# Designing a constant value current source circuit having 500 mA output using FET Technology

Thread Starter

#### xaadkhan98

Joined Nov 10, 2019
6
Hello Everyone , I am working on a small project, in which i have to design a constant value current supply/source circuit having 500 mA output using FET technology for the design.
NOTE: I Do not understands FETs Very well.

#### ericgibbs

Joined Jan 29, 2010
11,593
hi 98,
Welcome to AAC.
Do you have a circuit diagram you could post.?
E

Thread Starter

#### xaadkhan98

Joined Nov 10, 2019
6
Thank You Eric !
No , our teacher has just given the following condition of using FET while desiging a ciruit of 500mA,

#### Alec_t

Joined Sep 17, 2013
11,798
As this is Homework, please post your efforts so far at a design.

Thread Starter

#### xaadkhan98

Joined Nov 10, 2019
6
Actually this is our school project thats why i posted in project category, and yeah i need its design

#### ericgibbs

Joined Jan 29, 2010
11,593
hi 98,
We can only help with college assignments, not give a final solution.
So post your best attempt at solving the problem, a simple sketch would be a start.
E

Thread Starter

#### xaadkhan98

Joined Nov 10, 2019
6
it is giving me 499mA, but i am not sure about this.... wheteher this is right or not and secondly i have 5 and 12 v powers how can i reduce it to 3.728 or any alternate method.

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#### ericgibbs

Joined Jan 29, 2010
11,593
hi,
You cannot control the Drain/Source current to be Constant value by using a direct Gate voltage.

The other point that you have to decide will the 500mA load current device be in the Drain or Source of the NMOS FET.?
E

Thread Starter

#### xaadkhan98

Joined Nov 10, 2019
6
So how can i control it to be a constant cùrrent source ,

The other point Drain ,

#### ericgibbs

Joined Jan 29, 2010
11,593
hi,
You need some method of control the current thru the MOSFET.
Consider that you had say a low value resistor in the Source eg: 5R
What voltage would the Source pin measure to 0V when passing 500mA.????

E

Update:
Look at these two clips from the MOSFET datasheet.
Check the MOSFET Vgs for 500mA Drain current.

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Last edited:

#### Alec_t

Joined Sep 17, 2013
11,798
So how can i control it to be a constant cùrrent source
This is where you do a bit of research. Google 'constant current source' and I'm sure you will get a lot of useful info to get you started.

#### crutschow

Joined Mar 14, 2008
26,006
Can you use any other active components besides a FET?

Thread Starter

#### xaadkhan98

Joined Nov 10, 2019
6
Nopes Crutschow , only fets and resistors.

#### crutschow

Joined Mar 14, 2008
26,006
Nopes Crutschow , only fets and resistors.
Look up JFET constant-current source.

#### danadak

Joined Mar 10, 2018
4,057
The basic idea is you need to be able to sense I, with some form of feedback and
gain, and use that to alter the pass transistor control, in your example the Vgs of a
MOSFET.

So you need to detect I and control Vgs, how do you do that ? As I goes up we
need the Vgs to go down, to drive I back down, a way of offsetting I change...what
does an R exhibit when a I is passed thru it ? Next where do we place it to change
Vgs in the right direction ?

Regards, Dana.

#### MrAl

Joined Jun 17, 2014
7,886
Nopes Crutschow , only fets and resistors.
Hi,

Ok so what have you designed in the past so far?
We need an idea what you are used to doing to give you a good reply.
If you only can use FET's and resistors then you may have to construct a differential amplifier first.
Are there any other constraints, like overhead voltage allowance or similar. This is important too.

#### danadak

Joined Mar 10, 2018
4,057
Notice MOSFET characteristic curves. When Vgs is held at a value
what is happening to Id with Vds changing a lot ?

Regards, Dana.

#### Dodgydave

Joined Jun 22, 2012
9,854
Nopes Crutschow , only fets and resistors.
then use a Jfet like a J113, or 2N3819 in Constant Current mode.

Try a 1K potentiometer for R.