Constant current source

Thread Starter

drago

Joined Aug 12, 2009
14
I have built constant current source using ua741 amplifier and npn BJT bc639. Circuit gives constant current of 100mA. My new problem is how to control that output current from let say from 20 to 100mA by output voltage. The whole project goes like this; the circuit has to give constant current (max. 100mA) which amount has to be regulated from outside with voltage.
 

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SgtWookie

Joined Jul 17, 2007
22,230
Post an image of your schematic, preferably in .PNG format.

You can attach it to your first post by using the Edit button, and then click the Go Advanced button below the text box. On the next screen, click the Manage Attachments button, navigate to your image on your computer, select it, and then click the Upload button.
 

GetDeviceInfo

Joined Jun 7, 2009
2,192
constant current sources are typical in that they regulate voltage over a fixed resistance. It then becomes a matter of summing a variable reference voltage into your circuit.
 

Thread Starter

drago

Joined Aug 12, 2009
14
The load is changing between 0.5 and 2kΩ. the basic scheme and file for multisim simulator is in attachment.the current through load is constant and the problem is to control the amount of that current (max 100mA) with voltage.i think that should be an another circuit that control current trough the load.

i have no idea how to build that and mentor will not help me he said the task is:constant current source max 100mA the load variate between 0.5 and 2kΩ the amount of that current must be regulated by voltage 0 to 1V from another source, something like input signat to the circuit.
 

bertus

Joined Apr 5, 2008
22,270
Hello,

When you say 2 K Ohms at 100 mA the voltage must be at least 200 Volts for the driver circuit.
This can not be done with the simple 741, as that can work on max ± 15 Volts.

Greetings,
Bertus
 
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SgtWookie

Joined Jul 17, 2007
22,230
To get 20mA current through a 2k resistor will require a Vcc of 40v.
To get 100mA current through a 2k resistor will require a Vcc of 200v.

You wouldn't be able to use the current source schematic, as the 741 wouldn't tolerate a high enough Vcc.
 

Thread Starter

drago

Joined Aug 12, 2009
14
Vcc for bc639 is 15V.Vcc for 741 is +-15V. Rc is 100Ω and Re vary from 0.5k to 2kΩ. The current Ic is constant and i am getting 11mA. What am i missing...
 

JoeJester

Joined Apr 26, 2005
4,390
Let's see ... check your measuring device's specification ... or ... it's probably out of calibration.

15V / 2000 ohms = 7.5 mA ... it's not called Ohm's suggestion, it's Ohms LAW


Post your diagram.
 
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JoeJester

Joined Apr 26, 2005
4,390
Mu86neer,

Nice that you are using virtual devices. Try it with a spice model of something you'd really use ... complete with the +/- voltages.

If using the virtual devices were allowed, the sky would be the limit ... if any limitations were imposed.
 

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Mu86neer

Joined May 1, 2009
23
WOW that was impressive..hehe
well, could u plz tell me what if i feed the input with 12V whereby the gain is -10 so isn't it irrational to get an output of -120Volts...?????
i could have been killed in the lab if it was so.!!!!!
 
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