Is it safe to run 12v DC LED from 8v AC bell transformer?

[Mikefromlondon]

Why not do it properly?
http://www.ebay.co.uk/itm/78L12-Pos...hash=item2100dd8bfd:m:mFxc4-15sookGnOm7DIiCcw

Sorry mate, but its nothing to do with doing it properly, it would be insanity to use a 3 pin voltage regulator to just light up an LED, seriously it would be considered as a very unusual and unnecessary use of a 3 pin regulator just to drive an LED, besides, you then have to use a couple of high frequency decoupling caps both at input and output in case you didn't know for supressing any possibility of emf emissions and to stop regulators having instability. Most circuits using LED's do not require regulated power supplies, such as Xmas flashing light circuits, also 3 pin voltage regulators can get pretty hot when operating in linear mode and there is a lot of voltage to drop, completely unnecessary and not a common practice.

Of course there would be absolutely no harm in using one, and if a circuit was already using a regulator because of need for one, then it is no problem running LEDs off that regulated power.

 

[haggis999]

I have now obtained a packet of Vishay W02G-E4/51 bridge rectifiers and a selection of resistors. However, I have just realised that my Wiring Option C requires three conductors to be routed to the pushbutton. I have therefore generated the attached Wiring Option D diagram, which puts the bridge rectifier in series with the bell solenoid and thus needs only two conductors to go to the pushbutton. Have I got this right?

 

[Mikefromlondon]

Yes it will work good, and if you add a capacitor of say 100uF 25v you could then achieve no loss of LED light during short press of the switch, up to a certain length of time, LED would slowly fade away if button was kept pressed for too long, after button is released, the capacitor charges up again rapidly, ready for next press immediately, how long the LED continues to illuminate depends now on your capacitor value as well, a 100uF would provide illumination for a good few seconds, and also as you would be using a bridge rectifier, the capacitor will not drain back into the bell switch as the bridge rectifier diodes blocks reverse flow. Go ahead and do it now, get your soldering iron out, I hope you have a decent iron. Jesus Christ i am still 95! thanks!

 

[haggis999]

I thought you meant your age - and then I clicked the Thanks button...

 

[Mikefromlondon]

I am not far off that, lol! subtle hints work! lol thanks Remember next time to help all your mates too, this what goes on here, I noticed, there is a definitive pattern, you scratch my back and i will scratch yours, no wonder many have over a 1000 thanks. so we might as well learn something from them and put it in practice.

 

[JohnW2]

... there is a definitive pattern, you scratch my back and i will scratch yours, no wonder many have over a 1000 thanks. so we might as well learn something from them and put it in practice.

It has actually become a fair bit more difficult to accumulate 'Thanks' since the forum software changed a while ago. It used to be 'Thanks' or nothing, but now a lot of what would then have been 'Thanks' (particular amongst the 'regulars') have now become 'Likes'.

Kind Regards, John

 

[endecotp]

Have I got this right?

Use a voltage regulator.

 

[Mikefromlondon]

It has actually become a fair bit more difficult to accumulate 'Thanks' since the forum software changed a while ago. It used to be 'Thanks' or nothing, but now a lot of what would then have been 'Thanks' (particular amongst the 'regulars') have now become 'Likes'.

Kind Regards, John

and wasn't there a time when you would only score once per thread, any further thanks given would simply not add up to increase the score.

 

[haggis999]

When connected directly to my 8v AC transformer in a test setup, the bridge rectifier delivers 13.6v DC with no load.

Connecting that DC source to the LED of my pushbutton via a 1k resistor gives an acceptably bright white light, though I guess it will dim a little when the bridge rectifier is in series with my bell solenoid. The DC voltage reduces to 12.8v when driving the LED. There is no flicker.

 

[Mikefromlondon]

I guess this is without you having added any capacitor, when you do add any value capacitor across, the voltage would jump up to 19-20v dc, so beware, and if you are happy as it is, if the LED doesn't appear to shimmer, then just don't add any cap when it is truly not needed for an LED in this circumstance.

Can you measure voltage across the 1K resistor as it is in your test circuit, this will enable me to determine what value of current your LED is consuming since LED would dissipate across it 2.2V, so 13.6v - 2.2v gives us 11.4v , and if assuming 1 K has 6 volts across it, then we know 11.4 - 6 gives us 4.4volts across internal resistor, and we can then work out the value of the internal resistor without having to cut open the switch to see what value they used.

Once we determine the value of the internal LED resistor, we can then work out from LED's recommended 12V what the actual current LED has been designed for . so for instance, if we determined that internal resistor value is 500R, and at 12v supply voltage, we would have 12-2.2 giving us 9.8v, so that means 9.8 divided by 500R will give us LED current which in this example comes to just under 20mA, however by adding a cap, this value can go up, so say we do add a cap of around 22uF, and the voltage rises to 20v dc, so now we have a new situation 20-2.2 =17.8, so we now divide 17.8 by 500r internal resistance it now gives us 35mA, and again this is unlikely to damage your LED, as they can tolerate a lot more than 35mA, many can stand 50mA continuously.
#
finally you don't really want it to be so bright, and why make yourself visible from miles away, thieves would think Oh wait a minute have we seen a door bell like it before, and so they will think this owner must have something and let us play knock knock ginger with him!

 

[JohnW2]

and wasn't there a time when you would only score once per thread, any further thanks given would simply not add up to increase the score.

That's certainly how it works with some forums (one's Thanks' only count once for each recipient in the same thread), but I don't think that's ever been the case with this forum whist I have been a participant. Maybe it was earlier on - but I see that you've been a member for a few months less than myself. Some forums, and maybe this one in the past (can't remember!), present 'Thanks' (or whatever they call them) as, say, "XXX Thanks in YYY threads" - so, in effect, also gave an indication of how many threads one had been thanked in (even if by multiple different people within one thread).

As for the 'mutual back scratching' you previously spoke of, I think that at least some of the major players are too 'competitive' to engage in a lot of that - since they may well not wish to give 'points' to their 'competitors'

Kind Regards, John

 

[haggis999]

It's 6.4v DC across the 1k resistor. Actually, even though I asked for a 1k, it appears to be a 1.1k Ohm resistor (colour code is Brown, Brown, Black, Brown, plus Brown for tolerance).

As you say, there is no need for me to light up the entire street with my LED. All I need is something that makes the pushbutton location obvious after dark.

 

[haggis999]

BTW, when you say that a capacitor would enable the LED to stay lit for a few seconds while the button is being pushed, do you mean the same capacitor C1 location shown on my wiring diagrams for suppressing any flicker?

 

[Mikefromlondon]

yes, as it would now retain charge for a longer period due to full wave rectification and blocking action of the diodes, so the only component that would drain it slowly away will be the LEd itself.

Whereas previously not using a bridge, the switch contacts would short across the cap and (if it was being used to smooth flicker) and kill the charge on the cap instantly, also reduce switch life)

Can I now give you a Diploma in electronics

 

[Mikefromlondon]

It's 6.4v DC across the 1k resistor. Actually, even though I asked for a 1k, it appears to be a 1.1k Ohm resistor (colour code is Brown, Brown, Black, Brown, plus Brown for tolerance).

As you say, there is no need for me to light up the entire street with my LED. All I need is something that makes the pushbutton location obvious after dark.

Yes I wish they did that to my car door lock, mine is a 20 year old car, when remote locking or unlocking was not common, in dark i have treal problem seeing where the keyhole is!