Hi
For a charger in an automated system, I'd like to have a LED when my battery goes above 4V. How can I do this as simple as possible?
I can use a reference and a opamp etc, but if I want to really minimize current and components?
Any ideas?
A zener and a LED is not the option, with less voltage the Zener will still conduct. I could use a TLC431 or something....?
WBR Sonnich
A TL431, 5K pot, 100 ohm resistor & red LED, charger & battery:
+------+---------+----------+ | | | | | | [100R] | + | | | B C | [LED] A H | | T A \ ___|__/ T R / / / \ E G 5K \
There ya' go!
Just use a higher impedance pot, and the CMOS version of the TL431, and you'll have a low-drain monitor. ...Jim Thompson
Another option is a transistor in place of the 431, and a zener and resistor from base to +ve.
NT
l 3.3v zener
NT
Just to be safe, consider what happens above 4 volts.
John
Consider using an ADM803J. Product page and data sheet here:
This ultimately simple but reasonably accurate solution is a 3-pin package available as SOT-23 or SC70 size package. The output will go low whenever the voltage is below 4V (3.93 to 4.06 volts @ 25C). Connect the LED anode to GND. The cathode of the LED would connect to the output of the ADM803J and also to the bottom side of a pullup resistor. Select the pullup resistor so that it can bias the LED on when the ADM803J output is off. When the voltage in the VCC pin of the ADM803J is lower than 4V the output will go on and short out across the LED thus shutting it off.
Two main design considerations. The voltage pin (VCC) of the ADM803J has to be kept below 6V so use only with a battery solution that was 6V or less. The output of the ADM803J can sink a maximum of 20mA so make sure to select the pullup resistor and LED bias voltage rail so as to keep the current below the 20mA level.
What is wrong with using a 3.3 volt zener as the "emitter resistor" of a plain old NPN transistor, base to the voltage being measured, and collector through a current limiting resistor and the LED to the positive supply > 8 volts or so?
Jim
How accurate? If you're charging a lithium battery, you might be concerned about accuracy/stability. Unless you hire someone to watch the led 24/7, it won't do much good... or you can just monitor the fire department callout to see if it did.
The problem with zeners is that they need quite high current.
S"mike
** Son of a Bitch Jensen is far to thick to ever get a joke like that..... Phil
my
ill
Better to use a string of diodes, plus add an R from the bottom fo this string to ground.
NT
I'm starting to agree with other posters in this group; you are honking nuts.
Jim
It is for a solar mobile system for camping, I just want to know when it is fully charged. When full I can take it down.
What's wrong with a volt meter?
That's what he want's, a one-bit volt meter. ;-)
You can get a simple d'Arsonval meter that takes a few microamps and has more information output than an LED, and is lots easier to see in bright sunshine. The LED will take up a few thousand microamps, and requires support circuitry (a voltage reference) that takes more.
There are power supply monitoring chips, intended for power-good indications, that can do a very accurate job at this, but the swinging needle of an analog meter is a better human interface.
I googled and found he was quite right.
NT
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