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I am converting white LED Christmas lights (outdoor rated) into over head outdoor lighting. The box says I can connect up to 45 strings. I will only need about 10.

Since they flicker, I made a functional rectifier using 4 x 1N4007 (2 x 2 series and 2 x 2 parallel). During the test, I used a capacitor rated 4.7uF and 400 V. [All 5 components I desoldered from old phone chargers.] All of the lights turn on and stopped flickering.

Each full wire (70 LEDS) is 4.8 W and 0.04 A, and each LED is 3.4 V 68 mA. Each string consists of 2 x 35 series LEDs. My home is standard US 120 V.

Once I create this bridge, how much should I decrease the DC to allow for 10 strings (700 LEDs) and be safe?

When I finish, all will be encased in weatherproof housing.

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  • \$\begingroup\$ Several years ago I did a similar thing, I used a SMD bridge and no resistors or caps. This replaced the controller and I had no blinking problems with them. Mine were 120VAC and after the mod I connected them to 120VAC. \$\endgroup\$
    – Gil
    Commented Jul 10, 2021 at 23:20
  • \$\begingroup\$ Was your output VAC or convert it to VDC? \$\endgroup\$
    – Biocellguy
    Commented Jul 13, 2021 at 23:30
  • \$\begingroup\$ It was DC with lots and lots of ripple there was no filter. \$\endgroup\$
    – Gil
    Commented Jul 14, 2021 at 3:04
  • \$\begingroup\$ VOTE TO CLOSE people - the question has an answer AND is entirely understandable. \$\endgroup\$
    – Russell McMahon ♦
    Commented Jul 26, 2021 at 10:58

1 Answer 1

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If each LED is 3.4V and you connect 35 in series, you end up with a nominal voltage drop due to LED threshold of 119Vdc.

If you rectify full wave (Graetz bridge) connected to single-phase 120Vac you have ideally Vdcpk=170Vdc peak, and average voltage Vdcav=0.637Vdcpk=108Vdc. The ripple is reduced using the tank capacitor you mention, and the average dc voltage is increased: you can calculate it more accurately, but considering that the loading is quite small, you can assume that the ripple will be small and the average output will be close to the peak, having subtracted the voltage drops in the diodes. Let us say VdcavC=95%*Vdcpk=160Vdc.

Now you have a dc source of 160Vdc to connect to your LED strip (35 items), with many strips in parallel. You want to control the current for, first, not to blow the LEDs, and, second, to balance the luminosity. You can add a series resistor for each strip (R1, R2, ... Rn), each one limiting the current for that strip. The current is approx 40mA, the voltage difference at nominal values is (160-119)=41V, so you end up with R=1 kohm. All resistors equal. Now to balance luminosity you can put a second resistor in series (a trimmer) of about 10% value, so 100 ohm (or maybe 200 ohm) and you can balance each strip.

Power dissipation in the resistors is RI^2, so not an issue if you use a power resistor: 5W is advisable, 10W is better. 1 kohm is a standard value, so there is plenty of choice. For the trimmer, you will dissipate 160mW with the 100 ohm model, so better not choose the 200 ohm model, that will end up too hot for sure.

You speak of 700 LEDs, so that 35 set is replicated 20 times. Please, note that you cannot use 70 LED in series, because there is not enough feeding voltage. If your socket were 230 Vac, that would be possible. Overall consumption is 40mA x 20 = 800 mA.

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  • \$\begingroup\$ Thank you. My main concern is safety.I was on www.diychatroom.com & was talked down to when I asked this. I was told how dangerous it is to output that high of DC,&if someone accidentally touched it, it is more hazardous(deadly)than the corresponding AC.I know my bridge works & allows additional strings.I am more concerned with the safety followed by concern with more strings. Also, one full string is actually a manufactured two strings of 35.At the end of the full 70 (2 x 35), is a two prong outlet designed for daisy chain.[With the rectifier, I have to make note of the rectified polarity.] \$\endgroup\$
    – Biocellguy
    Commented Jul 11, 2021 at 22:15
  • \$\begingroup\$ 1) DC is not more dangerous than AC, the opposite (see IC 60479-1). 2) If you put it outdoor all connections (joints) must be protected for environmental factors with something equivalent to IP67 (at least IP66): so resistance to temporary submersion or resistance to heavy rain in bad direction. This also guarantees electrical safety, as there is no direct contact. \$\endgroup\$
    – andrea
    Commented Jul 13, 2021 at 6:56
  • \$\begingroup\$ 3) For peace of mind you could put a replaceable fuse on each LED line but margins are low: 40 mA absorbed, 120 mA in short circuit, and 50 mA if person touching (person is about 1-1.5 kohm). So, fuse should be very carefully selected and it's a bit "chancy". 4) Definitely with 50 mA and a touch voltage of 60-70V (accessible voltage after 1 kohm) you do not kill anyone. \$\endgroup\$
    – andrea
    Commented Jul 13, 2021 at 6:57
  • \$\begingroup\$ @Biocellguy I see now, you have to split the LED into max 35. 70 in a row cannot be fed with only 120Vac, you need 230Vac => move to Europe! (or borrow a socket there :) ...). If it is hard to separate them you can arrange them as 2 strips by 35 each i parallel to feed at the same time. Then, in the answer above you need to change the 1 kohm into 500 ohm, and the 100 ohm trimmer in something smaller (too much current). \$\endgroup\$
    – andrea
    Commented Jul 13, 2021 at 7:00

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