Martin Brown has brought this to us :
They are 30mA LED's.
They do not light at such low currents..
I can see the mains flicker, but that is not an issue - it is just a diagnostic aid.
That is offensive. You don't know my qualifications, so where does the inept come from?
You are clueless. Modern LED dies are visibly lit on 10's of uA. You only need a higher current if they are in sunlight.
Your original posting. You have no idea what you are doing.
Martin Brown submitted this idea :
Really! Just for reference - I designed and wrote the software for the very first micro-based home central heating system controller. It did all of the temperature sensing and timing in the one unit, it included variable timed temperatures as well as the basic on off and frost protection. This was long before the Internet appeared and in the very early days of home computers.
The fact that what I suggested works and continues to work, is cheap to implement sort of proves that I do know what I am doing. Your comments suggest you are a troll.
Mine (as best as I can recall) went:
L...2k2...33nF(or maybe 47nF)...LED in antiparallel with cheap Si diode...N (Earth in the case of the top floor "Protected Mains" twin socket where the cct was rearranged as L...2k2...LED & Si diode...33nF...E purely for ease of mounting the components - the resulting 2.5mA flowing to earth being an acceptable compromise, especially in the absence of ELCB protection on a cct intended to power PC kit with its own earth leakage issues via 47nF caps (at least one per PC ATX psu)).
I'd have sketched the original cct on a scrap of paper about a decade ago... a quick look through my permanent collection of such scraps of paper and a notepad failed to find such a cct diagram so memory it has to be (that plus a sanity check with an on line reactance calculator).
The 3mm dia LEDs were 1990s vintage, recovered afaicr, from scrapped IT kit. Despite their vintage, they give a bright enough indication despite the simple half wave rectifier cct being used.
If you care to use blue LEDs of any vintage (by definition, recent vintage), you could probably use 4.7nF and a 10K resistor with a halfwave silicon diode in antiparallel across the LED. :-)
Just how *bright* an indicator lamp did you need?[1] Even 100nF seems like 'overkill' imo (ISTR using 47 or 33nF caps for my own LED indicators).
Using a current inrush limiting resistor of only 220 ohms was likely the main reason the LED blew up (3.2A peak worst case[2] being possible on switch on) but the 16mA average current flowing through the LED may also have contributed towards its demise.
[1] From your next post, I gather you didn't really need it to be quite as bright as all that. :-)The 1.2K resistor was a move in the right direction but I chose a larger value again, 2.2K (peak inrush of c 300mA worst case scenario), or possibly 6.8K (I couldn't see all the colour bands on the resistor when checking an unused "protected supply" socket whilst the cct was still live).
[2] Worst case scenario in this circuit being that the power is disconnected just as the cap is charged to peak voltage followed by being reconnected during an opposite peak voltage event resulting in a peak voltage of some 700v being applied across the resistor.The larger the capacitor used, the wider the destructive pulse of current going through the resistor and LED/diode. A 1 amp rated diode might be ok with such surges but the LED won't (with a 1 in 2 chance of it being fried by such a pulse).
Even using a 1k2 resistor is going to result in a 583mA pulse of current. I have a sneaking suspicion I may have actually used 6k8 inrush limiting resistors in my own cct. A 6k8 resistor would have dropped 24v and dissipated 84mW, leaving the bulk of the volt drop (almost 90%) to the lossless capacitor dropper part of the circuit with a 47nF capacitor. A 6k8 resistor in this case would have limited the worst case peak current to 109mA.
In general, with narrow and low enough duty current spikes, diodes and LEDs can withstand current peaks 2 or 3 orders of magnitude greater than their maximum average ratings. You need to examine the data sheets to best decide on the current limiting resistor if you want to keep resistive losses to a minimum without needlessly endangering the diodes and LEDs chosen for this type of circuit.
To be fair and by your own admission, your foray into the design of LED based mains indicator lamps seems to have been more experimental than theoretical. At least you got to a working model (by my count) in only 3 iterations. :-)
By that example, I would say you *do* have *some* idea of what you were doing, contrary to Martin Brown's thesis. It just seemed to me that you CBA with those pesky calculations to check out the consequences of your original choices of component values.
Now I, when it comes to powered circuits, am of a rather nervous disposition so tend to calculate the consequences of component choices before allowing "The Experiment" to go 'Live'. :-)
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