# Understanding, Optimization/Realization of the voltage controlled current source.

Discussion in 'General Electronics Discussion' started by DX400, Jan 15, 2019.

1. ### DX400

6
0
Oct 26, 2017
Hi,

I am unable to make a voltage controlled constant current source.
So I am using the following scheme (Sorry for bad sketch drawing):

The constant control voltage varies in the range of 0 to 5 V is applied by constant voltage supply (like HY3002 from e-bay) or (in what follows) by DAC output of PC-controlled board. So the control voltage is rather constant and linear.
The desired output parameters: constant current on load from 0.5 to 150 mA.

So, as a result, at the load resistance I have got the following signal.

The questions:

What is the source of this large noise in output signal and the signal spikes and drops?

It is possible to tune the scheme to obtain constant current or the used scheme is completely incorrect for constant current supply?

2. ### WHONOES

1,124
311
May 20, 2017
Your circuit is wrong. The load should be in the FET drain circuit whilst the source of your FET should be connected to the resistor "U CONTROL". That resistor should be a constant value and not variable. The 300Ω pot should be a fixed value and should go to the negative input of the LM324. The input signal should go to the positive input of the 324. The top of 5K1 resistor should go to 0V and not +24V. The value of "U Control" resistor should be scaled to have 150mA flowing through it at 5V, use Ohm's law to determine.The LM 324 is not the best device to use as it is quite noisy. Also if you are using just one of the four amplifiers in the package without tying their + inputs to ground and connecting their - inputs to their outputs, they may well oscillate which is what you may be seeing on your scope. They will also need some power supply de-coupling capacitors across their supply pins. Also, the DAC might be a bit noisy as well.
Other than that, it is not too bad!!!

3. ### Bluejets

4,748
999
Oct 5, 2014
Don't think it will withstand 48v either (unless that is a drawing error....+24v , -24v)

4. ### WHONOES

1,124
311
May 20, 2017
Ta, missed that bit.

5. ### DX400

6
0
Oct 26, 2017
Of course, this is my drawing mistake. The power supply provide 24V and the LM324 is used in the unipolar power regime. The “Vcc1 (24V-)” at the bottom should be removed.