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  • Author Author: jw0752
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A New Approach to a Bench Top Ammeter

jw0752

     When I first found this Chinese Ammeter kit I bought it, as at $4.62 and free shipping it was a no brainer.

 

https://www.banggood.com/ICL7107-Digital-Ammeter-DIY-Kit-Electronic-Learning-Kit-p-977902.html?rmmds=search

 

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At +/- 1 mA resolution the specifications were very acceptable to the level of precision that I usually need. I love these inexpensive kits, not only for the low $ that I have invested, but also for the challenge that is introduced by their low cost. Instructions are often wrong, poorly translated, and occasionally the circuit is in need of redesign. My first challenge with this one was the instructions which I was able to download from the sales site. While labeled as English they in fact were a hybrid between English and Portuguese. While I can not speak Portuguese, my second language Spanish, allows me to read it with an acceptable degree of understanding. The need for detailed understanding however was not an issue with this kit. The screen print on the circuit board was well done and identified all the components properly. In fact I rather regretted printing the entire assembly manual as the step by step illustrations used up a lot of my green printer ink in the 11 pages of illustrations for placing each individual component.

 

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This was the first page of the instructions

 

The meter was fun and easy to assemble with only one final mV calibration between pins 35 and 36 of the main IC necessary. The circuit is powered by a 5 volt source and will output a reading of +/- 0 to 2 Amps with the supplied 0.1 Shunt Resistor. I noticed that there were solder pads that can be bridged to move the decimal point and one entry in the manual indicated that the meter was also being marketed to read 0-20 Amps and 0-200 Amps. Apparently a 0.01 ohm shunt will give a 0-20 Amp range and a 0.001 Ohm shunt will yield a 0-200 Amp Range.

 

I thought briefly about making the circuit multi-range by switching in the appropriate shunt resistors but some quick experiments showed that due to lead resistance and switch contact resistance I would not be able to obtain the needed accuracy.

 

I hooked the raw finished meter up and ran some trial tests to compare it against my other moderately accurate test equipment. It worked very well displaying one more decimal place than I had on the Fluke 177 and it seemed to be consistent over the 0 to 2000 mA range. Many of my little experiments utilize a mA meter and I suddenly had a memory flash from the past of the meters we used to use in High School Physics Class when we played with electricity.

 

 

 

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Why not make a modernized bench digital Ammeter in the same spirit as this moldy oldie? I looked through the selection of salvage cases from various dental equipment and found one that used to be part of a monitor for a Sterilizer. It was perfect except that it was constructed of 1.5 mm thick steel. An hour and a half later with lots of drilling, sawing and filing I had finally cut the 4 X 7 cm hole needed for the meter. The holes for the binding post were easy and I had the meter mounted and soldered to the binding posts.

 

My next decision was how was I going to power the meter. There was lots of room inside the case. I think they call meters like this lunch boxes for all the extra room. While it would have been very easy to put a small AC to 5 volt DC power supply into it I decided that I didn't want to add the need for another power wire to my experimental setups. Since the unit runs on 5 volts my attention was pulled to a small cell phone auxiliary charging battery that I had on the bench charging my cell phone.

 

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I checked the mA hour rating on the battery and it said 2200 mA hours. My guess is that this is a gross exaggeration but even if it is half that amount in reality it should power the 35 mA draw of the meter for 30 hours which is more than adequate for my convenience. Further more all I have to do to recharge the batter is plug it into a telephone re-charger. Here are a few pictures to show you the final product.

 

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Here are a couple pictures of the inside.

 

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Finally we have the meter monitoring a current through a power resistor:

 

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Not counting my time (hint: not worth much at this stage of life) the meter cost $12.00 including the battery. I had a good time putting it together and I anticipate the pleasure of using it in my next experimental setup requiring the monitoring of a current.

 

John

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