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Using 120VAC with LED’s – Why not use more LED’s versus large Current Limiting Resistor?


Jasen Betts

Jan 1, 1970
First off… Thank you for all of you that are being helpful.
I now understand the points that you’ve made about my original
question. Since then, I have dug into a more active circuit using
LM317 chip in constant current mode.

Here is a picture of the basic circuit off the Internet.

Here is an Internet calculator I used to size the resistor.

Here is a picture of my circuit (while running) and being lit by the bunch of LED’s.

And here is a close-up of the circuit.
I used calculator above which indicated I need the 56 ohm resistor
to give me an output current of 22.3 ma. With an input of 120.1 VAC,
I measure 107.1 VDC out of the full bridge. I also measured 106.6 VDC
coming out of the LM317 with no load connected. When connecting to
the load of 40 LED’s, I get a current of 10 ma. I kind of expected
this since the voltage drop over 40 LED’s is about 140V.

WHOA! You can't use an LM317 with more than 40V.
As I connect fewer and fewer LED’s the current goes up as expected.
However, I kind of expected the LM317 to start kicking in and keep the
current around 22.3 ma. In the picture above, you can see that at 36
LED’s, the current was showing near 30 ma.
Can you tell me what I’m missing? Some key words would be very helpful for me to research.

search this:

LM317 datasheet

On the natinnal semiconductor data sheet the 40V limit is at the top of page 4

An example current regulator circuit can de seen on page 18, it's
basicallythe same as yours.

The 40V limit means the voltage difference between your LEDs and the
input should never exceed 40V. regular 110V AC can have DC peaks
up around 170V somewhere so your LED string is constrained to be one
that drops 130V or more else you risk damaging the LM317 and incorrect

don't forget the fuse.


Jan 1, 1970
I’ll be looking into your other component suggestions, but for now, I’dlike to understand what’s going on with this circuit if for no other reasons, than educational purposes…


Ok… I think reading your two’s posts, you’re basically saying the same thing. I knew that the LM317 was limited to a 40V differential. The problem was… I should have known better about the output of the bridge. I was just reading the DC on the meter and not rationalizing the 107 V wasn’t really DC.

OK… so I found a 200V, 10uf capacitor lying around and added it across the DC output of the bridge. I’m now reading 165VDC. So… even if I’m seeing 165V, I should assume it still has an AC component and is really peaking between 160 and 170 volts. I don’t have an oscilloscope, so I’m guessing it would looks something like this…

So is the following a valid rationalization?

If I take the LOWEST voltage and divide by the nominal LED voltage drop, I would get a number of LED’s (160 / 3.5 = 45). This should be a kind ofthreshold for the LM317. At this value, the LM317 is always restricting the current to the LED’s and I should see the 22.3 ma?

And further… that at the high peak 170V, would only be using 170 - 45*3.5= 12.5V of the 40V differential allowed on the LM317?

And that… the AC power could go all the way up to (45*3.5 + 40) = 197 Vp-p before the LM317 would be compromised?


I started reading about the zener dode you are suggesting. If I understandcorrectly, this protects the LM317 from seeing more than it’s rated 40 Volt differential. Does it serve any other functionality? Considering thatits rated for 1.5 amps, what kind of failure mode should I expect from running the LM317 with too much voltage through it, but only at 30ma?