# LED on AC Mains

Discussion in 'LEDs and Optoelectronics' started by smartmachine, May 14, 2013.

1. ### smartmachine

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0
May 14, 2013
Hi!........

I saw a circuit on the instructables.com website, which described a simple circuit of how to connect a LED on AC Mains, on the following link:

http://www.instructables.com/id/LED-on-AC-Mains/

The circuit consisted of a Diode (1N4007), a resistance (resistor) of 1 watt rating of value anywhere from 20 kilo ohms up to 100 kilo ohms, and of course an LED (5mm or 10 mm of any colour). The circuit was assembled in such a way that, the Diode (1N4007) was connected with its black anode to the Cathode of the LED and the resistor was connected on the anode side of the LED.

I built this circuit with using Bridge rectifier (combining four 1N4007 diodes) and it is working fine. The problem (and my Question of course) is this that, the resistor in this circuit ALWAYS gets VERY HOT! I first used the resistor of 1 Watt rating of value 22K ohms, then used 27K ohms in place with 2 Watt rating, and now used 10K ohms of 5 Watt rating, the problem remains the same! The circuit still works, but the same resistance gets sooo hot (but never burns out, in all above cases and in this 5 Watt case of the resistor also, I've noted it for a very long time connecting the circuit with AC Mains) that u can't even pick it up with your two fingers to sense the temperature of the resistor! WHY IS THIS SO??..... and How to overcome this problem?

Plz provide help in this context for which I'll be grateful.

Last edited: May 14, 2013
2. ### CocaCola

3,635
5
Apr 7, 2012
That is the absolute WORST way to run LEDs off mains, and people that suggest doing it that way should be ashamed, it's a fire hazard and basic all around don't do hazzard...

Of course the resistor gets HOT, consider this for a moment (simplified), take an LED rated for 3.5V @ 20mA... You are pumping in 110V that means the resistor has to destroy 106.5 Volts... Simple math says that resistor is dissipating over 2 Watts as heat!

My advice unplug these and don't plug them back in, it's a huge safety issue...

3. ### CDRIVEHauling 10' pipe on a Trek Shift3

4,960
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May 8, 2012
I've been wanting to say this for quite some time. That website is a lawsuit looking for a place to happen! Absolutely the worst crap science found on the net!

For the most part it's the place to go to learn how NOT to do something!

Chris

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Aug 13, 2011
I often refer to them derisively as "destructables".

Better methods for powering small DC loads from AC main voltage without a transformer are shown here:

http://www.zen22142.zen.co.uk/Circuits/Power/tps.htm

Last edited: May 14, 2013
5. ### KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
Hmm. Yes.

I emailed them a while back asking if they wanted me to review their electronic instructables (for free) for obvious usability issues (after I saw a horrible design there for some voice-activated switch or something). I didn't mention safety because it didn't apply to that particular project. Anyway, the gist of their response was:
They don't, and can't, "authenticate" projects. Their site is for people to "share" and improve their "creations". Some projects are "exploratory" or prototypes. I can leave a comment on the crappy instructable, or make my own instructable and link to it from the crappy one.

No doubt their site has a disclaimer saying that they're not responsible for safety issues, copyright issues, issues relating to incompetent designers, etc.

I've looked at a few projects there, and most of them (the electronics ones, at least) seem to be of low quality.

I did post a response to the instructable that started this thread, but it seems to have disappeared. Perhaps it was too long. I want to edit it, to mention that using that design would probably void your fire insurance. That was a safety issue I hadn't thought of.

Last edited: May 14, 2013
6. ### CDRIVEHauling 10' pipe on a Trek Shift3

4,960
652
May 8, 2012
Since it's not uncommon for a newbie to begin a post with "I found this on Instructables..." maybe we need a early warning system.

Chris