**Description**

The LED circuit below is an example of using 25 white LEDs in series
connected to the 120VAC line. It can be modified for more or less LEDs
by adjusting the resistor value. The exact resistance will depend on the
particular LEDs used. But working out the resistor value is a bit
complicated since current will not continously flow through the
resistor.
In operation, the output of the bridge rectifier will be about 120 DC
RMS or 170 volts peak. If we use 25 white LEDs with a forward voltage of
3 volts each, the total LED voltage will be 75 volts. The peak resistor
voltage will be 170- 75 or 95 volts but the resistor voltage will not
be continous since the input must rise above 75 before any current
flows. This (dead time) represents about 26 degrees of the 90 degree
half wave rectified cycle, (asin) 75/170 = (asin) .44 = 26 degrees. This
means the resistor will conduct during 90-26 = 64 degrees, or about 71
percent of the time.

Next we can work out the peak LED current to determine the resistor
value. If the LED current is 20mA RMS, the peak current will be 20*1.414
or 28mA. But since the duty cycle is only 71 percent, we need to adjust
this figure up to 28/0.71 = 39mA. So, the resistor value should be
95/.039 = 2436 ohms (2.4K) and the power rating will be .02^2 *2400= .96
watts. A two watt size is recommended.

Now this circuit can also be built using 2 diodes and resistor as
shown in the lower drawing. The second diode in parallel with the LEDs
is used to avoid a reverse voltage on the LEDs in case the other diode
leaks a little bit. It may not be necessary but I thought it was a good
idea.

Working out the resistor value is similar to the other example and
comes out to about half the value of the full wave version, or about
1.2K at 1 watt in this case. But the peak LED current will be twice as
much or about 78mA. This is probably not too much, but you may want to
look up the maximum current for short duty cycles for the LEDs used and
insure 79mA doesn't exceed the spec.

**Circuit Diagram**

**Source**http://www.bowdenshobbycircuits.info/

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