Adjustable Low Voltage Indicator - Page 1
Forum section: electronics / beginners

author: nathanpc
author_rank: Unknown
timestamp: April 27, 2013, 09:26:29 pm
content: I was thinking about building a low-voltage indicator to integrate into my power supply project to check if the input voltage is enough to power the voltage regulators I'll be using, because I'm using a LM317 too, I wanted my low-voltage indicator to be adjustable, so I could adjust depending on the minimum input I need for the desired LM317 output. To start Isearched on Googlefor a simple circuit with a great explanation (since part of the goal with the power supply project is to learn more), sadly I couldn't get any good results.Could anyone provide a good source of information about adjustable low-voltage indicators?

author: mariush
author_rank: Super Contributor
timestamp: April 27, 2013, 09:47:27 pm
content: There are ready made chips that monitor a voltage and output something on a pin when the voltage goes below or above a threshold.Just search for "voltage monitor" or "voltage supervisor" on digikey.com or other specialized storesFor example:http://www.digikey.com/product-detail/en/MC33161PG/MC33161PGOS-ND/919013http://www.digikey.com/product-detail/en/MB3771PF-G-BND-JNE1/865-1021-ND/2039278etc etcI don't know something "analogue" aka made with just some passives, maybe you could do something with some zener diodes and an npn/pnp transistor but I can't be bothered to think of something.If you want something cheaper, you could always take the "overkill" approach and use a 0.4$ pic16f chip or an attiny , use the adc function in it to read the voltage and do something if the voltage is too low.But the microcontrollers expect to be powered from 2.5-5v so you'd need a reliable supply for this chip and you'd have to use a voltage divider if your voltage may be above 5v. The monitor chips above are more expensive but they work at up to 40v.

author: ResR
author_rank: Regular Contributor
timestamp: April 27, 2013, 09:56:15 pm
content: Maybe this helps, just needs to be regulated to detect voltage you need and you can left out some led's, the chip still works.http://www.electronicecircuits.com/electronic-circuits/lm3914-12v-battery-monitor-circuit

author: nathanpc
author_rank: Unknown
timestamp: April 27, 2013, 10:42:39 pm
content: Interesting... I'll order some MC33161 and LM3914, play around with them, do some tests, and choose the best one for my project. Thanks very much for the suggestions!

author: Kevin.D
author_rank: Frequent Contributor
timestamp: April 27, 2013, 11:18:22 pm
content: Why would you want to do this anyway.? dont you know what your input voltage is ,isn't it a fixed value.?Use a comparator and bias the - side at (lets say 3V below the reg input voltage if the reg dropout was 2.5 V )by using diode drops or a zener and resistor to get the required fixed voltage drop below input voltage ,then tie the compare + side to the ouput side .So the comparator goes high when they are equil ,then use that output to drive your "warning approaching dropout led" directly. I am assuming your input will always have some minimum voltage available to drive your led and comparator.You could do it simpler with a single transistor biased across the input and output so it is normally on and driving a led ,but then it will switch OFF when the voltage difference drops below a certain threshold you have set with a biasing zener,or diode in the base .So this would be opposite than with the comparator ,here the LED being on when ok then switching off when approaching dropout voltage.Quote :- "so I could adjust depending on the minimum input I need for the desired LM317 output"The minimum input you will need will always be your required output + dropout voltage of the regulator which is constant . (~2.5V for lm317) so you dont need to adjust the warning reference.

author: nathanpc
author_rank: Unknown
timestamp: April 27, 2013, 11:32:06 pm
content: My input voltage isn't fixed. I want to be able to power it from a 9V battery or a 12V wall adapter (or anything with more than 6.5V and less than 15V). I'll try to build the circuit you've suggested, thanks very much for the tips.

author: w2aew
author_rank: Super Contributor
timestamp: April 28, 2013, 12:42:08 am
content: You may also want to checkout one of my favorite little chips, the LM10. The data sheet shows an application do a low voltage detector.

author: nathanpc
author_rank: Unknown
timestamp: April 28, 2013, 01:17:24 am
content: Just had a glance at the datasheet's examples and I liked their low voltage detector circuit. I'll definitely buy some LM10s too. Thanks very much!