LED off at half current?

Thread Starter

Poor old sod

Joined Jul 25, 2017
128
what max current, when halved, will turn off a red 3mm generic LED, pls. viewed in shaded daylight. it's acting as a cheap sla charge indicator.
 

Thread Starter

Poor old sod

Joined Jul 25, 2017
128
voltage variation charged to flat 26.4v to 24v, less 2x 9 volt zenners, less 3v LED, less a resistor voltage. as battery volts fall, so does current thru LED. pls answer question. i know an led/resistor alone would not do it. general indication only req'd.
 
Last edited:

KeithWalker

Joined Jul 10, 2017
1,583
voltage variation charged to flat 26.4v to 24v, less 2x 9 volt zenners, less 3v LED, less a resistor voltage. as battery volts fall, so does current thru LED. pls answer question. i know an led/resistor alone would not do it. general indication only req'd.
In the circuit you describe above, the voltage across the resistor and LED will vary between 8.4V and 6V. With the right value of resistor, that could vary the brightness of the led but the value of the resistor will have to be picked by trial and error. Even then, the results will vary if you change the LED for one of the same type because of the spread of its characteristics. The value you are asking us for will probably be somewhere between 5Kohms and 100Kohms depending on exactly which led you are using. They don't all give the same amount of light for the same current!
 

Thread Starter

Poor old sod

Joined Jul 25, 2017
128
thx want discerable difference between both events in overcast daylight, with shade. don't know relative eye sensitivity in such daylight. if on op was rough == 3w fillament bezel, and off was roughly half that lux, then would i see the difference?
 

Papabravo

Joined Feb 24, 2006
15,481
The slope of the IV curve for a diode is exponential and near vertical when the LED is on. The current can drop to a small percentage of it's on value and the LED will still be visible. Non-linear behaviors are really tough to get a handle on. Try it on a breadboard if you think you're onto something.
 

wayneh

Joined Sep 9, 2010
17,031
Why don't you just use a comparator and either have the LED on or off?
Second vote for a genuine comparator. In fact I'd use a quad LM339 and use the other 3 comparators to indicate other things such as proper connection, rate of charge, whatever seems useful.
 

Thread Starter

Poor old sod

Joined Jul 25, 2017
128
i'll need to set a fixed Vak =Vg+0.7 and Vcc =2x10v[?] zeners to Vbb, R1 as a [zener?] diode, R2 fixed provides the voltage swing for Vg [2.4v] and the Rl + LED are the cathode load @ 10mA.
 

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