Designing Electronics for an RMS Voltmeter

[drewcrew]

Hello,

I recently had my midterm and my professor asked the following question:

Design 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.

Apparently, this is a very common interview question. I left it blank because I didn't know where to begin. Unfortunately, he doesn't provide solutions to tests so I have to learn this on my own and go to his office hours or something after some attempts.

I don't know where to begin and i'm embarrassed that a sophomore engineering major doesn't know how to do this. Please help.

Thanks.

 

[betwixt]

Analyze the question carefully and break down each thing it asks you.

1. It has to measure RMS voltage - you can do this in software but by far the easiest way is to use an RMS to DC converter IC.
2. It has to use a microcontroller - a good choice would be one with a built in ADC but you could use an external one with a digital interface to it.
3. If it has to work 'standalone' it must have some sort of display - consider whether LED or LCD is most appropriate and whether it can be driven directly by the microcontroller.
4. To work as a module in a larger system, it has to have a digital interface of some sort. This could be a bus based system (parallel) or it could be a serial link, see which would be most appropriate.
5. No voltage range was specified - maybe amplifiers are needed at the input to select the range, should the micro adjust the range automatically?

When each of these requirements has been investigated, the range of possible solutions starts to emerge. You really need more information on what exactly is being measured but the question may be trying to judge your analytic skills rather than getting you to produce a working meter.

Brian.

 

[FvM]

the question may be trying to judge your analytic skills rather than getting you to produce a working meter.

That can be expected in this case. Usually, you'll start by sketching a block diagram, defining the individual functions and specifying it in greater detail when needed. (A real design takes the same way).

In terms of electronic measurement theory, the analog front end and different´methods to implement a RMS characteristic are probably the key point.