# Constant Current power supply

Discussion in 'The Projects Forum' started by abicash, Nov 6, 2008.

1. ### abicash Thread Starter New Member

Oct 18, 2007
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Hi all

I will explain exactly what I am intending to do.

I am building a power supply for a resistive load.

The load under consideration is a kind of nichrome fuse,of which I have to test some parameters with my equipment.
Current should be variable from 1A to 5A in steps of 1A constant.
Voltage need not matter and I would be building a CV of 15V (I feel would be sufficient)
Now I have to program a one time pulse starting at 50ms up to 500ms in steps of 50ms.
After everything is set and I press 'FIRE' or something I will switch on a MOSFET to give a path to the current through the Load.

I am unable to understand a few things here though.

I primarily will set current with a fixed resistor bank and using a short circuit instead of load.
e.g. For 3A=15v/5E so would use a smaller R at say 4.5E to get a higher current. 15V/4.5E=3.33A (considering unknown load)
After this is set,I input value of pulse at say 50ms.

After this,I put the load on O/p and press 'FIRE'.
Now during this 50ms time I should limit the current w/o any overshoot at 3A by using some kind of variable resistor.
If the Load value is 5E then current will come down to 1.5A,which the circuit should compensate by either increasing the voltage or by a constant current source.
If I use a transistor as a variable resistor,wont the error correction take some finite time?which will not be permissible.

I hope I am clear enough..
Thanks and please get back with your valuable suggestions!!!

2. ### abicash Thread Starter New Member

Oct 18, 2007
7
0
Hi

After thinking for a bit I assume I can build this with 2 nos of LM338.(?)
I could build two power supplies for this,first one as a CV source and the second one as a CC source with 5 selectable resistors (1A to 5A).
Am I correct?

If I am correct,where should I add the MOSFET switch? After the CC source?
So that Load is not fed with the CC.In such a case what would happen to the O/P Voltage of the CC source? Wont it try to rise to the max permissible voltage?
I mean,when i turn the switch on,the load will see all of the max voltage and then the CC source will try to maintain the constant set value.In how much time?

(EDIT: I see for a 2V drop on load,the current is set in less than 5microseconds,likewise for overshoot)

Please get back.
Thanks and regards

Mar 24, 2008
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4. ### abicash Thread Starter New Member

Oct 18, 2007
7
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Thanks Bill

Quite a long post though..going through it!!

5. ### abicash Thread Starter New Member

Oct 18, 2007
7
0
Hi

First of all,a big thank you to Bill for pointing out the thread to me
It helped a lot.

I am now sure about using LM338 having the output current from its ADJUST pin,and selectable resistors between ADJ and O/P pins.
Please correct me if I am wrong!!

I also need to figure out how to select the bank of resistors for variable current.
I mean, for 1A i have to select just one 1.25ohm resistor,likewise for 2A I need to have either a 0.625ohms or two no's of 1.25 ohms paralleled and likewise for higher currents up to 5A.Any ideas how I can achieve this using some kind of switch.
All help appreciated
Thanks and regards