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If I used 500 ohms it'd drop 15V at 30mA - which wouldn't let me get
30mA into the LED string with low line in - and it'd dissipate 450
milliwatts when there was 30mA through it.
The trick to why it works with 100 ohms in there is that the LM317
doesn't start waking up until the voltage differential between the
input and output is a volt or so, so until then it acts pretty much
like a saturated NPN series pass transistor and the voltage across the
load will follow the input voltage minus a volt or so.
Then, as the voltage across the load rises, the current through it
will increase until the voltage dropped across R2 increases to 1.25
volts, when the LM317 will start limiting the current into the load to
30mA until the voltage across R2 falls to less than 1.25 volts later
on in that mains half-cycle.
Doing it that way only drops 3 volts across R1 and causes it to
dissipate 90mW with 30mA through it.
Also, the value of C1 was chosen to allow it to charge only to the
voltage required to get 30 mA into the string at low line, with the
LM317 doing the required limiting to keep the current in the string
relatively constant from low line to high line.
I've posted some photos to abse:
news:ng0ug7hf2jrhh49sn1gppa0betc05juu55@4ax.com
I didn't post any of the output voltages into the LED string, and I
want to get this on its way, so I'll do it later on this afternoon
when I have more time.
>Also, I'm a wimp where it comes to running LEDs near their ratings,
>so I'd use 51 ohms or more for R2 to keep the current lower than
>30 mA.
You're correct, and it's much appreciated. I have no idea where I erred on the higher value for R1, except that I may have typoed the calculator. Typo or not, it's a glaring error. :-(
I know your post made it to ABSE, because I see the post where JT asked about it, and your response to him. The bad thing is that your post didn't show up on news.eternal-september.org Makes me think I'd better get a different newsgroups provider.
In my prior reply I said: "I have no idea where I erred on the higher value for R1, except that I may have typoed the calculator. Typo or not, it's a glaring error."
Now I realize what I did - and I was correct, 100 ohms for R1 is too low using the LM317 and figuring worst case Vin high, but I didn't think of the high voltage version of the LM317. Your circuit using the high voltage version is better. :-)
The wasted power will always be the dissipation of R1 plus the dissipation of the LM317, so it really doesn't matter how much is wasted in R1 in terms of circuit efficiency.
On Fri, 13 Jan 2012 14:04:35 -0500, ehsjr wrote:>>
--- True, but with the mains varying +/- 10% about the 120V nominal, R1's being equal to 470 ohms won't allow the current - over the range of mains variations - into the LED string to be the 30mA the OP asked for.
With R1 equal to 100 ohms, and a 4400 ohm load resistance, (the equivalent resistance of the LED string) the voltage across the LM317 never exceeds the absolute maximum rating at high line and, with no heat sink, the temp of the LM327's tab never exceeds about 65C with a
25C ambient, so the circuit's pretty safe with a standard LM317.
Transient protection is something else, again, and the HV may have an advantage there due to its inherent standoff.
Yup. That's why your 100 ohms is better than the 470 (which is too small, anyway, with the regular LM317) if using the HV version LM317.
Here's where I'm missing something: with AC in at 132, the peak after the bridge is around 185. With 3V across R1, that's 182 on the input side of the LM317, and 132 on the output side, exceeding the 37 V max Vin-Vout. Even the 470 is too small, dropping only ~14.1 at 30 mA and yielding Vin-Vout ~39.
Yes, but a different topology at a lot higher cost. I'd use a low voltage LED supply & several LED strings. The supply takes care of the transients and safety issues, and current limiting can be the 317 circuit or a resistor.
If using the mains-bridge-limiter design, a TVS across the 'lytic seems a good way to go, and a .1 uF should be there too.
Paul proposed a zener across the 317 and a cap across the LEDs which should help, and you can work on the AC in side prior to the bridge. I'd also worry about a switch placed between the cap and the 317.
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