vicious5id Posted September 1, 2006 Report Share Posted September 1, 2006 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. Quote Link to comment Share on other sites More sharing options...
audioguru Posted September 1, 2006 Report Share Posted September 1, 2006 You should keep this dicussion in your original thread about a current source.A tiny change of only 0.1mA in a current of 52.2mA is a change of only 0.2%. The values of your resistors probably change more than that. Quote Link to comment Share on other sites More sharing options...
aProgrammer Posted September 2, 2006 Report Share Posted September 2, 2006 The precision of this circuit is limited bythe transistor's Hfebecause Ic is not egal to Iea mosfet transistor will be betterbecause Idrain = Isourcealso V1 should be regulated because if V1 drop by 1%,the current will drop also by 1% Quote Link to comment Share on other sites More sharing options...
vicious5id Posted September 3, 2006 Author Report Share Posted September 3, 2006 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? Quote Link to comment Share on other sites More sharing options...
aProgrammer Posted September 3, 2006 Report Share Posted September 3, 2006 Yes, Vin+ can be regulated instead of V1.possibly a zener,better replace R3 by a 78LXX or TL431 Quote Link to comment Share on other sites More sharing options...
vicious5id Posted September 4, 2006 Author Report Share Posted September 4, 2006 You want me to replace resistor R3, with a voltage regulator?, but the thing is these babies have minimum inputs and outputs that exceed 1 V, and 0 to 1 V is all I need at VIN+ in my circuit! Quote Link to comment Share on other sites More sharing options...
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