Ping for Andrew Gabriel

[Rick Hughes]

Hi Andrew ... in the post below you asked me for data on the LED Standard W10 Silicon Bridge Rectifiers ...

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Can you point to the datasheet for your LEDs? >My calculation assumed 40mA non-repetitive peak current. >What design constraints have you got (such as max power >dissipation per indicator)?

Here is datasheet on Br Rectifiers ...

LED are standard 5mm LED (100mcd) ... forward Voltage 1.8-2.2V

20mA nominal current, 40mA maximum.

Capacitor are X2 rated, Polypropylene MPX 0.47uF Withstand Voltage 275V AC Rating Voltage 250V AC

No specific design constraint on display, just something that will illuminate them.

I have purchased the capacitors, Br Rectifiers & LED so keen not to waste these ...

Were you able to take a look at cct to see if there is suitable a resistor or modification that allows use of these components, while reducing the risk you (thankfully ) pointed out of blowing the LED's

One suggestion was to increase resistor to 680R

Welcome your advice.

[fred]

In article , Rick Hughes writes

Delve a bit more into the data for the LEDs to find their peak rating, I suspect the values given are the nominal and maximum _continuous_ values.

If you can't find it on the sheet then give us a link to the data sheet.

There are other issues as well but one step at a time.

[Rick Hughes]

I did try that, and contacted seller ... all he has is what he provided ... he has no other data, he bought a job lot and has no data sheet. All he could give me was:

100mcd ... forward Voltage 1.8-2.2V 20mA nominal current, 40mA maximum.

I'm happy if we set 20mA as the design criteria, need indication only not light the room :-)

[fred]

Without full data for the LEDs it's difficult to say what the pulse handling capability of the LEDs you have is. Some are better than others and the different materials used to generate the light make a huge difference, red generally being more pulse capable than other colours.

After looking at some typical device data and with finger in the air, I'll say that your nondescript leds will take 75-100mA as an occasional short duration peak at switching time.

240V peak is 338V which would need 3k9 or 4k7 to limit switch on current to 87 or 75 mA.

Now consider dissipation in the res with led on:

You have gone for a 470nF dropper which gives a hefty 35mA rms current. This will be pushing 5W in each resistor which is really too much (even with a 10W res, it would be too hot to touch). In addition, you will be running at a peak led operating current of 50mA which again is too much.

Best to see if you can get sufficient illumination with a lower operating current, even if it means scrapping those leds (Experiment with a dc supply and a few resistors).

Lets try a 100nF dropper (Xc of 32k).

That gives 7.5mA rms or 10mA pk which I'm guessing will give you reasonable brightness indoors.

That will give just over a quarter watt in a 4k7 pulse limiting res which will be fine for a half watt resistor (always best to derate).

Next, peak power dis in the res at switching, 75mA in 4k7 gives 26W peak meaning that we really need this to be a pulse capable resistor such as carbon composition (yes they do still make them).

We also need the res to withstand at least the peak volts of the mains (338V) plus a bit for transients so a 350-500V resistor.

These parts would do the job at a cost of 45p per chain:

100nF 300Vac X2 4k7 0.5W 350V Carbon composition

Bear in mind that the above is entirely theoretical and should be used only as a starting point for experimentation.

In your place I would set up one of these on a switching rig (one red, one green or a few of each) and absolutely hammer them to make sure the leds and other components survive. At its most simplistic, this could be running them connected to an old fashioned mechanical plug in timer with alternating 15min on-off periods.

Starting from scratch I would probably search for some really low current (say 2mA) but bright and pulse capable leds which would result in reduced component stress, values and hence cost.

Finally the safety stuff:

1. Safe distances between L & N (and any series components carrying same) on a board or through air (creepage and clearance) is 4mm. That will mean removing intervening tracks if using veroboard.

1. Same for between L or N and Earth on an earthed (Class 1) setup is

6mm. 8mm to exposed unearthed metal parts on Class 2 I think.

1. Only use X2 rated caps and 350V or above pulse capable resistors.

2. IMO, it would not be safe to mount the LEDs in a metal panel (even if earthed), the insulation will not have been specified for mains use. Use a plastic panel and mount so that even if damaged, the LED junction would be some way behind the front face of the panel.

Are you sure you don't want to scrap it all and use some skinny neons ;-)

Otherwise, good luck.

[Rick Hughes]

Box will be all fully insulated nylon ...

Never use neons ... what would cct look like with these .... although I would prefer to use what I just bought .... (if it is possible)

The whole reason for going to capacitor dropper was to avoid dissipating heat in resistors which many of the simple ccts do.

Just so annoying I followed the design as per here ....

(I have had contact from page owner he insists with 220R it works fine, but for additional reliability it could be increased to 680R )

[fred]

In article , Rick Hughes writes

If you want to test the reliability of the circuit with the original values then set up a circuit to switch the mains to it every second or so and if it lasts more than 24hrs then stick with those values to monitor your occasionally switched heating zones, if they don't last then you have your answer.

220R will result in 1.5A peak LED current when switched on at the peak of the ac waveform, 500mA for 680R. I'd say that the former will pop all but the toughest pulse capable LEDs in short order and that the latter will be a 50-50 risk.

You still risk popping the LEDs for excessive forward current when lit, things last longer when derated, which is why they have a nominal and a max rating, they'll last 10-100 times longer when operated at nominal over what they will when operated at max.

