running led off 220V mains

[Johan Wagener]

Is it possible to run a led off 220V mains supply without using a
transformer? It draws very little current so I suppose a 1/2 W resistor with
a very high resistance would do the trick? also do I need rectification
considering the fact that a led is a diode

 

[UncleWobbly]

sure- you'll need to put it back to back or block it with a normal power
diode (350v forward or reverse - whatever you decide) coz the reverse
voltage will fry the LED otherwise, but work out the resistor as peak
voltage-2 / 10mA so with 220V RMS, we get (to get peak from RMS divide by
0.7071)

220/0.7071 = 311V peak

less 2V for the LED (practically you can ignore the LED voltage when driving
from HV sources) driving at 10mA... 309V/0.01A=30900ohms so any resistor
above 30K will do the trick - of course it will flicker (impersceptably?) at
the mains frequency

remember also that the other diode (you could always use two LEDs) will be
shunting the current if mounted back to back (by which I mean the two diodes
are mounted in parallel but opposite polarities) and it must be rated at
350V forward voltage. If you decide to use a diode to block (by which I mean
the two diodes are mounted in serial with the same polarities) you must
ensure it has a reverse breakdown of 350V or more otherwise it'll all go up
in smoke ) remember also that 300V @ 10mA is 3Watts of power - resistor
and power diode please take note. If you can get away with lower current
you'll be better off - consider using a hyperbright LED and drop the current
(by raising the resistor value) as much as possible. These LEDs are still
very bright at just a few mA. You could half the current (and so the power)
by doubling the resitor and still get a perfectly acceptable brightness from
the LED - perhaps even more. Play around and get it right....be very careful
rectifying mains - you'll get 320+volts of DC and across fingers it stings
*big time* - Any voltage at that level (but particularly DC) from one hand
to the other ***will kill*** even at very low currents - Don't take chances
with it!!!

"Johan Wagener" <pil(nospam)@webmail.co.za> wrote in message
news:3f75eb1f$0$64721@hades.is.co.za...

Is it possible to run a led off 220V mains supply without using a
transformer? It draws very little current so I suppose a 1/2 W resistor
with
a very high resistance would do the trick? also do I need rectification
considering the fact that a led is a diode

 

[petrus bitbyter]

"Johan Wagener" <pil(nospam)@webmail.co.za> schreef in bericht
news:3f75eb1f$0$64721@hades.is.co.za...

Is it possible to run a led off 220V mains supply without using a
transformer? It draws very little current so I suppose a 1/2 W resistor
with
a very high resistance would do the trick? also do I need rectification
considering the fact that a led is a diode

It's a matter of power. Most LEDS operate best at a 20mA. Neglecting the
voltage across the LED you need a 11KOhm 5W resistor to run the led. It will
become pretty hot. You also have to put another diode antiparallel to the
LED as the LED will not withstand the 220V (about 300V peek) reverse
voltage.

As the LED conducts for only one half cycle the real current is only about
10mA. To improve the circuit you can place the LED in a diode bridge made by
four 1N4148 for instance to get the full current.

To reduce energy loss, you can lower the current. Most LEDs light at small
currents of 1-2mA already. You may look at high efficiency ones to get more
light for the current.

The circuit I advise for the LED uses thee 4k7 0.25W resistors in series, a
four 1N4148 diode rectifier bridge and a high efficienct red LED.

For more light you may replace the 1N4148 by LEDs.

But... for simplicity you can better use a 120kOhm 0.5W resistor and a small
neon light.

pieter



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[default]

On Sat, 27 Sep 2003 21:55:07 +0200, "Johan Wagener"
<pil(nospam)@webmail.co.za> wrote:

Is it possible to run a led off 220V mains supply without using a
transformer? It draws very little current so I suppose a 1/2 W resistor with
a very high resistance would do the trick? also do I need rectification
considering the fact that a led is a diode

Very practical and easy to implement. I have an LED night light that

has been in continuous operation for over five years.

The circuit I used came out of a Siemens optoelectronics book.

Use a capacitor instead of a resistor to save power. Calculate the
voltage drop of the LED(s), rectifier diodes, subtract from 220. With
that voltage and the current you want to push through the LEDs
calculate the Capacitive Reactance needed in ohms then solve for
Capacitance needed to achieve that at your mains frequency.

I wanted three LEDs with a full wave bridge rectifier and calculated
..5 microfarads in series with the LED/rectifier. Back to back LED's
will work also and eliminate the rectifier. Bear in mind if you do
use back to back LED's each led is only on for 1/2 the time so the
current should be doubled to maintain brightness. Double the current
means a cap with twice the capacity - cheaper and smaller to use a
full wave bridge.

Use a small 100 ohm resistor in series to limit inrush current and to
act as a fuse. I used an ordinary carbon film 1/4 watt and when I
shorted the cap it opened without burning - but you'd want to make
sure it doesn't burn. (shorting the cap also opens the LED's so it is
destructive testing).

Insulate it well and you can make it safe. The voltage rating of the
cap should be 3X higher than the mains if it is DC rated, in a good
quality mylar, non-polar cap. Or use a cap with a 220 VAC rating. I
use 500V DC mylar caps for 120 volts.

Another idea would be to get a hold of an arc snubber, resistor and AC
cap in one package sold to prevent arcing on power switch contacts.

The capacitive reactance causes no heat or real power loss. There is
a small reactive current flowing, but it is out of phase with the
voltage so doesn't use power.


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[Brian]

The problem with using a series capacitor is that any transients will pass
through the cap at full voltage. They tend to fry electronic components. So
if using this method it is essential to include another capacitor across the
electronic item. Usually around one tenth of the value of the series cap
will be a good starting point.

