can leds meant for ac be run with dc?

I plan on running some led christmas lights on a mosfet controled by an arduino so that I can dim them. Will it work?

Perhaps, but making the attempt will depend upon your ability to find out what the situation with your LED strings is - knowing what you've got.

Do a web search for "christmas lights hack" and there's a lot going on.

http://www.dirtside.com/led/hacking.html
The LEDs there are all in series, so it's HV DC (170 V) after the rectifier with his particular product.
He could have hacked the string into more manageable lengths and used a lower voltage supply.
Yours may be different.
If he had a 170V supply, he could have connected that to the AC plug.
There are high-voltage transistors and MOSFETs.

He provides a link with notes on the LED string that he used (again, yours may be different), explaining their arrangement as made. He re-arranged them all in series. Anyway, it's not an as-simple-as-falling-off-a-log proposition

jameshutton:
I plan on running some led christmas lights on a mosfet controled by an arduino so that I can dim them. Will it work?

That depends totally on how the LEDs are wired. It's difficult to see that from here.

fungus:

jameshutton:
I plan on running some led christmas lights on a mosfet controled by an arduino so that I can dim them. Will it work?

That depends totally on how the LEDs are wired. It’s difficult to see that from here.

Well I would just be hooking it up to led christmas lights the same way you would ac. But just running them with dc instead

jameshutton:
Well I would just be hooking it up to led christmas lights the same way you would ac. But just running them with dc instead

What voltage will it need?

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What voltage will it need?

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It would originally take 120v ac...

jameshutton:
It would originally take 120v ac...

Right...so how many DC volts does it need?

If they’re like the lights* that the dirtden dirtside guy hacked, then you could light half of them with a 60V supply.

* Target LEDs

Right...so how many DC volts does it need?

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I have no idea. That's kinda what I'm asking. Is there a difference between an ac led or a dc led? Will ac led christmas lights even turn on with dc

See my previous reply.
(There are no "ac leds".)
Read the link in the previous reply, too.

Actually, those are white LEDs, and I was thinking red (lower forward volts.)
"120vac"
30 white * 3.5V ea. = 105v
120v - 105v = 15v
15v / 750? = 20ma

So you'll need a 3W, 120VDC supply.

Oh ok! Sweet!
So the more leds that I hook up, will that affect yhe current or voltage? I am hoping to make a box that I can just attach different amounts or types of led christmas lights too, so its not exactly an exact amount of lights or exact type that I will always be using. Kind of like a universal dc variable power supply to leds

jameshutton:
So the more leds that I hook up, will that affect yhe current or voltage? I am hoping to make a box that I can just attach different amounts or types of led christmas lights too, so its not exactly an exact amount of lights or exact type that I will always be using. Kind of like a universal dc variable power supply to leds

There’s no issue of “universality” here.
I think you have the wrong idea.

Still, applying HVDC to the AC plug, as I believe you propose, would result only lighting, possibly, half the LEDs - if they’re like dirtside’s (and I don’t insist that they are). What to do depends on what you have. There’s no “universal” here, none.

Ok, just one last quick question if you don't mind.
Say I hooked up led Christmas lights to mains, but just wanted to dim them manually, would I use a variable resistor rheostat or a variac?

I really appreciate the help

A "variac" is the better option.