Mains LED with blocking cap

About 16 henries?

But what is the cost of a 36 Henry inductor compared to a 0.28uF capacitor?

-Bill

"John Fields"

** Totally irrelevant to the switch on transient situation. ** Total nonsense.

** Imagine the LED has 200 to 300 volts momentarily imposed on it ?

This is the situation when the switch is closed near a voltage peak.

What resistance does it exhibit then ?

Wot - no maker's data published for that ?

Try your idea out.

Betcha one dead LED - first shot.

..... Phil

I haven't done the sim, but let's look at this qualitatively. The calculations above treat the LED as a linear resistance which it is not. If it has a drop of 3.5V at 20mA, at 10mA it may be 3.4V, giving an equivalent resistance of 340 ohms. At a surge current of 1A, the drop may be around 4V, giving only 4 ohms. And so on. The fallacy here is in ignoring the highly non-linear behaviour of diodes. A fast turn-on will see the capacitor as a very low series impedance, resulting in a heavy inrush current.

"Pimpom"

** Precisely.

The situation is the same as charging that 0.28uF cap to 200 or 300 volts and dumping the charge into the LED.

The tiny chip will likely give just one flash of light - and no more.

.... Phil

I made mains LED lamp one a decade ago, with 1K 1W resistor for surges,

220nF cap and inverse parallel series string of 10 LEDs, thing ran continuously no problem. Others along the same principle still running too.

Only problem I noticed was the older blue LEDs fading away to no light over several months.

The resistor is required for spike suppression, it was a 1W for the voltage rating (240V mains here).

Certainly I'd not run a LED from the mains without the resistor, and I'd use a higher value than the 75R suggested upthread.

Grant.

How does it protect against turn-on surge? (Sorry your netlist did not work for me...)

[...]

I don't see how stray inductance generates very high currents (inductance normally works to stabilise current). High voltage, potentially, but that is what the antiparallel diode is for.

"Turn-on debounce" won't prevent the initial inrush current, and soft-turn off is not required AFAICS. A simple RC feeding the leds should work (as Larkin pointed out).

--

John Devereux

--
Why argue if you haven't done the sim?

Which, BTW, contains LED models which I'm sure meet the criteria you
set out, above.

--
Run the simulation.

If its results are wrong and I haven't screwed up setting it up, then
your quarrel is with Linear, not with me.

"John Fields" "Phil Allison"

** Pointless.

The real world always rules.

.... Phil

--
The impedance of the RC formed by the LED and the cap limits the
current from the mains even when turn-on is at one of the peaks.

Sounds counter-intuitive, I know, but run the sim.

It's easy enough to set up using the ASCII diagrams above, and the
LEDs I used were Nichia NSPW500BS from LTC's library.

I'll probably put something together in the real world today as a
reality check, and when I get the data I'll post it.

"John Fields"

** How many LEDs do you have to ruin to be convinced ??

.... Phil

On Sat, 12 Mar 2011 06:43:30 -0600, John Fields wrote:

--- Oops...

My error; I started out with a sinewave delayed in phase to simulate startup at a peak.

Appears LTspice considers that always a sinewave and just calculated the current through the impedance without considering startup.

Putting a switch in there:

Version 4 SHEET 1 880 680 WIRE -96 32 -208 32 WIRE 112 32 -16 32 WIRE 256 32 112 32 WIRE 384 32 336 32 WIRE 496 32 448 32 WIRE -208 176 -208 32 WIRE -80 176 -80 80 WIRE 112 176 112 32 WIRE 272 176 272 80 WIRE -208 288 -208 256 WIRE -80 288 -80 256 WIRE -80 288 -208 288 WIRE -32 288 -32 80 WIRE -32 288 -80 288 WIRE 112 288 112 240 WIRE 112 288 -32 288 WIRE 272 288 272 256 WIRE 272 288 112 288 WIRE 320 288 320 80 WIRE 320 288 272 288 WIRE 496 288 496 32 WIRE 496 288 320 288 WIRE -208 384 -208 288 FLAG -208 384 0 SYMBOL cap 128 240 R180 WINDOW 0 24 64 Left 0 WINDOW 3 24 8 Left 0 SYMATTR InstName C1 SYMATTR Value 2.77e-7 SYMBOL voltage -208 160 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value 325 SYMATTR InstName V1 SYMBOL LED 384 48 R270 WINDOW 0 -36 32 VTop 0 WINDOW 3 -39 32 VBottom 0 SYMATTR InstName D2 SYMATTR Value NSPW500BS SYMBOL sw 0 32 M270 WINDOW 0 32 15 Left 0 WINDOW 3 32 44 Left 0 SYMATTR InstName S1 SYMBOL sw 352 32 M270 WINDOW 0 32 15 Left 0 WINDOW 3 32 44 Left 0 SYMATTR InstName S2 SYMBOL voltage -80 160 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 -45 9 Left 0 SYMATTR Value PULSE(0 1 0 1e-6 1e-6 1 0 1) SYMATTR InstName V2 SYMBOL voltage 272 160 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 -45 9 Left 0 SYMATTR Value PULSE(0 1 1.1 1e-6 1e-6 1 0 1) SYMATTR InstName V3 TEXT -184 352 Left 0 !.tran 3 TEXT -184 312 Left 0 !.model SW SW(Ron=1 Roff=1G Vt=0.5Vh=0)

and turning it on at 90° fixes the error and _clearly_ shows the inrush current.