 

[Costas Vlachos]

"Johan Wagener" <pil(nospam)@webmail.co.za> wrote in message
news:3f75eb1f$0$64721@hades.is.co.za...

Is it possible to run a led off 220V mains supply without using a
transformer? It draws very little current so I suppose a 1/2 W resistor
with a very high resistance would do the trick? also do I need
rectification considering the fact that a led is a diode

You'll need a diode to half-wave rectify the mains, and a series resistor.
Like this:

.. ___
.. o------>|------|___|------>----.
.. 1N4004 R I |
.. | /
.. 220VAC LED V /
.. RMS -
.. |
.. |
.. o------------------------------'


The half-wave rectifier protects the LED from excessive reverse voltage, and
R limits the current. For 220V RMS, there will be about 310V peak across R.
So, for I = 20mA peak, you'll need a 15 kOhm resistor. Because of the
half-wave rectifier, there will be about 155V RMS across R, so it will
dissipate about 1.6 Watts. Note that the average LED current will be much
less than 20mA, but most LEDs will light up just fine. You can also use
low-current LEDs to minimise heat dissipation across R. Finally, BE EXTRA
CAREFUL, you can get killed by directly handling live mains like this!

cheers,
Costas

 

[default]

On Sun, 28 Sep 2003 02:04:32 GMT, "Brian" <win@biggerpond.com> wrote:

The problem with using a series capacitor is that any transients will pass
through the cap at full voltage. They tend to fry electronic components. So
if using this method it is essential to include another capacitor across the
electronic item. Usually around one tenth of the value of the series cap
will be a good starting point.
Sure makes sense. A more fool proof method would be to clamp the

voltage across the rectifier diode with a low voltage mov or back to
back zeners. I like the idea.

In five years I haven't had a failure in either of two night lights so
perhaps the resistor serves as a current limiter? I'm in an area that
has frequent electrical storms - three fried modems in five years
(underground phone lines and mov on the phone line) and the occasional
near miss resets my BIOS.



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[cpemma]

"Johan Wagener" <pil(nospam)@webmail.co.za> wrote in message

news:3f75eb1f$0$64721@hades.is.co.za...
Is it possible to run a led off 220V mains supply without using a
transformer?
UncleWobbly wrote:
sure- you'll need to put it back to back or block it with a normal
power diode (350v forward or reverse - whatever you decide) coz the
reverse voltage will fry the LED otherwise...

Actually it doesn't, at least not quickly. I've run a led happily on 60v AC
for several days (led didn't pack in, I did). On reverse voltage, the led
will hit its avalanche (?zener?) region, and *provided the current is
limited* will survive, just as a zener diode does. Current does need to be
limited to a few mA, or the watts generated exceeds the led rating.

That said, it flickers perceptibly on 50Hz half-wave, so a
resistor/capacitor current limiter with a FW bridge rectifier system to the
led is much better.

 

[Don Klipstein]

In article <bl7ct8$qoe$1@news8.svr.pol.co.uk>, cpemma wrote:

"Johan Wagener" <pil(nospam)@webmail.co.za> wrote in message
news:3f75eb1f$0$64721@hades.is.co.za...
Is it possible to run a led off 220V mains supply without using a
transformer?
UncleWobbly wrote:
sure- you'll need to put it back to back or block it with a normal
power diode (350v forward or reverse - whatever you decide) coz the
reverse voltage will fry the LED otherwise...

Actually it doesn't, at least not quickly. I've run a led happily on 60v
AC for several days (led didn't pack in, I did). On reverse voltage, the
led will hit its avalanche (?zener?) region, and *provided the current is
limited* will survive, just as a zener diode does. Current does need to
be limited to a few mA, or the watts generated exceeds the led rating.

I am afraid it can happen slowly. Some LEDs fade (apparently to me from
diffusion of active ingredients) as a result of some combination of
temperature and bias voltage (what is specifically mentioned in Agilent's
relevant application brief is a combination of duty cycle and
temperature). This makes me think that reverse bias, especially with
heat, can be bad for some of them.

Some other LEDs, as in GaN and InGaN types in general (in general UV,
violet, blue, blue-green, non-yellowish-green, white, and Nichia's yellow)
develop partial shorts nearly enough instantly from reverse breakdown.

That said, it flickers perceptibly on 50Hz half-wave, so a
resistor/capacitor current limiter with a FW bridge rectifier system to the
led is much better.

- Don Klipstein (don@misty.com, http://www.misty.com/~don/ledx.html)

 

[Pieter Hoeben]

On Sat, 27 Sep 2003 21:55:07 +0200, "Johan Wagener"
<pil(nospam)@webmail.co.za> wrote:

Is it possible to run a led off 220V mains supply without using a
transformer? It draws very little current so I suppose a 1/2 W resistor with
a very high resistance would do the trick? also do I need rectification
considering the fact that a led is a diode
What current? Low current leds can do with 2 mA, normal ones 20mA,

and big ones even much more. I will presume 20 mA here.

Do not use a resistor, but a capacitor in series. For 20 mA you need
an impedance of 220V/20mA = 11000 ohm. This would mean a
capacitor of (at 50 Hz) 0,2894 u, lets say 270 nF / 400V
To limit the inrush current when you switch on at the peak of the
50Hz sine I recommend a series resistor of 2k2, 2 watt or so.

Only a series resistor for 20mA would be 11kohm, 4.4 Watt

And then, use a rectfier bridge that can handle these
voltages and connect your led to the + and -.

Success
Pieter