Looking for a simple circuit for power regulation and led indicator light

[pityocamptes]

Actually you are right, wasn't even thinking. So what would be the best method to show only the single red led for under a certain source voltage? Plus this would save power from the green led consuming power when it does not need to. Thanks again!

 

[MikeMl]

...So what would be the best method to show only the single red led for under a certain source voltage?..

Back to the single LM431 circuit in the other thread. Just set up the voltage divider (pot wiper) to produce 2.495V when the input voltage is 9V.

 

[Mike odom]

You still need two comparators, either the dual LM358 in an 8pin DIP package, or two of the LM431s.

Why are you using LM358? These are opamps. Why not use the LM393 dual comparators? These are open collector comparators and you wouldn't need D1.

 

[pityocamptes]

Oops double post

 

[pityocamptes]

Back to the single LM431 circuit in the other thread. Just set up the voltage divider (pot wiper) to produce 2.495V when the input voltage is 9V.

You mean the lm358? Confused now, as you used the 358 in the schematic. Can you explain what exactly why do I adjust the wiper to 2.495v? What does that do? Sorry for the ignorance...

I'm sorry to ask this but which parts would I exactly remove from the schematic you posted if I only use one LM358?

 

[MikeMl]

Why are you using LM358? These are opamps. Why not use the LM393 dual comparators? These are open collector comparators and you wouldn't need D1.

Likely 'cause I had about a 1000 Lm358s on-hand, and I would have had to go buy a LM393

 

[MikeMl]

You mean the lm358? Confused now, as you used the 358 in the schematic. Can you explain what exactly why do I adjust the wiper to 2.495v? What does that do? Sorry for the ignorance...

No, forget the LM358 circuit. Use the one shown here. I simplified it. The Led is on above 8.9V; off below. The Green trace shows current through the Led as a function of the input voltage (the x-axis). Note that at the point where the LED is turning off (~8.9V), the voltage on the REF pin (Red trace) is 2.49V.

To modify this circuit for other voltages, you need to make the voltage divider R8 and R6 produce 2.495V when the voltage at CC is whatever you want it to trip at. 2.495V is the internal reference voltage inside the LM431 (read the data sheet). Also note that LM431 and TL431 are the same part.

 

[Mike odom]

Likely 'cause I had about a 1000 Lm358s on-hand, and I would have had to go buy a LM393

And THAT, my friend, is the BEST reason of all (same reason I use the 1N4007).

 

[Mike odom]

Also note that LM431 and TL431 are the same part.

OMG, not the TL431... this actually is a better choice, and I never would have thought of it in this application off hand.... very good choice (and strangely, I am familiar with this part).

 

[pityocamptes]

Hey thanks! Just for experimentation, would it be possible to modify the circuit so that a red light comes on when it drops below 9 volts? Just trying to play around with ideas. Thanks again!

 

[MikeMl]

Hey thanks! Just for experimentation, would it be possible to modify the circuit so that a red light comes on when it drops below 9 volts? Just trying to play around with ideas. Thanks again!

I made a conscious decision to have a Green LED on when the voltage is higher than the trip point because that is when you have some excess power to burn. Lighting the LED after the voltage sags below the trip point means that you are hastening the demise of the battery. The original version of the circuit I linked you to twenty posts ago used an extra transistor to turn on the LED when the voltage drops.

 

[pityocamptes]

Ok. Thanks!