Hi all. This is my first possible project involving circuits. Its not my first experience with electronics. I am an auto technician by trade so most of my experience involves 12v DC systems. What I am trying to do is build a digital ammeter that will read up to 20 amps. I have a box I made to load test circuits that is adjustable from 2.5 to 15 amps and normally I just throw an inductive amp clamp over one of the leads and get to work. My thought is it would be slick to have a digital amperage readout at all times built into the box. What's everyones thoughts? Too big of a beginner project? In open to all comments. Thanks, Jon

 

There are several free projects already published with firmware included in the net.

You could look for them to make you an idea about where to start from.

 

Thanks for the reply. I will have to do some searching tonight.

 

My first idea looks something like this:

R2 and R3 are needed to drop the maximum of 20V = ILoad * R1 to a factor of 10, because I don't know if the micro will like that voltage at input. I did not ever make a project like this, and I don't remember very well the projects I found in internet.. If microcontrollers allow that value of voltage at input, maybe try to just limit current with R2 of a proper value..

Keep in mind that is just an idea where maybe you can start from..
If something is wrong, maybe some experts have more better suggestions than mine.

Regards,
simo

 

Dropping 20v across the 1Ω current sense resistor will be problematic for an automotive circuit with a 12v supply. Picking a 0.25Ω resistor will provide 5v which can be fed directly into a μC's ADC (analog) input, but even 0.25Ω/ 5v is still 10 or 20 times larger drop than what you'd want. Filtering and spike protection will also be needed.

You could try one of these:

http://www.devicecraft.com/haefcuse1.html

No voltage drop. Use a cheap (<$10) 200mv digital voltmeter for a readout and a nice regulated 5v wallwart for power. It will need to be calibrated so a small trimpot between the sensor and the meter will be required.

This is just one way, there are numerous others which you will no doubt see here.

 

Wow I really like the inductive deal. The more I have been thinking about it the more I get concerned with the current spike on initial startup since I'm using 6 2.5 amp Light bulbs as my loads. But I think the inductive probe would solve that issue.

 

Out of curiosity is there a way to use an LCD screen mounted in the already made box with some sort of interface?

 

That sensor's not inductive, inductive current transformers are strictly AC devices, not DC. This sensor is a Hall device, uses the "Hall effect" to sense current.

As long as the current inrush is under 80 amps, it won't hurt the sensor. It's probably good for much more than that.

You can mount the meter anywhere. Here's a typical one:

http://www.bgmicro.com/MET1031.aspx

The current sensor puts out 23mv/amp so 20a = 460mv. Use a trimpot to reduce that to 200mv and feed it into the meter set for the 200mv range. Set the DP to display xx.x amps.
bg will also have a 5v power supply too.

 

Excellent! Sorry about the terminology. In the automotive world they tend to generalize things for simplicity, like if it's a tool that senses current without contact it's inductive. But anyway this is getting exciting becaaue it's looking like it's going to be very doable

 

Very doable from many different approaches.
You can build your own hall sensor from components, or buy one.
You can build your own 110vac to 5vdc power supply, or buy.
You can build a 12vdc to 5vdc supply with battery clips to make it kinda self powered.
You can program a $2.50 microcontroller to read your sensor and feed and drive an inexpensive LCD display of your choice, or buy the $10 meter.

Just about all you need to know is in the ebooks of this site or the forum members.
If you chose the μC route, like PICAXE, their forum will be more than glad to help too.

You will need a good DMM that reads DCV and amps to test your project.

 

Thank you so much for all of your input. Especially since im new here it's a very warm welcome. I'm going to get this project rolling asap and I will document it on this thread. I have a few good dmm's at my disposal so that will be helpful. I'm just excited to make my already useful homemade tool even better