Mea culpa. :-(

--- JF

--
As many as it takes for me to prove to myself that I'm wrong.

Probably just one.

In this case, however, none, since I found the error I made with the
simulator, fixed it, and learned something in the process.

--
And, with 2 LEDs:

Version 4
SHEET 1 880 680
WIRE 160 -112 128 -112
WIRE 272 -112 224 -112
WIRE -96 32 -208 32
WIRE 32 32 -16 32
WIRE 128 32 128 -112
WIRE 128 32 96 32
WIRE 160 32 128 32
WIRE 272 32 272 -112
WIRE 272 32 224 32
WIRE -208 176 -208 32
WIRE -80 176 -80 80
WIRE -208 288 -208 256
WIRE -80 288 -80 256
WIRE -80 288 -208 288
WIRE -32 288 -32 80
WIRE -32 288 -80 288
WIRE 272 288 272 32
WIRE 272 288 -32 288
WIRE -208 384 -208 288
FLAG -208 384 0
SYMBOL cap 32 48 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName C1
SYMATTR Value 2.77e-7
SYMBOL voltage -208 160 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value SINE(0 325 50)
SYMATTR InstName V1
SYMBOL LED 160 48 R270
WINDOW 0 -36 32 VTop 0
WINDOW 3 -39 32 VBottom 0
SYMATTR InstName D2
SYMATTR Value NSPW500BS
SYMBOL sw 0 32 M270
WINDOW 0 32 15 Left 0
WINDOW 3 32 44 Left 0
SYMATTR InstName S1
SYMBOL voltage -80 160 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
WINDOW 0 -45 9 Left 0
SYMATTR Value PULSE(0 1 10e-3 1e-6 1e-6)
SYMATTR InstName V2
SYMBOL LED 224 -128 R90
WINDOW 0 -41 34 VBottom 0
WINDOW 3 -38 36 VTop 0
SYMATTR InstName D1
SYMATTR Value NSPW500BS
TEXT -184 352 Left 0 !.tran .1
TEXT -184 312 Left 0 !.model SW SW(Ron=1 Roff=10G Vt=0.5Vh=0)

Like any diode, an LED can take quite a current overdose.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

"John Fields" "Phil Allison"

** I ruined five last evening - all old stock with leads a bit tarnished.

A pair of 3mm red LEDs are now merely 1.2 ohm and 10 ohm resistors after one dump from a 330nF cap charged to 240 volts. A 5mm red LED is now a 1520 ohm resistor that gives no light after two dumps.

But a pair of 5m amber LEDs proved tougher - even after multiple dumps each still produces some light.

BTW: Each application of the charged cap produced a spark similar to shorting the same cap.

With reverse parallel LEDs, I reckon you will kill two birds with one stone as the instantaneous voltage across the reverse polarity LED will be so far over its rated max.

... Phil

--
Nice. :-)

There are two to deal with:

1: Inrush current - easy to solve with a series resistor to limit that to something that does not damage the LED.

2: Reliability of the capacitor and safety of the capacitor failing

The capacitor needs to have a decent AC voltage rating. A DC voltage rating significantly greater than the peak voltage is not sufficient to survive prolonged AC, even without any voltage spikes. Things get better if the capacitor is UL recognized or the like.

One thing that also helps: Have everything enclosed in an enclosure made of flame retardant plastic or the like, suitable for containing things going wrong. I would not consider chances of the current limiting capacitor failing being reduced to zero.

The resistor being used for limiting inrush current should be a flameproof type, preferably UL recognized or the like.

I would avoid filter capacitors, since they could receive full peak line voltage plus some of any voltage spikes if the LED(s) fails open. Electrolytics including tantalums could leak conductive liquid if overvoltaged. This is a concern especially if things going wrong includes the current limiting capacitor failing short - even if only temporarily shorting.

Sounds scary? How about using an LED that gives you enough light at a current low enough to make practical use of a resistor without a capacitor for current limiting? There are quite a few LEDs that are plenty bright for use as indicator lamps at currents anywhere from .2 to 1 milliamp.

--
 - Don Klipstein (don@misty.com)