Building a digital AC ammeter

[lilimike]

I am planing to build a digital AC ammeter 0-20Amp to measure current on 120VAC
I am looking at an ICL7135 but instead of the standard shunt resistor couldn't I just wind some magnet wire around one of the live wire?

If I could do this how would I calculate the length, turns and size of the magnet wire?
Maybe just make it generate more than I need and reduce with a capacitor in series?


Mike

 

[crutschow]

You'd be better off to buy a current transformer, designed and calibrated for such a task.

Building one would require winding the secondary on a split magnetic core to clamp around the wire to be measured, to get a measurable signal, and calibration of the unit after it's built.

 

[RCinFLA]

I did that for a 50 amp DC ampmeter for auto alternator measurement and worked pretty good. Only issue (beside isolation safety) is the copper temp coefficient. If you don't have instrument to fine calibrate the shunt then worrying about temp coeff. is probably mute.

Take a look at the hall effect sensors in link below. They are fairly inexpensive.

**broken link removed**

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=620-1239-ND

 

[lilimike]

The Hall Effect chip is over $6, I can get a shunt resistor for $5 and the reason I am looking at using magnet wire is to save a little.
I am still searching for current transformer, I found some at $2 and some at $75 and still looking.

Mike

 

[lilimike]

My final design will be enclosed in a box with a plug, the device to be measured will be plugged in so all the circuit will be in the box.

Mike

 

[lilimike]

I found this one a little over $2
This seams awfully small to support 500V @ 20A
Would I have to connect the primary (pin 8 and 7) in series or in parallel?

Mike

 

[RCinFLA]

I found this one a little over $2
This seams awfully small to support 500V @ 20A
Would I have to connect the primary (pin 8 and 7) in series or in parallel?

Mike

That one is for current sensing in a high frequency switching power supply. Will not work for 50 - 60 Hz AC.

 

[crutschow]

Perhaps this would work for you as a sensing coil.

 

[lilimike]

The Rogowski coil looks interesting, I could not find it already made in North America (searching by its name) but I found a few sites that explains how to make it so I think I will give this a try. Just to validate my understanding, the Rogowski coil will pickup current and convert it to voltage which I can calibrate and connect to the inputs of an ICL7135. Am I right?

Mike

 

[Reloadron]

The Rogowski coil looks interesting, I could not find it already made in North America (searching by its name) but I found a few sites that explains how to make it so I think I will give this a try. Just to validate my understanding, the Rogowski coil will pickup current and convert it to voltage which I can calibrate and connect to the inputs of an ICL7135. Am I right?

Mike

Regardless of the sensor type you choose to use there is going to be a need for some signal conditioning. If you have not see it yet in your searching this is a good read of what is involved in the calibration of a Rogowski coil.

Another option is as was covered using a CT (Current Transformer) which differs from the Rogowski design in that it has an iron core that may be split or solid. If you are curious as to the inner guts of a CT I am attaching a few images. The secondary winding is a parallel pair so it looks a little strange.

Using either the Rogowski coil design or a standard CT you will still have a need for signal conditioning. The CT would need to drive into a load. This link provides some good information on basic CTs like the attached images. When using a CT we need to place real close attention to "Burden" of the load on the output.

Finally there are the hall effect devices like the Allegro model linked to earlier. These are a very good choice as all the initial signal conditioning is already done and they provide a nice analog DC voltage level that is directly proportional to the AC current being measured. Personally I would lean towards one of those devices.

Ron

 

[lilimike]

Thank you Ron for the info,

I found some CT like you have for $35 to $45 which is a little over my budget. I have to built 50 of them so the price goes up really fast at this cost so a CT is off my list.

I started testing with a Rogowski and I have made a quick test:

but unfortunately feeding 300mA (powering two CFLs for a total of 37 Watts did not make the slightest difference on my scope.

Being that the hall effect device will save me lots of work and extra components, I think for around $6 it will probably be a good choice.

I should also thank RC for pointing this out as well.

Mike

 

[crutschow]

I started testing with a Rogowski and I have made a quick test:
View attachment 46755
but unfortunately feeding 300mA (powering two CFLs for a total of 37 Watts did not make the slightest difference on my scope.

Did you feed one wire or the wire pair from the CFL through the center of the coil?

