LED with AC Current

[scarrier]

hello, I am working on a little indicator that will light a led 20mA green led. I want it to be fully lighted at 25 amp. the input is a CT with 1520 ratio

I have come to this design (see attachment)

But i have a lot a difficulty on how to do the math on that circuit. when doing simulation i have realized that the voltage at the CAP point is clipping.

[Simon]

what is your input voltage ?

All you need is a 1N4007 diode, your LED and a suitable resistor, the 1N4007 diode will protect from the reverse voltage cycle and the resistor will regulate your current.

[scarrier]

it actually an input ac current and with the 100 Ohm it makes a voltage, I intend to play with that resistor to change when the light go on 

I put the cap because I tought the led would flicker if the cap isn't there.

For a 25 AMp primary i have 16.5mA because of the 1520 ratio  now with the 16.5mA I can put a 100 ohm like on the design to have 1,65 Veff or a 200 ohm to have 3,3Veff

[Simon]

what ratio ?. please start from the beginning.

Where are you getting your supply voltage from ?

[hlavac]

I don't think you have enough power in the circuit to light the LED and charge the (big!!!) cap.
You need ~2.1V to light a green LED, and at least a few mA of current. These will steal the current from the 100R resistor and voltage will drop across it. It already is only ~1.6V with no current stolen...

If you were to steal 10mA from the shunt resistor and have 2.1V across it with the remaining 6.44mA going thru it it would have to be 330R or so. Put a smaller cap there. 1u or so.

It would be interesting to know the open circuit voltage on secondary at 25A - is it higher than 2.1V at all?

[Simon]

oh i get it your using a current sense transformer.

just supply your LED ? if it's a current source you don't have to worry about dropping any voltage

[hlavac]

I'd suggest simply putting the LED across the secondary, with a diode antiparallel with it. Done

[Simon]

yea quite, although i get the impression the Op is trying to get the LED to light when 25amops are reached, this will require circuitry.

[scarrier]

yes Simon you are getting what I try to do, I can have almost any voltage depending on the resistor i put across the Curent sens transformer. At the end I want to have 4 ranges the first is 25 amps, the second is 40, the third is 50 amps and the last is 65 amps, what i think will change between all those circuit is the resistor at the input.

The curent sens is : http://www.crmagnetics.com/Products/Assets/ProductPDFs/CR8300%20Series.pdf

what i am trying to sort out is how to determine the resistor value to know when the led will start to light up and when it will be fully lighted.

The other alternative I have found is putting a bigger zener diode for each range to compensate the higher voltage.

[scarrier]

hlavac

I'd suggest simply putting the LED across the secondary, with a diode antiparallel with it. Done

I could probably do that but the led would flicker at 60 Hz an this would be annoying, I would like to have a steady on state, and off when there is less than 20 amp at the input

[Simon]

if you are looking to detect a certain current flow then you need a comparator based circuit with appropriate resistor for the voltage.

You won't see the 60Hz flicker, if you do then a small capacitor in parallel will do it

[scarrier]

there is no external source of power i would like to keep it passive, i think the circuit at the first post would work it is basically what you describe the revese diode, and the cap in parallel with the led and a resistor for the current. Is there a formula to know what is the voltage that will be in the cap see the attachment, The voltage clip and I don't know why and how to calculate this.

[hlavac]

hlavac

I'd suggest simply putting the LED across the secondary, with a diode antiparallel with it. Done

I could probably do that but the led would flicker at 60 Hz an this would be annoying, I would like to have a steady on state, and off when there is less than 20 amp at the input

Then make a bridge rectifier and put the LED after it, makes it flash at 120Hz (not very visible), with added load symmetry as a bonus