A New Approach to a Bench Top Ammeter

29 Sep 2017

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

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.

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.

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.

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.

Here are a couple pictures of the inside.

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

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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jw0752 over 7 years ago

After having a few days to think about it I realized that I was approaching the idea of expanding the ammeter to a dual range version all wrong. Sometimes I get a wrong idea in my head and it takes a while to see past it. In this case I was envisioning having to switch different shunts into the circuit in order to have a dual range device. Yesterday the light in my head finally flickers and brightened a bit so I realized that the shunt should be a fixed 2 stage voltage divider and that I only need switch the input of the meters ADC from one point on the shunt to another while also switching the display decimal point one place. Here is a schematic for what I decided to do:

Now in theory the resistors are the values that are printed on them and everything turns out pretty but this seldom happens, at least in my world. In this case the theory only got me to the point where one scale was off from the other by about 10 percent. If I had a reading on 2 amps on the 2 amp scale the 20 amp scale would read 1.8 amps. After an hour of adding various resistors to the Trim resistor I finally had both scales on the same page. Here a re a couple pictures of the display reading 318 mA

2 Amp Scale

20 Amp Scale

Here is a picture showing the messy shunt with the extra 2 trim resistors across the 0.1 ohm shunt.

Note that the input to the ADC starts with a series 1 M resistor. At least by switching the ADC and not shunts the resistance of the switch contacts are irrelevant.

This is a different unit from the one that I built above in the black case. I am still trying to decide if I am going to make that one into a dual range as I did not plan for a range switch and there isn't a really good place to put one. Perhaps I will have to wait for the lights in the old head to flicker on again.

Last picture shows the whole raw ammeter and the switch. I am currently planning on building a one channel 30 volt bench supply with an auxiliary ammeter and this one will get used in that build.

John
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jw0752 over 7 years ago

After having a few days to think about it I realized that I was approaching the idea of expanding the ammeter to a dual range version all wrong. Sometimes I get a wrong idea in my head and it takes a while to see past it. In this case I was envisioning having to switch different shunts into the circuit in order to have a dual range device. Yesterday the light in my head finally flickers and brightened a bit so I realized that the shunt should be a fixed 2 stage voltage divider and that I only need switch the input of the meters ADC from one point on the shunt to another while also switching the display decimal point one place. Here is a schematic for what I decided to do:

Now in theory the resistors are the values that are printed on them and everything turns out pretty but this seldom happens, at least in my world. In this case the theory only got me to the point where one scale was off from the other by about 10 percent. If I had a reading on 2 amps on the 2 amp scale the 20 amp scale would read 1.8 amps. After an hour of adding various resistors to the Trim resistor I finally had both scales on the same page. Here a re a couple pictures of the display reading 318 mA

2 Amp Scale

20 Amp Scale

Here is a picture showing the messy shunt with the extra 2 trim resistors across the 0.1 ohm shunt.

Note that the input to the ADC starts with a series 1 M resistor. At least by switching the ADC and not shunts the resistance of the switch contacts are irrelevant.

This is a different unit from the one that I built above in the black case. I am still trying to decide if I am going to make that one into a dual range as I did not plan for a range switch and there isn't a really good place to put one. Perhaps I will have to wait for the lights in the old head to flicker on again.

Last picture shows the whole raw ammeter and the switch. I am currently planning on building a one channel 30 volt bench supply with an auxiliary ammeter and this one will get used in that build.

John
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