Led limiting resistor

[igeorge]

Hello,
I have a line where the current varies from 1 to 5 amps.
I would like to insert a resistor which will turn on an optocoupler rated at 1.5 volts , 10 mA
The issue here is that at 5 amps i need a 0.24ohms resistor , and at 1 amp i need a 1.2 ohm resistor.
Assuming that i use the 1.2 ohms resistor, at 5 amps will generate 6 volts and fry my opto.
I need help to solve this issue, which i do not know how to do it
Regards
Ion

[Demon]

I would start here:

http://www.centralsemi.com/PDFs/sele...t_is_a_cld.pdf

A Current Limiting Diode, also known as a “Current
Regulating Diode” or a “Constant Current Diode”, performs
quite a unique function. Similar to a zener diode,
which regulates voltage at a particular current, the CLD

limits or regulates current over a wide voltage range.

[igeorge]

Thanks Robert, but you missed the whole point of the question. For constant current ti is not an issue as we use Ohm's law to calculate.
The point is in this particular case that current change from 1 Amp to 5 Amps
I will try to make a sketch and post it soon.
Regards
Ion

[rsocor01]

I will try to make a sketch and post it soon.
Regards
Ion

Yes, posting the schematic will help, so we can have an idea of what you are talking about. Also, what optocoupler are you using?

[igeorge]

Hello,
I do not have any opto in mid yet, but i am flexible on what ever i can use.
Here is a concept of what i need
regards
Ion

[rsocor01]

What is the purpose of R2? I don't quite understand what you are trying to do. Are you trying to measure the current in the heater? Or, are you trying to switch it on/off?