hi,
i am charging a capacitor (4700uf) to 30v using a step up buck converter from ebay.
how can i use a red led and zener diode to light up when the capacitor is virtually charged up to say 28v .
a small diagram and zener diode value would be helpful
thank you
dave

 

To get good accuracy an inexpensive programmable voltage regulator is better than a Zener diode.
Below is the LTspice sim of an example circuit.

The TL431 turns on and lights the LED when the Ref voltage reaches 2.5V.
Here the values of voltage divider R2 and R3 provide that voltage at about a capacitor voltage of 28.3V.

 

thank you for that, i appreciate it.
please could you post a diagram of how it could be done with just a zener diode and resisitor and the values of each ?
i appreciate it will not be as accurate, but it has fewer components.
i understand your circuit, but i am not an electronics guru, as you may have guessed.

 

could you post a diagram of how it could be done with just a zener diode and resisitor and the values of each ?

Also needs a transistor and a few resistors, so won't be that much simpler than the circuit I posted.

 

ok,
thank you for the circuit you posted i will give it a try

 

Unless your charging current is very small, 4700uF will charge to >28V in next to no time. I'm curious as to why you want to use a LED to indicate full charge?

 

how it could be done with just a zener diode and resisitor

Very crudely, potentially inaccurate, with lousy temperature behavior, and possibly burning out the LED. That said use 100 Ohms, a 20mA rated red LED, and a 27V 1W zener all in series. Note that the zener is typically 5%-10%, so don't expect it to indicate the proper level of the capacitor, just somewhere close. If the zener is 5% low and the supply 5% high you might burn out the LED. You could buy a 1% zener, but the slightly more accurate circuit would still have lousy temperature behavior.

If you want something that works well and won't destroy itself follow Crutschow's circuit above. Add a 5K pot in series with R2 and now you can fine-tune the indication trip point. (Note that Crutschow added R5 simply to slow down the rate of charge and thus get a nice plot of function. Values for R1 and R4 are not critical, 4.7K-5.6K would work.)

 

iif you want to switch on the LED at 27.80 volts, this circuit will not work adequately. But if the intent is to see that the capacitoe is well on the way toward 28 volts, , a 24 or 25 volts Zener diode , a red LED with a forward voltage of about 2 volts at 20 milliamps, and a series resistor to drop either 3 or 4 volts at 20 milliamps is what will work.. The LED will start to illuminate before the voltage reaches 28 volts, probably starting to glow at 25 or even 24 volts.The zener current rating must be at least 25 milliamps at the rated voltage.

 

thank you all for your comments.
as a starter i am going to try crutchow's circuit.
alec-t , yes you are correct, at 30v and 100ohms the time constant is .47secs,so it should be fully charged in about 2.5secs if i am not mistaken.