want to add led indicator for "current limit"

[bengaltiger]

Hi,

I want to add LED indicator for "current limit" in the following circuit.

http://www.zen22142.zen.co.uk/Circuits/Power/l200.html

Please help.

Regards,
BT

[TerminalJack505]

There are a couple of ways to do this. 

• Measure the voltage difference between pins 5 and 2.  A difference of 0.45V will indicate that the regulator is limiting the current.
• Measure the voltage at pin 4, relative to ground.  A voltage less than 2.77V will indicate that the regulator is likely limiting the current.
  

The attached schematic is an example of a circuit that implements case #2 above.  It uses a TL431 as a comparator.  When the voltage at pin 4 drops below ~2.77V the LED will come on. 

You would likely want to place a small cap across R2 and would probably need to tweak R1's value for a real-life circuit.  Also, it might be necessary to either use an LED with a fairly high Vf or add a diode or two in series with the LED to ensure it stays off when it should.

Another option would be to use a comparator with a built-in voltage reference to implement a similar circuit.

[TerminalJack505]

I was bored so I drew up a schematic that implements case #1 mentioned in my earlier post.  As you can see it requires a few more components.  R1 would be Rsc, the current sense resistor between pins 2 and 5 of the L200. 

U2 can be replaced by a comparator with a built-in voltage reference.  This would eliminate the subcircuit that generates Vref as well.

[TerminalJack505]

I guess it probably doesn't matter since the OP seems to have created this topic and then immediately fell off the face of the earth but if anyone else is interested I built these circuits on a breadboard and they work fine.

[grumpydoc]

Well, Bengall Tigers are an endangered species....