# Voltage Controlled Constant Current Source

Discussion in 'The Projects Forum' started by drago, Sep 25, 2009.

1. ### drago Thread Starter Member

Aug 12, 2009
14
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My project is to make a Voltage-Controlled Constant Current Source. I have found two solutions This is first one

http://electronicdesign.com/Articles...ArticleID=9018

Problem is last sentence: For the circuit shown, a maximum current of 20 mA is feasible for a maximum load of 1100 Ω. using higher-voltage op amps and larger power transistors can increase these values if cost isn't a concern.
Cost is not much of a problem, but obtaining those components is hard in Croatia. So that solution is out of the game!!!

Second solution is this one and I will try to make it work

http://www.priorartdatabase.com/IPCOM/000007013/

I am new in electronics and I need help with choosing components for this circuit. This is my basic problem

Specifications are: Ic is constant and max 100 mA, that current is controlled by U1 and U2. The load is variable from 0.5k to 2 kΩ. Second amp will provide constant current because it is connected like Current Source with Floating Load and first amp will provide control voltage.

Circuit works as it should in multisim file is hire

and the picture is in the attachment...

All help is more then welcome...

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Apr 5, 2008
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3. ### drago Thread Starter Member

Aug 12, 2009
14
0
then my mentor is gone crasy i don't have 100 euros for amp.

I was wrong when i told that is 10mA in last tread. I was thinking he can't tell me 100mA it is too much but it is (thought i heard it wrong) i asked him now again..

i am stuck...

How i can build 200V supply... use what direct power supply or build something using capacitors even if i get that i still have money problem for amp.

4. ### JDT Well-Known Member

Feb 12, 2009
658
85
The simplest circuit is in the first part of my attached diagram.
Only the transistor needs to be high voltage. Low voltage op-amp.

If you need a grounded load you could replace the load in the first diagram with the current mirror as shown in the second part. All the transistors will have to be high voltage types. Also take care of power dissipation and "safe operating area". See the transistor data sheet.

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5. ### t06afre AAC Fanatic!

May 11, 2009
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I agree with JDT. You do not necessarily need a high voltage opamp to control your current. But some parts like the controlling transistor or fet need to be high voltage. You do not have to use transistor you can also use a fet. But anyway you should base your design on components that you know you have access to. Do not select your final design before you have confirmed that your components and spare parts are available.

6. ### drago Thread Starter Member

Aug 12, 2009
14
0
Transistors are no problem even if they are high voltage i can get supplier for them but high voltage amp are out of the question.

can someone tell me what transistors or fet to use that have low power dissipation and good "safe operating area", and can i use ua741 as a amp they are easy to get or something stronger?

JDT thanks for the advice it helped a lot... and the second part of my circuit is the same as yours in first part it is a simple constant current source and first amp in my circuit is for controlling amount of Iout. i think that concept of my circuit is not bad but my basic problems is still the same what components to use

7. ### SgtWookie Expert

Jul 17, 2007
22,183
1,729
Since E=IR, to get 100mA current through a 2k resistor will require 100mA x 2000 Ohms = 200 Volts, so you will need slightly more than a 200v supply.
If the load drops to .5k, then to get a 100mA current through the load will require 100mA x 0.5k Ohms = 50v.

If you're still using the 200+v supply, that will mean your current limiting transistor will have 150+v dropped across itself with 100mA+ current flow, and a power dissipation exceeding 75 Watts.