Neargh! Why won't it work?!
Apr 29, 2009 at 2:23 AM Thread Starter Post #1 of 13

Tridacnid

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Blah!

Okay, just when I felt that my building skills were getting to be useful, I run into this. I'm building Doug Savitsky's "L'espressivo" somewhere between version 1 and version 2. On his site, he recommends upgrading the CCS if anything else is upgraded. Anyways, I've built the CCSs and now I'm trying to set them to 25mA using a trimpot in series with a 10 ohm resistor.

Here's the basic schematic.
ConstantCurrentSourceSchem.jpg


Anyways, I can't get a reading on my multimeter when I connect the positive terminal of a 9V battery to the B+ in and then put my multimeter on the 200mA setting and place it in series with the "Plate" out and the negative terminal on the 9V battery. I've even tried reversing the polarity of the battery, just in case something is wrong with the battery. No matter what I do, I get a boring old reading of 0.00mA. I even borrowed a multimeter from my school, and it gives the same reading.

Now here's the strange part, to me at least. I can put a green LED in between the Plate out and the negative terminal and it will light up like a candle. Not only does it light, though, but it puts out enough heat to lead me to believe that it is being driven close to its maximum current rating (around 25mA).

I'm at a loss. In the picture above, I've numbered the Source, Drain, and Ground of the CCS chips. This is the only thing that I can think of being possibly incorrect. I must have connected the wrong terminal to something. Here is the datasheet for the chips I'm using. The numbers in the schem above correspond to the pin numbers I believe are correct.

Let me know what you think is wrong. I can post pictures, give a better description, chase groundhogs for you, anything, just help me figure this out.

Thanks,
Tri
 
Apr 30, 2009 at 2:46 AM Post #7 of 13

luvdunhill

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Quote:

Originally Posted by Tridacnid /img/forum/go_quote.gif
Across a 100 ohm resistor, I'm getting a difference of 1.24 volts on both meters.


now use Ohm's law...

does your DMM have separate place to plug in the leads when measuring current?
 
Apr 30, 2009 at 3:05 AM Post #8 of 13

Tridacnid

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I=V/R. So I'm looking at 12.4mA, correct? I even ohmed out the resistor, and it was a perfect 100.0 on the better multimeter.

Yep there are two separate plugs. I have tried it, though, so that doesn't fix anything. Blah.
 
Apr 30, 2009 at 3:06 AM Post #9 of 13

luvdunhill

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Quote:

Originally Posted by Tridacnid /img/forum/go_quote.gif
I=V/R. So I'm looking at 12.4mA, correct? I even ohmed out the resistor, and it was a perfect 100.0 on the better multimeter.

Yep there are two separate plugs. I have tried it, though, so that doesn't fix anything. Blah.



perhaps a blown fuse in the meter?
 

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