LED Dimmer Circuit

This circuit will adjust the brightness of one or more LEDs from 5% to 95%.

                          Power supply: 5 volts
                          RP = 47 Ohms
                          Max num of rows: 33
                          Max num of LEDs: 33

                          Power supply: 9 volts
                          RP = 68 Ohms
                          Max num of rows: 33
                          Max num of LEDs: 66

                         Power supply: 12 (13.5) volts
                         RP = 47 Ohms
                         Max num of rows: 26
                         Max num of LEDs: 78

                         Power supply: 15 volts
                         RP = 20 Ohms
                         Max num of rows: 26
                         Max num of LEDs: 104

For each LED connection above, i give some characteristics. First of all, the power supply. This is the power supply that the controller operates with. As i told you before, only the LED array will change with the power supply.

Then the RP. The RP is the protective resistor for each row. Each LED row must have its own protective resistor. Although multiple LEDs connected in parallel can use just one resistor, this must be avoided. LEDs should always be connected in series with resistors. Each row composed by a number of LEDs and a protective resistor (RP) can be multiplied in parallel, as many times as the 2N2222 permits.

The next number shows exactly this: the number of rows for each connection that the 2N2222 can handle. For example, when powering with 12 volts, the transistor is capable to drive up to 26 lines of 3 LEDs each (and of course a protective resistor on each line) and that is the current limit of the 2N2222. If you think of pushing the transistor to the edges, then you should first measure the real current that your LED array draws, and do not exceed the maximum 800mA that the transistor can handle. Remember that the transistor may need heat-sink if it gets hot.

The last number, indicates how many LEDs in total can be controlled with each connection. For example, when powering with 15 volts, the transistor can control up to 26 rows of 4 LEDs each, that makes 26 x 4 = 104 LEDs in total.

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