Is the center coax cable conductor shorted together in the loop? I can't tell from the picture. If so, that could affect your measurement.

 

[lilimike]

I fed only the common wire through the loop, I figured common or neutral wouldn't make a difference.
I had pulled out the center of the coax cable, I only inserted one inch of it to keep the two ends together.
This was build from a length of about 5.5 inch of coax cable and I used #29 or #28 magnet wire and the diameter is around 1.75 inch.
I didn't count the turns but my fingers couldn't take any more turns as I had first planned to make it 12 inches long.

Mike

 

[crutschow]

What is the nature of the current to the CFL? Is it AC or DC?

 

[lilimike]

It is 120V AC 60Hz

 

[lilimike]

Something I am not understanding here.
Today I went shopping to a surplus store and I was hunting for anything with a winding and a hole in it. I found this thing that looks like a choke, it has 3 pins but one doesn't seam to go anywhere. I passed my common (120VAC) feeding a 22 watts CFL and also tried adding a 100 Watts incandescent bulb for a total of 122 watts and look at the results I got in the picture:

I was expecting the opposite!

Mike

 

[lilimike]

I switched the ICL7135 for an ICL7107, built it on breadboard and working accurately measuring DC volts low to high.
I just received my ACS756.
I can only make circular plated holes on a PCB which makes pin 4 and 5 about 5 mm in diameter, if this is my only choice I guess I will do it but before I do I was wondering if anyone put this device into a PCB. I would be curious to see how you did it.

Mike

 

[crutschow]

To experiment with making an inexpensive current transformer you might try starting with a ferrite toroid such as one of these and then wind a number of turns of fine wire on it (100 turns should give you about a 100:1 reduction ratio for example) for the secondary. I think one with a high µ would work best. Run one wire of the AC you want to measure through the center of the core. Then check the current output (not voltage) with an AC current through the center and an ammeter connected to the secondary.

To avoid possible saturation problems with the core, it's best to run the output into a current to voltage (trans-impedance) op amp circuit. Simply connect the transformer secondary directly to the negative op amp input. Use a feedback resistor from the op amp output to the same negative input to get the required output voltage for the secondary current. For example, if the transformer output current was 10mA rms (1A primary and ideal 100:1 transformer), then a 1kΩ feedback resistor will give you 10V rms op amp output. Since the signal is AC you will need a plus and minus supply for the op amp.

Note: To protect the op amp when unpowered connect two inverse parallel diodes from the input to ground.

 

[lilimike]

Carl,

I think I will first start testing the ACS756 which seams to be quite simple.
My only issue with this device is to physically incorporate it into a PCB but I am sure I can manage to find a way, worse case scenario would be to make 5 mm plated holes for pin 4 and 5.

Thanks

Mike

 

[lilimike]

I am getting more confused as I get into this project.
My first idea was to build an ammeter to measure 0-50 Amps on 120V 60Hz.

Since only small variations of the circuit can be made to measure AC / DC voltages and AC / DC Amps I have decided to include in my circuit a set of jumpers that can be set to provide required type of measurements.

The ICL7107 responds to DC voltage so measuring DC is simple and accurate. This step is now behind me.
My next step was to measure AC voltage, so I simply added a bridge rectifier at the inputs and adjusted the ICL7107's resistor accordingly.
I have not tested low AC voltages because the purpose of this device it to measure regular home voltages so between 110V to 130V is all I need to be accurate for.

Where it gets complicated is the Amp part. I am not as well equipped as I would like to be but I am testing with incandescent light bulbs and I have 3 x 50 Watts and 1 X 40 Watts. I first measured the current using my multimeter and then compare the voltage at the output of ACS756 and these are my values:

Vrms voltage is 128 AC

50 W = 400 mA / ACS756 output = 16 mA
100 W = 830 mA / ACS756 output = 33 mA
150 W = 1.240 A / ACS756 output = 49 mA
190 W = 1.56 A / ACS756 output = 61 mA

The problem I have now is I can not feed the ACS756 output voltage through diodes to convert in DC since the voltage is too low.
Is an OpAmp my only solution?

I would appreciate some guidance here, am i going the right way?
I was hoping to build a small circuit but it keeps growing!

Mike