Constant current source

Discussion in 'Homework Help' started by drago, Sep 21, 2009.

Aug 12, 2009
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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.

• current source-sink.PNG
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Last edited: Sep 21, 2009
2. SgtWookie Expert

Jul 17, 2007
22,183
1,728
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.

3. GetDeviceInfo Senior Member

Jun 7, 2009
1,571
230
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.

Aug 12, 2009
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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.

Apr 5, 2008
15,796
2,384
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

Last edited: Sep 21, 2009

Aug 12, 2009
14
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sorry guys it is 10mA

Last edited by a moderator: Sep 21, 2009
7. SgtWookie Expert

Jul 17, 2007
22,183
1,728
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.

8. SgtWookie Expert

Jul 17, 2007
22,183
1,728
So, what is the new current range? 2mA to 10mA?

Oh, is this a school homework assignment?

Aug 12, 2009
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Yes it is may first home work!!! and yes again 2mA to 10mA Vcc is +-15V

10. beenthere Retired Moderator

Apr 20, 2004
15,815
283
The numbers still won't work. I = E/R. 15 volts and 2000 ohms can only produce 7.5 ma.

11. Wendy Moderator

Mar 24, 2008
20,772
2,540
Drop the emitter resistance. Use the + side of the op amp as a control signal (voltage in, current out).

12. JoeJester AAC Fanatic!

Apr 26, 2005
3,393
1,211
There is a way to get 10 mA ....

see attachment

• Current_source_sink.pdf
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Aug 12, 2009
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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...

14. JoeJester AAC Fanatic!

Apr 26, 2005
3,393
1,211
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

Last edited: Sep 22, 2009
15. Mu86neer Member

May 1, 2009
23
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HERE IT IS ,,,
try to built the circuit in the attachment and then i can show you the calculations.

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16. hgmjr Moderator

Jan 28, 2005
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Isn't the gain of the first stage -10 rather than -11?

hgmjr

17. Mu86neer Member

May 1, 2009
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yes ,as long as its connected to the non inverting input of the opamp ( Av=-Rf/Rin)

18. hgmjr Moderator

Jan 28, 2005
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214
If you take a look at this presentation the inverting gain is Av = -Rf/Rin while the non-inverting gain is Av = 1+ Rf/Rin. I think you may have it backwards.

hgmjr

19. JoeJester AAC Fanatic!

Apr 26, 2005
3,393
1,211
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.

• Virtual_Devices.pdf
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Last edited: Sep 23, 2009
20. Mu86neer Member

May 1, 2009
23
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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.!!!!!