mark said:
hi, i want to make my own lead light for workin on cars etc.... i have
ordered 100 , 10 mm white leds.
what is the best way to connect them up ? in series or in paralell ? i
will be using around 70-90 leds.
As LED's are driven with current rather than voltage, it's generally best to
connect multiple LED's in series, together with a current-limiting
resistor.
The number of LED's you can connect in series depends on the available
supply voltage and the type op LED's used.
A white LED has a forward voltage drop of approximately 3.6 volts, so if you
connect two in series, you need a supply voltage of at least 7.2 volts;
with three in series the required supply voltage is 3 x 3.6 = 10.8 volts
minimum and so on. Also, you have to add a few extra volts for the
current-limiting resistor.
So if you have a 12V supply, and you want to operate each LED at 20mA of
current, these are the calculations:
- You can connect three white LED's in series, resulting in a 10.8 volt
forward voltage drop, with 1.2 volts remaining (12 - 10.8).
- To get 20mA of current with a 1.2 volts voltage requires a resistor of
1.2 / 0.02 = 60 ohms. So you cold use a 68 ohm series resistor.
With 90 LED's in total, this means that you have to make 30 strings, each
composed of three white LED's and a 68 ohm resistor connected in series.
These strings are connected in parallel to the 12V supply, resulting in a
total current of 30 x 0.02 = 0.6A.
For 24 volts, you can double the amount of LED's in each string:
6 x 3.6 = 21.6 volts. To get 20mA you divide once again the remaining
voltage by the desired current: (24 - 21.6) / 0.02 = 120 ohms series
resistor. Now you need only 15 strings of 6 LED's and one resistor each.
The total current drawn is 15 x 0.02 = 0.3A.
I would strongly recommend not raising the voltage any higher. And note that
this is all based on a DC supply. If you have an AC supply, you could
double the amount of LED's per string, with two LED's instead of one per
3.6V voltage step, connected anti-parallel, and approximately half the
resistor value to double the current (as each LED is switched for on only
half the time). Also note that LED's don't take reverse voltages very well.
Richard Rasker