# Offset-adjusted ammeter for up to 25 Amps

Discussion in 'The Completed Projects Collection' started by cmartinez, May 7, 2015.

1. ### cmartinez Thread Starter AAC Fanatic!

Jan 17, 2007
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Here's a little project that I made that may be a bit laughable for the real experts in this forum, but that came in pretty handy for me when I was trying to measure current with an oscilloscope.

What it is: It's a little circuit that makes it easier to measure current with an oscilloscope.
What it does: It subtracts the voltage offset from the signal being generated by an ampsense AMP25 sensor so as to bring it within range of the oscilloscope.
Why: Because an AMP25 sensor generates an output signal between 0 and 5V depending on the current that passes through it, but it has a 2.5V offset. That means that negative current values will be reported as a voltage between 0 and 2.5V, and positive values will be reported as a voltage between 2.5 and 5V
Why is this an issue?: Because oscilloscopes can adjust for signal offset only within certain limits, and if I want to read a small current signal (like when the AMP25 reports a value of, say 25mV) then I will run into trouble. For example, to more or less accurately read 25mV, the oscilloscope's vertical divisions should be set to 10mV. But since my oscilloscope has only 8 divisions in its complete screen, and the middle of the screen is 0V, then the maximum offset that it will let me adjust at that scale is ±100mV... but the problem is that the AMP25 zero-Amp signal is reported as 2.5V, which is way above the maximum 100mV that the oscilloscope will allow me to offset adjust on the 10mV/division selected scale.

So without much further ado, here's what I did: I built a simple, adjustable voltage subtractor circuit that subtracts 2.5V from any voltage entering it, conditioning the AMP25's output to make 0A = 0V ... and now negative current values fall in the range of -2.5V to 0V, while positive ones are in the range of 0V to +2.5V
The circuit is powered with a standard wall-wart set at 12VDC output. This voltage is then lowered and regulated to 5V with an off-the-shelf 7805. After that, I used an ICL7660S chip configured so that it generates -5V from the 5V being fed into it. This allows us then to power an OpAmp (I used an OPA244) configured as a voltage subtractor. A simple 50K trimpot connected as a voltage divider serves as an adjustable 2.5V reference for the OpAmp.

Last edited by a moderator: May 10, 2015
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2. ### cmartinez Thread Starter AAC Fanatic!

Jan 17, 2007
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Here are the datasheets of the important parts, and an AutoCAD 2010 drawing containing the complete schematics.

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• ###### Current Measurement.zip
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3. ### nsaspook AAC Fanatic!

Aug 27, 2009
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Sweet, I love those little AMPLOC hall sensors. The interface works perfectly with a 0-5vdc ADC input.

4. ### ScottWang Moderator

Aug 23, 2012
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Will you label the values of R in the circuit?

5. ### cmartinez Thread Starter AAC Fanatic!

Jan 17, 2007
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Sure thing... I'll prepare a complete diagram and post it in a couple of days.

6. ### ScottWang Moderator

Aug 23, 2012
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If the Vcc powered with a standard wall-wart set at 12VDC output, have you measure the noise of the output of op amp?

7. ### cmartinez Thread Starter AAC Fanatic!

Jan 17, 2007
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I'll also post a 0V reading from the circuit that'll probably answer that question

8. ### ScottWang Moderator

Aug 23, 2012
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Sorry, I just can't trust the low cost switch mode power using in the amplifier (whatever audio amp or op amp...)

Did you used the switch mode power supply?
Have you measure the output of the power? (scale at ±10 mV or less)

9. ### Lestraveled Well-Known Member

May 19, 2014
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C.
I am having trouble opening the .dwg file (schematic). Could you post a jpeg of it. Thanks

10. ### cmartinez Thread Starter AAC Fanatic!

Jan 17, 2007
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Scott, I'm not using a switch mode power supply to feed the circuit, but rather a 7805 linear regulator. As I said before, I'll get back here later with the complete schematics.
Down here's 2:00 A.M. so nighty night, for now...

11. ### ScottWang Moderator

Aug 23, 2012
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He was shown in 4,5 photos, the zip files was for reedit not for watch.

You may use this to see the online *.dwg file.

12. ### ScottWang Moderator

Aug 23, 2012
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So could you show the standard wall-wart?
I still have some AC110V/60Hz to 12Vdc conventional transformer adopters, and they have a good ripple rate, and I also bought some switch mode power supplies as AC110V/60hz to 5Vdc/12Vdc and different current spec.

13. ### Lestraveled Well-Known Member

May 19, 2014
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Thank you Scott. That is a nice app.

14. ### cmartinez Thread Starter AAC Fanatic!

Jan 17, 2007
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There you go Scott, here's the diagram I promised

Last edited: May 8, 2015
15. ### cmartinez Thread Starter AAC Fanatic!

Jan 17, 2007
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And here's a sample reading taken from the circuit's output.

The green trace was measured using a 100X probe, and it's measuring the voltage across a valve's coil. The yellow trace is measuring current going through the coil, using a 1X probe connected to my circuit's output.
As you can see, CH2's grid (yellow trace) is set at 50 mV per division. And since the AMP25 sensor delivers 37 mV per Amp at its output (when it's powered at 5V), the peak current stands at about: 270 mV/(37 mV/A) = 7.30 Amps

I blame the yellow trace's thickness on noise, which considering the scale of things is not so bad.

Last edited: May 8, 2015
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16. ### Wendy Moderator

Mar 24, 2008
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Nice work.

Is their something different about the PCB material?

17. ### cmartinez Thread Starter AAC Fanatic!

Jan 17, 2007
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Thank you.

Nothing special about the PCB really... it was made using the toner transfer method that I mentioned in this post, using red ink. And BTW, you may want to consider placing that post in the completed projects forum as well. Soon I'll also post plans and pictures of my etching tank.

18. ### Wendy Moderator

Mar 24, 2008
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Start a new post with an eye toward that forum.

19. ### cmartinez Thread Starter AAC Fanatic!

Jan 17, 2007
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Tell you what... I'll copy that post and place it along with the etching tank plans and pics in a few days... that way it could be considered as a "more complete" project.