Serial Voltmeter - Page 1
Forum section: electronics / projects

author: CodyW
author_rank: Contributor
timestamp: October 27, 2013, 08:28:22 am
content: Hi Everyone,I'm currently designing my own serial voltmeter so I can analyse signals in Matlab (or Scilab). Does anyone who has built one have any tips, tricks or things to look out for when building one of these?Also, does anyone know of an ADC that I could use that can handle around 20v 0.5A?Any help or advice would be greatly appreciated.Cody

author: w2aew
author_rank: Super Contributor
timestamp: October 27, 2013, 04:09:12 pm
content: Not sure what you mean by "serial voltmeter". From your description, it sounds like you might be referring to a data acquisition system to sample a voltage over time, and send that data to MATLAB for further processing - sort of like your own scope or analyzer. Can you be more specific? What do you need to measure? What sample rate is required? What accuracy and resolution are required? etc. etc.

author: tszaboo
author_rank: Super Contributor
timestamp: October 27, 2013, 08:37:35 pm
content: Quote from: CodyW on October 27, 2013, 08:28:22 amAn analog to digital converter cannot handle that. If you are asking how to measure thoose two parameters, that is different.For the project, first you should define the requements. This is absoulutely the first step. Think of theese parameters, not exclusivity:Accuracy (0.1% ? 50ppm?)Resolution (3,5 digit? 18 bit?)Sampling speed ( 1/s? 1MSPS?)Method of measuring current (for this I highly recommend checking out current shunt monitor solutions on my blog)And most importantly: BUDGET!If you have some more numbers, we can probably help.

author: Unknown
author_rank: Unknown
timestamp: October 27, 2013, 09:34:35 pm
content: There are also various affordable DAQs that might be suitable. For example the various DMMs with serial interface, Labjacks, CEE, etc. I'm sure there are also some Arduino sketches that convert an Arduino into a DAQ. Though again, it's hard to make specific recommendations without proper specs.

author: codeboy2k
author_rank: Super Contributor
timestamp: October 28, 2013, 03:41:18 am
content: Quote from: CodyW on October 27, 2013, 08:28:22 amAs everyone else has already said, we need specs.There is no ADC that can directly read 20V (DC? AC?). You need to scale it with a resistor divider first. So, for example, if you know your maximum input voltage will be 20VDC you can use a divide by 5 resistor divider at the input, and now the max is 4VDC and you can use an ADC @ 5V Vcc with a 4.096V reference.For current (0.5A) you just need to convert that to a voltage first, with a current sense resistor. You don't need an ADC that can handle the current, just an ADC that can handle the voltage after conversion, which is never a problem, since the current sense voltage is very small anyways. If you use the same ADC as for voltage, you need to switch between the two at some rate, scanning. If you use a second ADC then you can read current and volts simultaneously and be able to compute instantaneous power, in watts.And since you said you are building your own serial voltmeter, I presume you want to read the data serially. It sounds like you might want a single chip solution in the form of a microcontroller with 2 ADCs built in. The only other major chip you will need is a reference, unless the micro you find has a built in reference. And everything still comes down to specs... if you can find a micro with a suitable reference built-in and 2 ADCs with suitable bits of resolution, then you're set to go.And if it's ACV you are reading, well that's another story

author: David_AVD
author_rank: Super Contributor
timestamp: October 28, 2013, 06:41:07 am
content: Even with the scaling and current shunt sorted, your test inputs are going to be ground referenced if you don't build in the appropriate isolation for the serial connection. That could lead to some interesting fireworks!

author: Jon86
author_rank: Frequent Contributor
timestamp: October 28, 2013, 11:40:30 am
content: I'm interested in doing the same thing actually, surely there must already be a cheap tool on the market for doing this, just a couple of inputs and serial out? Seems like it'd be a damn simple thing to make for a professional EE?

author: David_AVD
author_rank: Super Contributor
timestamp: October 28, 2013, 11:46:03 am
content: You can buy bench meters with RS232 or USB output.

author: codeboy2k
author_rank: Super Contributor
timestamp: October 28, 2013, 05:43:39 pm
content: Quote from: David_AVD on October 28, 2013, 11:46:03 amThis is the best option if time is important and you don't want to build anything. A cheap $50 meter with RS232 output should be fine of the accuracy and resolution suits the task at hand.