Led limiting resistor

[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?

[igeorge]

r2 is to turn on the opto.
all i need is to sense if the heater is burned out when the switch is closed.
I do nt measure current
thanks

[rsocor01]

r2 is to turn on the opto.
all i need is to sense if the heater is burned out when the switch is closed.
I do nt measure current
thanks

What about using a relay to detect if the heater is burned out?

[HankMcSpank]

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

iF you place a 450R resistor in series with the opto, that'll ensure the voltage doesn't exceed 1.5V/10mA at the opto for 5A through the element ....of course the opto will only then 'get' 300mV if the element current drops to 1A....but without knowing more about the opto blah blah