variable current source

[NARENDRA1234]

we are looking for a current source circuit which can supply a variable current from 5 to 15 or 0 to 10 mA current.how to design it?please help

 

[prcken]

add trimming bits to achieve variable current

 

[NARENDRA1234]

what do u mean by trimming bits??????????????????????????????

 

[prcken]

i meant you can add several legs of current which can be controlled by digital signals.

 

[NARENDRA1234]

we dont want such complicated circuit..we just want to control the current through resistor and transistor

 

[ssingh]

if ur current source is voltage controlled then u might use Operational Transconductance Amplifier where the input voltage controls an output current by means of the device transconductance, labeled gm. OTA is a voltage-controlled current source ........

 

[prcken]

do it in analog fashion? maybe a little bit tough.
as ssingh mentioned maybe you have to design a VGA to controll the VCCS.
anyway, how to change the variables, i dont know,heh

 

[keith1200rs]

This is a simple current source. The current depends on the input voltage and the value of R1.

Keith.

 

[NARENDRA1234]

thnks man just one doubt..van i use other op amp instesd of this?is it available in market?(op amp) i can vary current by changing the load resistor right...how to calculate input voltage?

---------- Post added at 17:54 ---------- Previous post was at 17:51 ----------

instead od bc848 can i use 548?

 

[keith1200rs]

The transistor and opamp aren't critical - most will work. The current is Vin/R1.

Keith

 

[jerkymotion]

please,can u help me in getting(variable and upto 6A) output current ..

---------- Post added at 23:06 ---------- Previous post was at 22:55 ----------

..can u please explain the circuit that u just posted in currentsource.pdf. in which mode does the transistor operate and how the circuit output current varies.

 

[ssingh]

the resistor R1 acts as an current sensing resistor(the one in feedback loop of op amp).The op amp automatically adjusts its output (Q1's base drive) to bring its negative input equal to the positive input. This means that Vsense = Vin. Since current is vin/R1 and Q1 delivers the current. And because Q1's currents are related by Ic ≈ Ie, the same current that that flows through R1 will flow through R load,hence the current delivered to R load is vin/R1..

 

[keith1200rs]

Ssingh,

Thanks for the contribution. Just to add, for 6A output you will probably need some more gain for the transistor depending on the opamp. A darlington transistor would be one option, along with a lower current sense resistor, of course.

Keith

 

[jerkymotion]

can we use mosfet operating in an active mode........to obtain constant current (not a current mirror)

---------- Post added at 00:30 ---------- Previous post was at 00:27 ----------

if yes ....then please help to design constant current source(say 6A) using Mosfet. can we change it's output current by varying the gate voltage

 

[keith1200rs]

You can use an NMOS in place of the NPN.

Keith

 

[NARENDRA1234]

here V-in means V1 right?

 

[keith1200rs]

Yes Vin = V1. Be careful about power dissipation if you want 6A. If you have a 12V supply and a 6A current source then that is 72W that has to go somewhere - into the load, the transistor and the current sense resistor.

Keith

 

[jerkymotion]

Does the mosfet used instead of npn transistor(in above circuit.....current source.pdf) operates in active mode. What if the mosfet begins to operate in other region rather than active mode.
please help me out in designing the current source using mosfet.

 

[keith1200rs]

The NMOS would be operated in saturation (not to be confused with saturation of an NPN). It would still work even if it becomes ohmic, but in that case it would probably be about to cease controlling the current as it means you haven't enough gate drive available.

Keith.

 

[jerkymotion]

please give some suggestion to keep the NMOS operating in Active region so that it behaves as an current source and make it variable one too..