LED Full Wave / Half Wave Rectifiers

**chelmuth** · 01-09-2011, 08:56 PM

I have a TON of half wave Multi-Color LED strings and I want to convert them to single color full wave strings so I’ll need to recalculate the resistor and had a few question about Full vs. Half Wave. I've been reading about resistor calculations for strings haven’t found a definitive answer for a question I have. RMS vs. Peak. My VOM and most others measure RMS, but I’ve been reading that for resistors in LED strings you need to calculate for Peak voltage and that is typically 1.4xxxx times the RMS voltage. Now my question is.. Full vs. Half wave do you calculate for different voltages. I.e. Half wave only calculate based on RMS because it’s not being rectified and then when using a rectifier for full wave do I need to calculate for Peak? Or should I always be calculating for Peak and never RMS.

**Penfold** · 01-10-2011, 01:45 PM

In my experience the only time that you have to change the resistor value is when you are cutting down the string or increasing the number of lights on it. Otherwise, just keep the resistor that is already on the strand and add your diodes or full wave rectifier before the resistor on the strand.

Of course, this is my experience so I could be wrong. Anyone concur with this?

**griffixdc** · 01-10-2011, 03:10 PM

from my newly experience putting a rectifier or 4 diode setup before anything on the string lights is the best. you can even make a rectifier plug tap that the string of half wave led's plug into....the only problem with this is you cannot add incandescent or full wave led's after....so if you want to do that you should just put the rectifier/ 4 diode setup before the resistor/resistors but closest to the first led as possible. RMS and Peak have nothing to do with calculating resistance; this is per-say your adding resisters....you calculate rated on the voltage the LED's are rated at and the millamp draw they use...they are going to have a specific tolerance % value in them usually no more than 10% because of course no voltage source is exact and does fluctuate sometimes besides if you were to dimm them these values all change anyways use this guide to calculate resistance for LEDs...this guide is only needed if you are going to create your own LED light strings or retrofit incandescent strings to LED.

Guide.....http://led.linear1.org/led.wiz

**se7enred7** · 01-10-2011, 03:51 PM

If you convert to a full wave string, the resistor (and led's) will dissipate twice as much power as with a half wave. The resistor value does not need to change. However, the power rating of the resistor could become a concern, as a full wave string uses twice as much power as a half wave string.

For calculating power dissipation: Do not use peak voltage regardless of half wave for full wave, as the peak voltage is only applied for a short period of time. The RMS voltage is what should be used when calculating power. I believe your resistor power dissipation for a full wave string should be something close to

[120V-(#of LED's*LED Forward Voltage)]^2/(Resistor Value) = Resistor Power

It would follow that half wave power dissipation would be half the above value. I would suspect there is some margin built into the resistor that came with your half wave strings, so you could try running full wave and keeping an eye on the resistor for a while to see if it gets too hot.

**griffixdc** · 01-10-2011, 04:00 PM

from my experience...even that is not needed se7enred7....the power demand is so small that the resisters can handle the doubled power demand....although with an ammeter the power really is only multiplied by 1.4-1.8, which doesn't make sense to me but hey. but as se7enred7 has stated your pwer has essentially doubled once converted to half wave to full wave and the resisters on the light string were originally rated for the original power demands not the new...so for longer life span of your light strings converting the resister to a more tolerant resister will help

**se7enred7** · 01-11-2011, 04:11 AM

Doh! I neglected that there is a stack of diodes, and not just a single diode. So power would be dissipated only over a fraction of the wave, with the fraction depending on the forward voltage of the entire diode stack.

**chelmuth** · 01-12-2011, 07:41 PM

Well what i'm planning on doing is taking a whole bunch of multi LED strings and making them single color.. ie. 5 Multi's into 5 strings of R, G, B, O, Y.. so my resistor values will change because they were based on multi color.. I'm thinking forward voltage of ROYG are about 2-2.2 volts so strings of 60 should be pretty close to 120v and i should be good with a 1Ohm resistor. my concern is more for the blues.. them being 3.3v i'll end up with almost 200v forward so should i be shortenting the blue strings to 35-40LEDs to get close to the 120V. Currently the strings are wired with 12 of each color meaning about 12V for each set of color repeated 12 times for total forward voltage of 140v..

**griffixdc** · 01-12-2011, 07:48 PM

Originally Posted by chelmuth

Well what i'm planning on doing is taking a whole bunch of multi LED strings and making them single color.. ie. 5 Multi's into 5 strings of R, G, B, O, Y.. so my resistor values will change because they were based on multi color.. I'm thinking forward voltage of ROYG are about 2-2.2 volts so strings of 60 should be pretty close to 120v and i should be good with a 1Ohm resistor. my concern is more for the blues.. them being 3.3v i'll end up with almost 200v forward so should i be shortenting the blue strings to 35-40LEDs to get close to the 120V. Currently the strings are wired with 12 of each color meaning about 12V for each set of color repeated 12 times for total forward voltage of 140v..

are you sure ou have to change the resistor values....what would the resistor values be on multi LED strings...i was going to do the same thing with mine

**chelmuth** · 01-12-2011, 07:51 PM

LOL no i'm not sure and i'm not sure of the current resistor value either.. That's partially the reason i was asking. Actually just now looking at the strings. .these don't have resistor pods.. maybe because they are 60LED's? they are rated over the 120v.??

**griffixdc** · 01-12-2011, 08:01 PM

Originally Posted by chelmuth

LOL no i'm not sure and i'm not sure of the current resistor value either.. That's partially the reason i was asking. Actually just now looking at the strings. .these don't have resistor pods.. maybe because they are 60LED's? they are rated over the 120v.??

thats what i was thinking...a lot of LEDs are standard at 2-3 volts so putting 60 makes roughly 120v to 180v's ...so i am guessing that they have to be real close to 2 maybe 1.8 no more than 2.2 volts....i guess just try each color one buy one to see if they are safe for 120v