[Capitol]

Thought a bit more about this. If these are low efficiency red leds, then the Vf will be about 2V---roughly. This is about twice the level of a 4148 diode which will withstand 1A for 1 second (data sheet). This indicates that the leds will probably withstand 300mA for 1 second so, I'd guess that 1A at 10mS would be survived. So personally, I'd go with 4(400V) resistors to make up 1K and survive any transient voltages. I'd also reduce the capacitor to 0.1uF. Over run leds don't last long, as the chinese are proving in lamps. My rule of thumb for capacitative droppers is 80mA/uF, so peak running currents of 12mA should be adequate. These are off the cuff thoughts, others may differ. I've seen

1500V transients on our house mains supply and I would normally scope this circuit to make sure that things are behaving as expected.

[Capitol]

Was the original designed for 110V?

[Rick Hughes]

No ... the guy has a page with his design (and theory) if you want a look :

I did reach out to him, and although he advises his circuit works fine he is adamant the 220R stops any excess in-rush current,though he did add that increasing that to 680R might be worthwhile.

It looks like I will have to buy a new set of capacitors ... seems to be form your & Freds comments hat 0.22uF will be better at ~20mA

[fred]

I'm sure this hole is getting bigger as we speak but please don't buy _sets_ of anything until you have fully tested the concept for light output and durability.

Your 470n caps are not totally wasted as you can combine them in chains to experiment with circuit values. If you weren't already aware, 2 x

470n in series will create 235nF so a suitable test for 220n or 5 x 470n will create 94nF which will test for 100n. Obviously more bulky than correct replacements but this is only for testing.

Note that if you go for 20mA then you will limit your pulse resistor size to about a quarter of the sizes I suggested (power = I sqrd x R), you will be limited to 1k or so make sure you test for durability of your non spec'd leds at those pulse levels before building in quantity.

Reet, that's enough digging, I'm out . . . .

[Rick Hughes]

OK went and ordered some 0.22uF X2 caps ... so when they arrive will try them out ... so with the aim to be at 20mA, am I following you correctly to use 1k 1/4 resistors ?

[Capitol]

The problem now is not the wattage, it's the voltage across the resistor. You need to use 4 off 400V rated resistors to avoid the coating breaking down when a transient hits the circuit. Check the resistor voltage ratings. There are some special high voltage reistors around, Philips used to make some, Multicomp do some 1W carbon film resistors which will withstand 1KV but only down to 1K I believe. You'd need two of those in series.

[fred]

Actually no, the values I recommended are those given in my second post in this thread[1] but you have chosen to ignore them.

My mention of 1k was a warning that that was the maximum value you could use at your new current level without the resistor bursting into flames, it was not a recommendation to use that value.

If you insist on using those values then you do so at your own risk but in the interests of safety for you and yours, I will point you at:

1k0 0.5W carbon comp 350V working, 700V permitted overload

1/4W is not suitable, nor is anything other than a pulse capable, high voltage resistor. [1] In my earlier post I suggested a 4k7 0.5W 350V Carbon composition resistor for use in a 7.5mA circuit but incorrectly gave the part number for a 680R resistor. The correct part number for 4k7 is:

[Rick Hughes]

No ... not intending to ignore value, apologies if you think that.. but you posted for 7.5mA which would seem too low, I want to get closer to

20mA nominal for LED ........ but accept your warning over 0.47uF capacitor being too big.

I had assumed in your post when you wrote: "if you go for 20mA then you will limit your pulse resistor size to about a quarter of the sizes I suggested (power = I sqrd x R), you will be limited to 1k "

Assumed that this meant use 1K as the current limiting resistor .... if this is incorrect, what value should be used with a 0.22uF capacitor ?

I will also try the 100nF & 4k7 option you give .... so I can test both

[fred]

Ok, fair enough.

The general point I was aiming for was that you want to minimise both the operating and transient current in the droppers where you have fragile elements (the LEDs and to a lesser extent, the resistors).

Without any real data available for the LEDs I made some assumptions (perhaps conservative ones) that 100mA repetitive peak would be a safe max current and that was where my 4k7 pulse/inrush/transient resistor came from (actually gives 75mA pulse limit) and that is what I think is a safe value so as not to pop LEDs or overstress the resistor under maximum switch-on pulse. This is the same for all LED operating currents (cap sizes) but you can't use 4k7 at operating currents above 7.5mA (rms) as the resistor will fry.

Reducing the transient limiting resistor will increase the risk of popping the LEDs so that is why I am not happy about it.

Capitol has suggested 300mA might be a safe peak limit and that may be closer to the mark but we just don't know.

Trying 300mA peak pulse gives 1k1 as a resistor value so 1k0 may be something you want to try. The 20mA you mention with 220n is actually closer to 16.5mA (rms) which will give 272mW in a half watt resistor which is safe. The peak power at switch on is a hefty 90W which is a lot or a half watt resistor to take so the res may pop as well as the LED (or neither . . . ).

What I meant was 1k0 is the max you can use (for resistor power dissipation reasons) when running at 20mA (16.5mA actual) and I didn't think that would be enough to avoid popping LEDs but if you want to give it a go then it may work (no guarantees).

Do try for the lowest current you can.

Designing this properly is tricky, if I was doing a product design with something like this in I'd be choosing my LEDs carefully from low current, high efficiency but pulse capable ranges and operating it at as low a current as possible, 2-5mA max, to reduce component stress.

[Rick Hughes]

Thanks for the details .... it was the extreme simplicity of the cct that was attractive .... I'm at the point of changing the whole concept and use Mains neon indicators instead ... :-(

Would also avoid need to create a PCB ...