View Full Version : Current Limiting Resistors to LED Ratio

07-19-2006, 02:23 AM
I have an attached schematic.

It is a Voltage Source that supplies 250mA.

There are 32 LED's attached to it.· I would like to limit the number of resistors on the circuit.· The resistors that I am using are 1/4W resistors.· With 32 LEDs on 250mA, Each LED will only be using about 7mA.

With this low of current on Each LED, and 1/4w resistors, shouldn't I be able to use just 1 resistor for say 8 LEDs?

I have attached the schematic showing 32 LEDs and 8 resistors (2 resistors for every group of 8).·

What do you guys think?· Am I pushing some Power boundaries, or does this look OK?

Sorry about all of the questions today.·


Mike Green
07-19-2006, 02:37 AM
The problem with this scheme is that there is some variation in the forward voltage drop in LEDs from unit to unit. If you connect two (or more) LEDs in parallel with a single series resistor, one LED may take most of the current. There will be substantial variation in brightness and, in cases where there are many LEDs in parallel or the currents are large, one LED may take enough current to overheat or destroy itself. Stick to one LED per resistor. If you have a high enough voltage, you can put several LEDs in series with an appropriate current limiting resistor. That works. It's even more efficient since more of the power is being used to produce light rather than just go to heat. Most red LEDs have a forward voltage of 1.3-1.7V and you could easily put 3 in series on a 6V supply. White and blue LEDs have the highest forward voltage (around 3.1V). You could easily run 3 in series off a 12V supply for example.

07-19-2006, 03:56 AM
It's okay to put LEDs in series, but not in parallel.
You don't say what voltage the power supply is ???


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