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 justhifi 06-21-2010 08:00 PM

How to make an ammeter

I am an absolute newbie to electronics design and I'm looking to make an ammeter to monitor the current in a small little set up that I have. I use a 9V battery to power my little rig and need to be able to monitor the current from across the room. I was hoping for some help on how to make one that uses some LEDs as the display. Perhaps one for each 1/4 mA . The current range that I am expecting is 0 to about 20mA.

Thanks in advance !!

 R!f@@ 06-21-2010 08:15 PM

Welcome to AAC.
For this U will need some complicated comparators circuits and an LM3914 IC's in cascade
Complicated in the sense means there resistor values have to be calculated to get the exact amount of threshold.
One more thing is a current shunt.
First tell us where you are from and how you can get these components.

 KL7AJ 06-21-2010 08:19 PM

Quote:
 Originally Posted by justhifi (Post 254036) I am an absolute newbie to electronics design and I'm looking to make an ammeter to monitor the current in a small little set up that I have. I use a 9V battery to power my little rig and need to be able to monitor the current from across the room. I was hoping for some help on how to make one that uses some LEDs as the display. Perhaps one for each 1/4 mA . The current range that I am expecting is 0 to about 20mA. Thanks in advance !!

Greetings....there are multitudes of ways of doing this, depending on how tricky you want to get.

All current meters function in essentially the same way, by measuring a small voltage drop across a known shunt resistance.

As far as an LED driver is concerned, you can build what's known as a "thermometer" decoder, which is simply a string of voltage comparators, each at a slightly higher threshold voltage. It's not really a decoder, because there's no real logic involved...each comparator just drives a separate LED. A couple of quad comparators will give you eight different levels.

Look at other AAC threads on voltage comparators.

Eric

 justhifi 06-21-2010 08:23 PM

I live in Canada, I don't know how I will get the components.... internet order ?

 mcgyvr 06-21-2010 08:28 PM

Canada is fine, there are plenty of places to order on the internet to ship there... He was just making sure you weren't from some where that might be hard to get parts like Maldives,etc.. :)

 someonesdad 06-21-2010 09:17 PM

It wouldn't hurt for you to read up on ammeters and how they work: http://www.allaboutcircuits.com/vol_1/chpt_8/4.html and http://www.allaboutcircuits.com/vol_6/chpt_2/4.html.

If it were me, the first thing I would consider is using an analog meter and a single shunt resistor. This could make your ammeter in record time. It's not too hard to find scrapped meters and once you find out what their full scale current is, it's easy to calculate what the shunt resistance should be. Sounds like you want to make the ammeter read 0 to 20 mA full scale. Some of the older meters are nice in that you can open them up and replace the scale with one custom-made. This can be done with pen and ink or with a computer if you know a graphics language like PostScript. Analog meters often come in full scale currents of 50 to 100 μA. Note that you can probably use almost any meter, even one marked for measuring voltage, by taking it apart and replacing or removing internal resistors. Of course, if you need help, take pictures and post details here and you'll get some advice.

Total time for this is probably shorter than any other method.

If you want to use LEDs, then there are ICs that can do the comparisons and light the relevant LED. One is the LM3914 -- and the datasheet shows you how to cascade them. Using such an IC will probably get you what you described, but it's going to be more work because you have to acquire the parts and build things up.

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