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So I thought I had enough knowledge to do a simple LED string, but now I'm wondering.

Basically what I did is got a constant current DC power driver (700mA, 15-42V output) and wired it in series with seven colored LEDs. the forward voltages of the 7 LEDs add up to about 21 volts, which I thought was within range of the driver and so would be okay.

a complicating factor is that we hired someone else to build an enclosure for what we're making, and he decided to wire it up without our permission and wired it up terribly. he put everything in parallel, and wired the 120 volt AC directly into the DC circuit. He then tested it several times putting all that voltage directly through everything.

When I got my hands on it and wired it up the way I had intended the green LED was not working. I assumed that he had burned it out, and ordered a replacement.

Today I went to put the green into the circuit, and everything was going fine, but my first few tests resulted in more problems. Now four of the seven LEDs are burned out, including the new green one I had ordered.

So what I'm wondering is, was my initial plan fatally flawed? Did I fail to account for something? Or is it possible that the incorrect wiring did damage to the other LEDs that is just now showing up?

I'm extremely confused, so TIFA for any advice.

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    \$\begingroup\$ Are the LEDs rated for 700 mA? I would discard all the LEDs that were exposed to 120 VAC, even if they appear to work. \$\endgroup\$ Oct 25, 2020 at 19:57
  • \$\begingroup\$ They're all rated for up to 1000 mA. \$\endgroup\$ Oct 25, 2020 at 19:59
  • \$\begingroup\$ Assuming 2-3V per LED, you can test them with a 9V battery or 3 AAs in series, and a high enough resistor in series to limit current to safe values even if the polarity is wrong, like 10 kOhms. Note if the constant current driver says 15-42V and there are not enough LEDs in series on the output to reach that voltage, it may not start up completely, or blink, or light very dimly. Can you post a schematic of the current state of things? \$\endgroup\$
    – bobflux
    Oct 25, 2020 at 20:14
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    \$\begingroup\$ Note it's normally not possible to blow LEDs with a constant current supply unless they're wired in reverse polarity. Most LEDs won't tolerate more than 5V reverse, the supply goes to at least 42V so that may be it. \$\endgroup\$
    – bobflux
    Oct 25, 2020 at 20:19
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    \$\begingroup\$ Are you using the power supply that got mains voltage connected to the output? Also, are you switching the LEDs while the supply is on? \$\endgroup\$
    – bobflux
    Oct 25, 2020 at 21:40

2 Answers 2

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700mA through an LED will generate a lot of heat. Make sure the heat sinks on the LEDs are adequate. If not, they will overheat and fail rather quickly.

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If 120 volt AC hit both the LEDs in parallel and the output of the power supply, it's very likely that everything has been damaged including the power supply. Your initial set up is good but you have to replace everything. You can verify the output voltage of the power supply with new leds. But if it's defective you risk to blow up leds again.

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