Constant current source using op-amp

Hi Umesh,

I will move this question to the precision amplifiers forum for further support. 

Regards

Hi Umesh,

An op amp based current source's output must be able to achieve a voltage level dictated by the constant current value, times the load resistance (I x R). You mention that you need a constant current of 10 mA delivered to a load that can vary from 10 ohms, to 5 k ohms. The output voltage range that the op amp output would have to be able to accommodate would then be 100 mV to 50 V.  Therefore the op amp's output range, which is referred to as an output compliance range, would have to be able to swing from  to 0 V on the low end, to at least 50 V on the high end.

Your planned power supply is +/-15 V. The immediate problem is an op amp's output swing can't be higher than its supply voltage. Therefore, a high voltage op amp with a positive supply operating higher than +50 V would be needed to support the +50 V output voltage. This approach would accommodate a 10 mA current through a 10 ohm, to 5 k ohm load resistance. A low voltage negative supply (~ -5 V) could be used for the HV op amp V- supply. Otherwise, the high-end resistance would have to be limited to something less than 1.5 k ohm for the +/-15 V supplies.

Regards, Thomas

PA - Linear Applications Engineering

Thank you very much Thomas
Is there any op-amp which provide supply voltage up to 50V?
Or at least 40 V. Because I want above 8mA value

Regards
Umesh

Hello thomas,

I changed the opamp from LT1002A to LM258AD  and it woks great . But now I have doubt about OP-AMP supply because in multisim software, nothing is shown at its supply pins as I shown by red arrows. 

So for proper simulation is there need of connecting V+ and GND of OP-AMP??

Hi Umesh,

The ideal op amp for the application is the OPA454. It is designed for +/- 50 V supplies and can sink/source up to 50 mA. In your application it would only be sourcing current so I would use +55/ -5V supplies if possible.

You can view the OPA454 datasheet here:

Regards, Thomas

PA - Linear Applications Engineering

 

Hi Umesh,

Below you will find a circuit diagram for a 10 mA Improved Howland Current Pump. It uses the OPA454 HV op amp. It appears the positive supply V+ is going to have to be set to at least 60 V to be able to provide the combined voltage drop across Rs and RL, while supplying 10 mA to the maximum 5 k load. If you only need 8 mA, then the circuit can be scaled and V+ will not need to be set as high.

I've attached the TINA Spice schematic. You can use it, or recreate it in your simulator software.

Regards, Thomas

PA - Linear Applications Engineering

OPA454_Impr_Howland_01.TSC

 

Thank you very much Thomas

Hello Thomas,
Your circuit works pretty good.
Thanks again for help

I made slight change in my previous circuit as :
replaced LT1002A with LM324AD and Supply for LM324AD is +12V and GND.
With this supply (only 12V) it is giving 8mA constant current for 5k ohm load also.
I wonder by seeing this in multisim simulation because from 12V supply can it possible to create 40V drop across RL?

Will also it work practically?

Thanks in advance
Regards
Umesh

This is my simulation result: current changes in few micro ampere only

Umesh,

LM324 is my device. With a 12V supply voltage, maximum output is 10.5V 

This would be just 8V on the 5k load for 1.6mA

Oh my god
Then how is showing 8mA for 5K load in multisim?

So unfortunately I think it will not work practically.

Thanks Ronald for Information.

What is the major difference between LM342A and LM324AD?
I am unable to find out datasheet of LM324AD

My circuit gives 8mA for 5Kohm when LM324AD is used. And it gives only 2mA for 5Kohm when I use LM324A

Umesh,

The LM324A and LM324AD are the same. The "D" means SOIC package.
The quality of the models are not the same. Where did you get the LM324 models?

Hi Umesh , 

Sorry for the late reply but got a couple of more ideas for you constant current here

We can get away with a low supply op amp by connecting an external high voltage source

 

I have done a Dc sweep by varying RL from 10ohm to 5kohm

Current is constant          

 

The draw back I see is

At low resistances  the Vce drop across the transistor is very high , hence the power dissipation is very high

Around 58* 10mA = 580mW

 

But we can take care of this by making the 60V supply  a variable supply whose voltage increases with resistance value so as to make sure that the transistor is always in linear mode and does not go into saturation

 

Hence at lower resistance we can reduce the voltage supply , thus making the Vce drop  just sufficient to keep the transistor in linear mode does regulating a constant 10mA current

 

One mode idea that i have is connecting the load across the boost converter 

 

Hello shreenidhi,

for 5K ohm load and  8mA current 40v is required, which is not possible with 12v supply, so I reduce my load from 5k to 500 ohms only.

And it works fine upto 660 ohms of load that means I am getting 660*8mA=5.2V which is less than half of supply voltage(12V). This result is for above howland current source. It is why because to maintain constant current the output voltage of op-amp would be 2 times that of the load voltage as shown in below fig.

So here I cant use full range of voltage across load. 

How can I modify this circuit to use the full range of output voltage

Or any other circuit is there to achieve this??