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Current-dropping current source! I don't get it! Please help!


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Greets everyone,

I am not sure if this is the right place to post this in the forum, but I have a problem with a circuit that I designed at the implementation stage.

^ That's the circuit that I have designed. The purpose of it is to behave as a current source that supplies a range of current from 0 to 120 mA, to a 20 Ohm load (R11 in schematic). I have simulated the circuit within software known as SIMetrix and it works as I want it to under high temperatures, long periods of operation and if the 20 Ohm load alters in value. However once I built the circuit and tried it out I was surprised.

When I begin to rotate the pot, and stop it at a point where I need it say 52.2 mA (?!), the current will immediately begin to drop by 0.1 mA. Over time it will keep on dropping (over time = ~10 mins) but the 0.1 mA reduction gets very slow. I thought that it's becuase my resistors or something else was getting hot, however if I cooled the transistor more current began to flow!

I really don't want the current to drift by 0.1 mA at all! I want the current to stay exactly where I set it regardless of how much temperature changes. Also the current I set should stay constant for +/- 10 Ohms in the load. So if I were to set the current I want output to 80 mA it should stay constant over a 10, 20 and 30 Ohm load.

Well anyway I think this post is getting long enough and I don't want to start mixing my objectives and confusing everybody!

I really appreciate any help you guys have to offer.

v5 0ut.

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Oh damn I forgot I posted something similar to this already in this forum, sorry, any moderator want to chuck this topic back into my old one?

Hmm, OK, how would I go about regulating VIN+? I thought about using a diode, or a Zener diode, in like a shunt configuration, or would it be better for me to build/buy a regulator?

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