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Constant current power supply help needed

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abicash

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Hi

I am building a CV-CC supply which would output a 50ms pulse of the set output current to a load which would be up to 5 ohms.
I will set a current of 1Amp (using fixed resistor prior to the load) by replacing the load by a short circuit,vary the voltage and then connect the load for actual experiment.
During the 50ms time,how do I ensure that exactly 1A current is flowing irrespective of load value?
I am thinking of varying the voltage,but error correction on output by sensing current feedback will take some time,which will not be permissible.Although the max current will always be limited by the fixed resistor+load resistance
How do I tackle this problem.
 
Hi

I am building a CV-CC supply which would output a 50ms pulse of the set output current to a load which would be up to 5 ohms.
I will set a current of 1Amp (using fixed resistor prior to the load) by replacing the load by a short circuit,
If its a true constant current source, varying the supply voltage will not change the current.
Connect a current meter in place of the load and vary the constant current sense resistor until the meter read 1 amp.

vary the voltage and then connect the load for actual experiment.
During the 50ms time,how do I ensure that exactly 1A current is flowing irrespective of load value?
Connect a low value resistor in series with bottom of the load resistor, measure with a scope the voltage across the known resistor, Use Ohms law.

I am thinking of varying the voltage,but error correction on output by sensing current feedback will take some time,which will not be permissible.Although the max current will always be limited by the fixed resistor+load resistance
The maximum current will be limited by the constant current source not the load+resistor.


How do I tackle this problem.

hi,
Whats the purpose of this test.?:)
 
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Doe it matter if the current decays exponentially or does it have to be a square pulse?

You could use a 50mH inductor with a shottky diode and a transistor.


Turn the transistor on and allow the current to build up in the coil untill it reaches 1A, interrupt the current in the coil and it will keep flowing through your 5Ω load and the diode. The time constant will be 50ms meaning the current will have decayed to 63% of its initial safter 50ms.
 
A LM317 used as a constant current source should work. It has a response time of a few µs so it should generate a good constant 1A pulse for 50ms. Page 8 of the National Semi data sheet for the LM117/317 shows the circuit. It only requires one 1.2Ω resistor.

You simply apply a 50ms voltage pulse to the LM317 circuit of a few volts higher than the maximum required load voltage to generate the current pulse.
 
Along the lines of crutschow, we used this circuit to pulse laser diodes. Since Ron for the MOSFET is not "0.000Ω", or if Q2 is a BJT, you can add a fast diode in series with the load to assure no current through the load between pulses..

Ken
 

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Hi
Thank you all so much for the replies.Much appreciated!!! :)

@ Hero999 : current needs to be a square pulse.But your idea is interesting and I am thinking on how it could be improved to suit my needs..(i will explain my exact circuit requirement at the end)

@ ericgibbs : I will be more clear on my requirement.Since this is not an experiment,was just trying to put it in easy words :)

@ crutschow : The current will be slightly higher (with 1.25V drop).I am looking at LM338 on same principles after your suggestions,since the requirement is changed to variable current up to 5A.

@ KMoffet : Interesting !!!


I will explain exactly what I am intending to do.

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 as always!!!
:)
 
I see your requirements have changed, but were you still needing a 1A pulse, it looks like you could do it with a single LM2941T. It's an adjustable 1A regulator with on/off control pin. A single current programming resistor like for an LM317, and your timing input...
 
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
 
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Hi
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.

Please someone get back
Awaiting your precious reply
Thanks and regards
 
I am now sure about using LM338 having the output current from its adjust pin,and selectable resistors between ADJ and output pins.

Fine. (The current really comes from the output pin, whereas the adjust pin is just a sense pin.)

Also, a constant current source like this does not output max voltage as you suggested in an earlier post, because since there is nothing ground referenced here, your load simply sees the programmed current. The voltage that ultimately get impressed upon your load is a function of its resistance. A direct short sees no voltage across the load. In that case the LM338 will see virtually all of the input voltage (less the drop across the programming resistor, of course). With a 5Ω load at, say, 3A, 15V will appear across your load.

Below is one method of wiring up a rotary switch with programming resistors:
 

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Thanks a lot saturn1bguy :)

I think I will go ahead with this now.

Regarding the rotary switch selection,isn't there a simpler or rather,cheaper way of connecting these resistors?.

I was thinking about only 5 resistors and progressively add each resistor to the previous arm.(I presume it will be a complex instead of simple method.)

Thanks anyways
 
Regarding the rotary switch selection, isn't there a simpler or rather, cheaper way of connecting these resistors?

Sure, do what you think best. You could do it with just five resistors and five slide switches, each one adding a resistor.
 

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Thanks a lot saturn1bguy :)

I think I will go ahead with this now.

Regarding the rotary switch selection,isn't there a simpler or rather,cheaper way of connecting these resistors?.

I was thinking about only 5 resistors and progressively add each resistor to the previous arm.(I presume it will be a complex instead of simple method.)

Thanks anyways

Wire 5 suitable valued resistors [ to give the required current steps] in series.

Connect the switch wiper to one end of the series resistor string and the contacts so that you short out each switch in turn.
Your switch needs to be rated at 2Adc
 
Wire 5 suitable valued resistors [ to give the required current steps] in series. Connect the switch wiper to one end of the series resistor string and the contacts so that you short out each switch in turn.

Unless I'm missing something, this looks like it demands some funky values, perhaps much harder to find.

In any case, here are several options, perhaps only four of many :)
 

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Hi again

Thanks for the tips.
I had thought that using just five resistors will have to have several values which will have to be custom made.So this exercise is out of question,and I would sacrifice simplicity over ambition :p

I was thinking on the lines of putting some relay contacts in between the arms and shorting out each arm as current proceeds.But again its the cost impact that is more harmful.

After this i thought putting diodes like this

SW (A)
o---> o-----^^^----- 1.25ohms (1A)
| |
---- |
/\ |
/ \ |----------> to load
------ |
| |
| |
o----^^^-------1.25ohms (2A)
(B)

I think you might get a crude idea of what I am thinking.
The switch SW will connect to position A for a 1A and to position B for 2A.I hope a volatge 1.25V+0.7V+Unknown V is generated at point A so that the current remains constant(?)

Likewise for 5 A this voltage should be 0.7Vx4=2.8V up plus the other necessary voltages.

Am I thinking right or its just gibberish..or a figment of my imagination?

The method 3 proposed by saturn1bguy is what I will build if everything said above is nonsense.

Today I tested the CC method on a LM317 where I drew 250mA and it was constant irrespective of load..yayy..

Now to finish it off..one more question..:eek:

What voltg is optimum as input to CC regulator for my requirements.I had proposed 15V but 75W seems to a very unreasonable dissipation.

Any help appreciated...

Again thanks for the patience required to read such a long post...

EDIT: Attached a better diagram
 

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