I'm now tucked up in bed doing the Daily Telegraph cryptic crossword, but I've just made a quick dash for the Thanks button...
Is it safe to run 12v DC LED from 8v AC bell transformer?
- Thread starter haggis999
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Not really, you've turned my nice even 200 into 201 !
I will just make it even number then.
For some strange reason, it seems that those buttons are always hidden until one moves one's cursor into the vicinity (not necessarily actually over) of where they are!
Kind Regards, John
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Into the thread, anyway.
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Now you're 203...Hang on, that's fixed it!
It's more than just 'into the thread' - at least for me (browsers may vary) it has to be 'into the message in question' and, with longer messages, it does not even seem to work everywhere within the message.
Kind Regards, John
Yes, sorry, I meant into the message. I hadn't realised it got fussy in the longer messages.
well worth bumping this thread, in the anticipation of offering genuine help. I gained 5 thanks.
As there is an element of doubt on what value of additional series resistor might be required to limit the current through the LED, how about using a trimmer potentiometer such as the following?http://uk.farnell.com/bourns/3296w-1-202lf/trimmer-25-turn-2k/dp/9353232
http://uk.farnell.com/bourns/3296w-1-202lf/trimmer-25-turn-2k/dp/9353232
http://uk.farnell.com/bourns/3296w-1-202lf/trimmer-25-turn-2k/dp/9353232
No, do it properly. It says it wants 12V, so use a 12V regulator like a 78L12.
Seriously, you are driving an LED, not a voltage sensitive CMOS Chip, that would be an overkill, and you do not need this and in particularly a 10 turn preset, that component is used in highly precision electronics, where you have to trim voltages or current to like 2 or 3 decimal places! An LED rated at say maximum 50mA can still sustain a lot of assault, like I said before they are far more tolerant than light bulb, you just cannot go wrong even if a 12v LEd was being driven by a 16v .
Choosing the value of the pot, again you will need some idea roughly what value you need the pot to be, such that it can dissipate power along most of its track, or at least 50% of its resistive surface, imagine if you get a 100K pot, and you wanted only say 200 ohms, that 100K pot would only need to be turned a fraction just a few degrees from one end and (since 100Kilo ohms divided by 10 turns gives you roughly 10Kilo ohms per turn, so to get 200 ohms from a 100K pot would practically mean you are just 10 degrees of your first turn, its not even one full turn! and that short space is going to dissipate 1/4watt, it would burn that track and since these pots are only rated at 500mWatt. (1/2 watt across the full track)
If you were experimenting and learning then by all means do it, for practical purpose a fixed value resistor is fine, besides when you buy these pots (presets) they come set mid way, and you could still get it wrong if you turned it wrong way.
Nor do you need a 12v regulator for an LED circuit.
Choosing the value of the pot, again you will need some idea roughly what value you need the pot to be, such that it can dissipate power along most of its track, or at least 50% of its resistive surface, imagine if you get a 100K pot, and you wanted only say 200 ohms, that 100K pot would only need to be turned a fraction just a few degrees from one end and (since 100Kilo ohms divided by 10 turns gives you roughly 10Kilo ohms per turn, so to get 200 ohms from a 100K pot would practically mean you are just 10 degrees of your first turn, its not even one full turn! and that short space is going to dissipate 1/4watt, it would burn that track and since these pots are only rated at 500mWatt. (1/2 watt across the full track)
If you were experimenting and learning then by all means do it, for practical purpose a fixed value resistor is fine, besides when you buy these pots (presets) they come set mid way, and you could still get it wrong if you turned it wrong way.
Nor do you need a 12v regulator for an LED circuit.
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It's encouraging to know that I am unlikely to burn my LED to a crisp unless I do something really stupid. I plan to order a bridge rectifier and a 1k Ohm resistor tomorrow and get this show on the road.
Why not do it properly?
http://www.ebay.co.uk/itm/78L12-Pos...hash=item2100dd8bfd:m:mFxc4-15sookGnOm7DIiCcw
http://www.ebay.co.uk/itm/78L12-Pos...hash=item2100dd8bfd:m:mFxc4-15sookGnOm7DIiCcw
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don't just order a single 1 k resistor, order some 560R, 330R and a 220R, see any of these trims your voltage to the final terminal of the led at the switch to about or between 10-12volts.
And another thing, you can build different resistor values by adding them in series and parallel, e.g if you have two 1 K resistors in series it will give you 2K total resistance, and in parallel it would be half of 1 k so it would end up being 500R (this is provided both resistors are same value when in parallel)
otherwise if they are different values, you have to use a formula R1 x R2 divided by R1+R2 so if you have 1K in parallel with 560R, this would give you
1000 x 560 / 1000 + 560 = 359R
I think by the time your door bell is ready to ring, you would be an expert in electronics.
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