Designing a display for Watts, Volts and Ohms.

[Browndustin]

I am working on a small project and having those values shown in real time on the device will be helpful. I don't have much room as it is. I currently have one of those little 1s-6s volt meters used by RC hobbyist and use my multimeter to check resistance. My goal is to have something around that size, but show all three values.

I have experience, but unfortunately it was mostly just having my boss hand me a schematic and a pile of parts.

A couple of engineer friends helped me some with theory but not much practically. They are Nuclear engineers. One of them frequents this website regularly and suggested you guys/gals for this.

I was hoping for an off the shelf part, but hours of searching came up with nothing.

Some specifics; The power is coming from two IMR LI ON 3.7 volt high drain batteries in series. A buck converter with a 20 amp/ 120 watt limit. The output voltage range is 3.3ish up to 6 volts. The variable resistance will have a low end of .3 ohms and an upper limit of about 1 ohm.

I know it's possible because I have seen it exactly in another device. I thought about cannibalizing it for the display, but it would cost about 100$ for a small piece of circuit and a tiny oled screen. Furthermore that's not a real solution.

 

[meBigGuy]

It isn't clear to me what, exactly, you are trying to do.

In order to display watts, you need to measure two things (V, I or V,R, or I,R) and operate on them.

In order to display resistance you need to connect an ohmmeter to each end of a part not in a circuit.

Now, given a specific circuit, you can measure things about it in different ways if certain things are known or constant.

Also, you want three displays? You want to move probes? You want it to cycle through the readings? You want to throw a switch?

I'm sure it is all clear in your mind. Write it down for me.

 

[Browndustin]

I really appreciate you helping! I apologize for not being clear enough. I understand Ohm's law and know this isn't a difficult idea.

What I want is one display showing all three values possibly all on screen at once or cycling through by up and down buttons. I want a circuit that is fixed in the device without having to move probes. The voltage will change from fresh batteries at 4.2/8.4 down to about 3.3/6.6 volts. The chip I'm using is a naos raptor by GE. THe 20 amp limit one. I will have to look at the data sheet to see if can pull a constant voltage down from one of the outputs. Right now I'm using a 200ohm potentiometer to have a voltage range of 3.3-6volts.

 

[Browndustin]

The resistance is a heating element that will change periodically.

 

[Browndustin]

The entire thing is controlled by an off/on Pushbutton switch. I would like it if the display could be operated independent of the switch.

 

[meBigGuy]

I still don't know what nodes you want to measure. But it really doesn't matter I suppose. The problem is how to convert circuit values to a display via some sort of user interface algorithm.

The most fun would be to build an arduino based (or other uC) display controller. The Arduino A/D converter could be switched to measure various things and display them as you desire. It's not a trivial project though.

You can start here
http://www.electroschematics.com/9351/arduino-digital-voltmeter/

Or you could use something like the icl7107 (google for digital voltmeter IC - there are quite a few) and control it with an arduino or other uP.

As you narrow the search feel free to ask more questions.

 

[Browndustin]

Exactly the kind of thing I'm looking for! Now to be able to measure the resistance of the heating element as well as power across it...

 

[dlgoff]

I agree with meBigGuy as to where to start but just to give you an idea of what's out there, Texas Instruments has this .pdf application report that's cool.

http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=snoa592b&fileType=pdf

Texas Instruments ADD3501 is a monolithic CMOS IC designed for use as a 3 (/2-digit digital voltmeter).

Edit:
Now the chip is manufactured by National Semiconductor. Their data sheet can be found here:

http://www.alldatasheet.com/datasheet-pdf/pdf/115505/NSC/ADD3701.html

 

[Browndustin]

I'm at work now, but I will definitely check out that link. One thought I had wa to gut a multimeter.

 

[Browndustin]

I have been looking at the dmm ic's. I think that is the ticket. Now to figure everything else out!

 

[davenn]

you realize you are probably not going to be able to directly measure resistance of the wire whilst there is power on it
But using a PIC processor you would be able to measure the voltage across the element and the current through it and do some maths in the PIC to calculate the resistance

I have http://www.powerwerx.com/digital-meters/dc-inline-watt-meter-power-analyzer-bare-wire.htmlbetween my PSU and my radio transceivers it gives me voltage, amps, Watts, Watthours etc
A very nifty device


cheers
Dave

 

[Browndustin]

That is a cool device and I appreciate the input, but unfortunately it is larger than I have space for. I will look into the PIC processors you mention.

 

[davenn]

that unit has a PIC or similar processor in it, and unless you are a magical electronics whizz in a hi tech Japanese company, I can pretty much garantee you are not going to make anything any smaller !

its only ~ two inches so long and ~ 1/2 inch wide
well just a tiny bit bigger
•Size: 3.3 x 1.7 x 1.0" (85 x 42 x 24mm)


Dave

 

[Browndustin]

Lol! It looks way bigger in the picture, but I guess I should have used the wires as a reference.

 

[Browndustin]

If I end up doing it, I'll be messaging you for those hi tech Japanese companies' phone numbers! Let them know I'm a whiz!

 

[davenn]

Not knowing your electronics skills level and going by your comments so far, honestly, I would be suprised if you could even make it as small as that one, you would have to be a very skilled designer and constructor
Size: 3.3 x 1.7 x 1.0" (85 x 42 x 24mm) and don't forget those are the outside case measurements
the circuit board etc inside is smaller.
I have had my one running for a couple of years now, a great little device.

Do you have SMD circuit design and construction skills?
micro processor programming skills ?
What PCB manufacturing background do you have ?

Im not trying to put you off, rather trying to help you understand that it isn't a trivial project and you just need to consider these things

cheers
Dave

 

[Browndustin]

I was being facetious. Like I say earlier in the thread, I have experience putting things where they go according to plans someone else drew up.
However, (in my life up this point) I have only failed once when it has come down to me wanting to accomplish something. It was not a winnable battle to start with, I guess twice if you want count the ex.

 

[Browndustin]

I have seen exactly what I want and it was about the size of a matchbook.