed that the supply voltage will be a healthy 14.6 volts, which is fine for a bike that is running - but what happens when the bike is not running? What happens is that the supply voltage will drop to nearer the standing battery voltage of around 12.4 volts or so (this depends on whether there is any other load on the battery, but we won't concern ourselves with that just now). Now we know that the LEDs have to have 2 volts across each one for them to work properly - and that will now leave us with just 0,4 volts across the limiting resistor. What effect does this have? Well, it means that the brightness of the LEDs will be seriously affected as the current through them will be a lot less than the 20 mA we calculated earlier, in fact it will be 0.4 volts divided by 130 Ohms = 3 mA!
Which is what the problem was that I mentioned at the start. The indicators worked fine with the engine running (nice healthy 14.6 volts) but not when the bike was not running (standing battery voltage of 12.4 volts or so). The problem is due to the simplistic current limiting achieved by using a fixed resistor - which is what you will find supplied with most after-market LED indicators.
Now, you would be correct in thinking that by using just 5 instead of 6 LEDs in series that the problem can be overcome and this would be true, but only up to a point. You always have to calculate the size of the current limiting resistor at the highest voltage you will encounter, and any lower voltage will always see a reduction in the brightness of the LEDs no matter how many there are.
Any questions?
Recently, I had a conversation with a member about the operation of after-market indicators, particularly when their bike was stopped (ie engine not running).
The scenario was that the LED indicators worked fine with the engine running but were "misbehaving" when the engine was stopped. The important thing here is to remember that the voltage of a bike's electrical system will be higher with the engine running than with it stopped providing, of course, that everything else it working as it should (alternator, reg/rec and battery).
Now, LEDs are curious things and not quite so simple as they first appear. To start with LEDs require the current through them to be regulated. If the current is too high there is the very real danger of blowing the LED completely or seriously shortening its working life. The other oddity is that LEDs have a fixed voltage drop across them that is dependent on their colour - I kid you not. (and for readers of Terry Pratchett the reason is "quantum" - honestly).
So, how does this affect our LED indicators? To start with the voltage drop across the LED - for Orange LEDs this is 2 volts. What this means in practice is that we can put a number of LEDs in series across a supply. If we put 6 LEDs in series, then the total voltage drop across them all will be 12 volts - nice and handy that figure, don't you think?
Now the other thing we need to do is to restrict the current through the LEDs and we can do this with a resistor of a suitable size. But hang on a minute, if we have 6 LEDs in series we already have a voltage drop of 12 volts so putting a resistor in series with the LEDs will cause problems, no?
Not really no (well, for now anyway). Since we know that the bike's "12 volts" is actually higher than a true 12 volts, then the "spare" voltage will be across the resistor. Let's take a realistic running voltage of 14.6 volts and work with this. So, we have 12 volts across our 6 LEDs which leaves us with a "spare" 2.6 volts across the limiting resistor. If we want to limit the current to, say, 20 mA (which is a very common current for a high-brightness LED) we will need (using Ohms Law) 2.6 Volts divided by 0.02 Amps (or 20 mA - same thing) which will give us 130 Ohms.
So, we wire our 6 LEDs in series with our 130 Ohm resistor and we have our LED indicator. Job done!
Not quite. We have assumed that the supply voltage will be a healthy 14.6 volts, which is fine for a bike that is running - but what happens when the bike is not running? What happens is that the supply voltage will drop to nearer the standing battery voltage of around 12.4 volts or so (this depends on whether there is any other load on the battery, but we won't concern ourselves with that just now). Now we know that the LEDs have to have 2 volts across each one for them to work properly - and that will now leave us with just 0,4 volts across the limiting resistor. What effect does this have? Well, it means that the brightness of the LEDs will be seriously affected as the current through them will be a lot less than the 20 mA we calculated earlier, in fact it will be 0.4 volts divided by 130 Ohms = 3 mA!
Which is what the problem was that I mentioned at the start. The indicators worked fine with the engine running (nice healthy 14.6 volts) but not when the bike was not running (standing battery voltage of 12.4 volts or so). The problem is due to the simplistic current limiting achieved by using a fixed resistor - which is what you will find supplied with most after-market LED indicators.
Now, you would be correct in thinking that by using just 5 instead of 6 LEDs in series that the problem can be overcome and this would be true, but only up to a point. You always have to calculate the size of the current limiting resistor at the highest voltage you will encounter, and any lower voltage will always see a reduction in the brightness of the LEDs no matter how many there are.
Any questions?