Can someone help me out on this one?  - Page 1
Forum section: electronics / projects

author: furnac3
author_rank: Newbie
timestamp: January 07, 2012, 12:10:21 am
content: I am really interested in designing electronics. I know I have to use a A/D converter, and I need help with the electronics part or possibly code.Electronics Design ProblemDesign the electronics for an RMS voltmeter that uses a microcontroller. The voltmeter must be able to function as a standalone system or as a module of another system.

author: Rerouter
author_rank: Super Contributor
timestamp: January 07, 2012, 12:23:56 am
content: ok, suppose the best thing to do is break down the task into areas,input:what voltages do you need to measure with it, AC, DC, someething like 2mV or something like 1KV,filtering:if DC then measuring rms is easy, if ac it gets a tad harder, however you could sample from peak to peak and work out rms that way, of if only sine mains ac, then you can cheat, measure the peak, and times it by 0.707 (rms value of a sine wave)measuring:lets say you go for a off the shelf spi based adc or even the inbuilt one, what resolution will you need and how wide a voltage do you want to measure at this resolution. and what kind of protection or buffering will you need to put in line with it to not kill the dac, e.g. a diode clamp after the high resistance input, buffered through an op amp,displaying:how will you display the rms value. lcd, seven segments, seriel communication output (serial may be handy for managing the "work as a module" part)from there it gets into the software domain and that i cannot help with

author: grenert
author_rank: Frequent Contributor
timestamp: January 07, 2012, 03:28:13 am
content: Quote from: furnac3 on January 07, 2012, 12:10:21 amYou are really interested in having others do your school assignment.

author: amspire
author_rank: Super Contributor
timestamp: January 07, 2012, 05:01:46 am
content: Quote from: furnac3 on January 07, 2012, 12:10:21 amThere are two ways to go. Get a RMS to DC converter IC - probably one of the Analog Devices ones, and use the micro to measure the DC output of the converter, or alternatively use the micro to take sample the AC waveform and calculate the RMS. The RMS IC's have good accuracy, excellent bandwidth, and are easy to use.RMS stands for "Root Mean Square". So to use a micro to directly measure RMS voltage instead of using a RMS converter chip, it has to take lots of samples, square each reading, average the squares (ie calculate the "mean"), and then take the square root to get the RMS value. For this to work well, you need a micro with a very fast A/D sampling aperture (the actual measurement window), and ideally a very fast sample rate. A micro A/D input often looks like a capacitor in series with a resistor, so you have a RC delay. Ideally you want the RC delay to be in the microseconds. You may need something like 100 samples or more per wave of the incoming AC to get decent accuracy. So if you can perform 1 million samples per second, you may be able to measure up to 10KHz fairly well. If you have a very small sampling aperture but a slow sample rate, it is still possible to get an accurate calculated RMS value if you use "equivalent sampling rate" methods, but this kind of reading can be fooled by an unfriendly waveform pretty easily. You really need something like a DSP chip to do this well. A standard microcontroller will have a pretty bad upper bandwidth when acting as a RMS converter.

author: codeboy2k
author_rank: Super Contributor
timestamp: January 07, 2012, 07:18:38 am
content: Quote from: furnac3 on January 07, 2012, 12:10:21 amThis is either someone's schoolwork, or one of those bids on a virtual work for hire website, where they outsource to mostly Russia or India.At the risk of sounding prejudiced, I have seen far too many contracts like this get outsourced to people who don't know how to do what they are asked to do,or what they have bid on, so they ask on the web for others to do there work for them.

author: 8086
author_rank: Super Contributor
timestamp: January 07, 2012, 02:54:24 pm
content: See:https://www.eevblog.com/forum/index.php?topic=6236.msg81316#msg81316