Constant AC current ?

[Externet]

Hi fellows.
The nature of AC voltage does not play well for obtaining constant current; but want opinions on this and other methods you may come up with...

CCAC.png (11.52 KiB) Viewed 6039 times

How would you do it ?
Which cheap 2 leaded IC for I1, I2 would be applicable ?
If they have built-in D1, D2, better.
Would a 'limiting' capacitor added in series with the AC supply work also ?

( For many number of white LEDs )

Would this work for I1, I2 devices ? :
NSIC2020BT3G ---> http://pdf1.alldatasheet.com/datasheet- ... 0BT3G.html

Miguel

[Bob Scott]

I'd just buy a cheap string of white Xmas lights.

A series resistor would get warm because it would dissepate power. A series capacitor would conduct harmful high frequency spikes directly to the LEDs. I bet the Xmas lights use some combination of resistors and capacitor.

[dacflyer]

i have usually seen a dioad and resistor, the lights typically run at 1/2 cycles ( 60 volts 30Hz )

[gerty]

I've used this before, at 120v . Not sure where I got the drawing

[Externet]

Thanks, gerty.
That does not maintain a constant current if you increase the number of LEDs in series.

By the way, in your picture, you can replace the 1N914 with another LED in counterparallel instead of wasting the opportunity to double the light.

[gerty]

[quote="Externet"]Thanks, gerty.
That does not maintain a constant current if you increase the number of LEDs in series.

By the way, in your picture, you can replace the 1N914 with another LED in counterparallel instead of wasting the opportunity to double the light. [/quote]


With the ac voltage crossing zero 60 times a second, I don't know how you would keep the current constant.
As far as an additional led instead of diode, I have used that arrangement for polarity detection (at lower voltages), but when ac is applied they both lit

[Robert Reed]

Have you considered constant current diodes? They are basically an FET with gate & source internally tied together resulting in a standard two lead diode package. I have never used one, but have been on the brink of ordering for past projects.If memory serves me right they were in the 20 ma range. Don't remember their PIV ratings, but might be worth checking out.

[Externet]

Hi Robert.
THAT was the question on the original post for I1 and I2. (see its last link)
Next time I order parts, will find out.
Miguel

[haklesup]

Ignoring the semantics of AC signal never really being Constant (DC) one can imagine a constant AC current. The strategy here is to put enough LEDs in series so that the voltage is dropped across a larger impedance with less total tolerance drift than a single diode alone. Naturally all LEDs in one string have the same current which stays relatively constant and adding a second string in parallel has a decent chance of being close to the first and not hogging current.

However, no matter what, if you put poorly tolerance devices like LEDs as the only load with a constant voltage supply the current will only be somewhat predictable. If you really do need constant current, you will have to use a current source and not a voltage source. If you want to discuss constant current LED driver chips or a current mode AC supply, that's a slightly different topic