Digital wireless portable power supply / ammeter / voltmeter - Page 1
Forum section: electronics / projects

author: eripaha
author_rank: Contributor
timestamp: January 01, 2014, 09:44:58 pm
content: I want to build digital wireless portable power supply / ammeter / voltmeter (wow alot of fancy words in a row). Basicly it would be and arduino controlling bunch of stuff.For power supply:Arduino would control digital 10k pot (MCP41010) that would control step down regulator (LM2596). The power supply would be able to be switched on and off by a mosfet. Im looking for <1amp currents (i would say max 750mA@ 5v) and voltages up to 24V.For voltmeter:Arduino analog pin with simple voltage divider with resistors in kilo-ohm range. Max voltage at 30V. I want to have diode protecting the polarity.For Ammeter (in series volt meter):Arduino would read cheap digital ammeter (ACS712 max.5a)For wireless communications a simple bluetooth module (HC-05)On the computer i want to have simple program to control and read all aspects of this box using serial (over the bluetooth). It would be able to log the read values into a file and be able to set voltages from file with time codes.The box would have power led and a power-button. Also want to have cheap voltage display on the box it self connected to the read and supply pins.The box would be powered by 18650 (1S) stepped up by two cheap step up converter to 5v for arduino and another to 28v for the power supply circuit.For the arduino logging side i want to implement first poll-like measurements and later on burst measurements.What kinda advice, ideas or comments you have for this project?

author: eripaha
author_rank: Contributor
timestamp: January 01, 2014, 09:49:18 pm
content: Simple image of how i mean to lay it out on the project box;

author: Unknown
author_rank: Unknown
timestamp: January 01, 2014, 11:24:20 pm
content: Quote from: eripaha on January 01, 2014, 09:44:58 pmY'know, the LM2596 has an on/off pin...

author: mariush
author_rank: Super Contributor
timestamp: January 02, 2014, 12:49:47 am
content: I would recommend going with a more efficient boost regulator that could also work in SEPIC mode. This way you can use a li-ion battery and be able to output less than the input voltage. A cheap ebay lm2596 is either going to be buck only or boost onlyI would recommend going with LT1308 or something similar, running at high frequency it allows you to use smaller inductors and capacitors.Im looking for <1amp currents (i would say max 750mA@ 5v) and voltages up to 24V.Yeah ok, just be aware that switching regulators are not really designed to allow you to modify the output voltage that much. If you want the best efficiency out of a switching regulator, if you want it to be stable and so on, you want to pick an inductor and shottky diode that are ideal for an output voltage and an output current range. An inductor selected for 5v @ 1A max is not going to work fine for 24v 100mA. Just because those ebay switching things allow you to adjust the voltage that much, doesn't mean that it's normal or 100% safe.Also keep in mind that there are some limitations - you can do 5v @ 1A but you may not be able to do 24v @ 1a which is 24 watts.. to output 24 watts with about 70-80% efficiency that means it needs about 30 watts at input - if the input is a 4.2v battery that means peaks of around 8 amps, which is not good. Even the battery may have problems with it, not to mention the chip itself and the inductors and the input capacitors and so on.I would set more realistic goals, like maybe restricting it to 12v max output, which would also allow you to use smaller 16v rated capacitors instead of large 35-50v capacitors (make the psu more portable).For powering arduino and the bluetooth module you could use a small and simple NCP1402 ( 1-5v in, fixed 5v out , 200mA max voltage regulator):http://uk.farnell.com/on-semiconductor/ncp1402sn50t1g/ic-dc-dc-converter-5sot2/dp/1652393Allows you to power your arduino separately from the lithium battery pack in case, for example from one or two aa batteries.The datasheet explains the circuit and the parts you need to make the regulator and how to determine the parameters you need for the inductor (peak current) or the shottky diode and even includes part names you can just pick straight away. Not sure if 200mA is enough if you include the bluetooth module, if it's not enough.. use another ic.If you feel adventurous, buy some FERAM, it's like EEPROM but very fast and doesn't have a limited number of writes and these for example works at 5v and through i2c so easy to read and write with arduino:2KBhttp://uk.farnell.com/fujitsu/mb85rc16vpnf-g-jne1/fram-16k-i2c-5v-8sop/dp/20702588KBhttp://uk.farnell.com/ramtron/fm24c64b-g/fram-64k-i2c-8soic/dp/207775032KBhttp://uk.farnell.com/ramtron/fm24w256-g/fram-256k-i2c-8soic/dp/2077752The larger memory chips you could use to upload voltage files to them from pc (instead of using the arduino built in eeprom which is small anyway), or you could use for logging voltages and currents. And being i2c (there are also spi versions of these chips) you can put several on the same wires and just change address of each one to be unique.Oh... i don't get why you'd want blutooth seems like something that would restrict your product. Why not a simple ftdi usb to serial chip and a couple of optocouplers to insulate your device from the usb and your computer? Would be 100% compatible with everything, not just with a bluetooth device and carrying for example a small usb cable with the power supply to connect in a phone wouldn't be a big deal, would it?

author: dannyf
author_rank: Super Contributor
timestamp: January 02, 2014, 02:09:35 am
content: QuoteIt may be helpful if you ask yourself why you want to build this thing. After that, it may be easier for others to help you to find a way to get there.

author: eripaha
author_rank: Contributor
timestamp: January 02, 2014, 09:01:42 am
content: Quote from: Tac Eht Xilef on January 01, 2014, 11:24:20 pmThank you! One less chip in the desingQuote from: mariush on January 02, 2014, 12:49:47 amThank you for the ton of good input! I have to rethink about the idea based on your comments. Part of the reason why i want to use bluetooth is that im familar with the chip in question.Quote from: dannyf on January 02, 2014, 02:09:35 amThe why is there. I dont always have the possibility to set up my projects next to my work desk because of lack of space. So they sometimes sit on the living room table or the kitchen table. They are usually something that dont need much power and i might just want to watch the battery level drop as the device runs. I dont need fast polling speeds nor that high precision on this things. I dont have the possibility to make a pcb for this right now so im stuck using Perfboard. So i want to be able to use as many preassembled boards as possible. This makes this project some of what ghetto tech.

author: dannyf
author_rank: Super Contributor
timestamp: January 02, 2014, 12:32:20 pm
content: QuoteSounds like a perfect situation for a portable battery bank. $2.49 from Radioshack or a little bit more from your friendly cellphone retailers.