Author Topic: How to design a true constant current source ?  (Read 1516 times)

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Offline Arsh ahmad

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How to design a true constant current source ?
« on: June 29, 2022, 10:00:43 am »
I want to design a current source in which even if the load resistance goes up the current remains constant, I need an accurate load current of 200mA. The source should maintain the same current level even if the load resistance is increased.
 

Offline perdrix

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Re: How to design a true constant current source ?
« Reply #1 on: June 29, 2022, 10:08:12 am »
What is wrong with the classic current mirror or other current sources - there's a lot of designs for this.

Just open a copy of most texts and you'll find lots.

D.
 

Offline Arsh ahmad

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Re: How to design a true constant current source ?
« Reply #2 on: June 29, 2022, 10:17:36 am »
I need my load to be varied from 1ohms to 100 ohms but I want the same 200mA through the load. In normal current sources, the current starts to drop when I increase the resistance.
 

Offline Codemonkey

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Re: How to design a true constant current source ?
« Reply #3 on: June 29, 2022, 10:32:20 am »
Is your supply voltage high enough ?
 

Offline Arsh ahmad

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Re: How to design a true constant current source ?
« Reply #4 on: June 29, 2022, 10:36:54 am »
I need one to be operated at about 5 volts and one more that need to operate at about 20-40 volts.
 

Offline RogerThat

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Re: How to design a true constant current source ?
« Reply #5 on: June 29, 2022, 10:39:39 am »
This: https://www.researchgate.net/figure/Circuit-diagram-of-the-constant-current-source_fig4_45899961

The op amp will make sure that R1 have the same voltage drop as the zener. Depending on your needs and precision, you could play around with other voltage references/transistors/resistors.
 

Offline Codemonkey

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Re: How to design a true constant current source ?
« Reply #6 on: June 29, 2022, 10:52:18 am »
No constant current generator is going to be able to put 200mA through a 100 ohm resistor when fed from 5V. See Ohms law!
 

Offline Arsh ahmad

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Re: How to design a true constant current source ?
« Reply #7 on: June 29, 2022, 11:00:55 am »
so can I have a mechanism that can adjust the voltage automatically if possible to keep the current constant.
 

Offline Terry Bites

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Re: How to design a true constant current source ?
« Reply #8 on: June 29, 2022, 11:04:08 am »
So you need acomplaince of 0.2*100R V. 20V. Thats not too hard to achieve with a supply voltage of say 24V, on 5V no way. Do you need unipolar or bipolar operation?
« Last Edit: June 29, 2022, 11:30:11 am by Terry Bites »
 

Offline ebastler

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Re: How to design a true constant current source ?
« Reply #9 on: June 29, 2022, 11:06:45 am »
so can I have a mechanism that can adjust the voltage automatically if possible to keep the current constant.

Sure. That‘s what every constant current source does.
 

Offline Zero999

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Re: How to design a true constant current source ?
« Reply #10 on: June 29, 2022, 11:07:09 am »
V = IR
I = 0.2
R = 100
V = 0.2*100 = 20V

You need a boost converter to generate >20V to power your current source.

The power consumption will be >4W, so your 5V power supply must be able to provide over 800mA, but 1A is probably more realistic.
 

Offline Arsh ahmad

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Re: How to design a true constant current source ?
« Reply #11 on: June 29, 2022, 11:19:04 am »
I am using 3A LDO as a 5v supply.
 

Offline joeqsmith

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Re: How to design a true constant current source ?
« Reply #12 on: June 29, 2022, 11:23:35 am »
I was curious to find out what a "true" CCS is.

I want to design a current source in which even if the load resistance goes up the current remains constant, I need an accurate load current of 200mA. The source should maintain the same current level even if the load resistance is increased.

I want one too.  For a true current source, it should output the same current independent from the load.  0-infinity.  Same current meaning just that, no change at all.   
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https://github.com/joeqsmith
 

Offline Terry Bites

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Re: How to design a true constant current source ?
« Reply #13 on: June 29, 2022, 11:40:06 am »
so you want a precise current source- well nothings perfect. What kind of precision do you want. See attached. You need only 3 precision resistors and low bias precsion Rail to Rail input opamps.

Or see how CERN do it, a modified version of the same. Note the use of a dc-dc converter to create the top supply rail.
 

Offline ebastler

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Re: How to design a true constant current source ?
« Reply #14 on: June 29, 2022, 12:01:07 pm »
Based on the type of their questions, I assume the OP does not actually need a high-precision current source, but just "any old constant current source that actually works, kind of".

@Arsh ahmad: Did you understand the explanations given so far?
  • Any current source will keep adjusting its output voltage as the connected load resistance changes, to keep the current constant, so that U(t) / R(t) = I_target. That's what a constant current source does.
  • But the current source can only adjust its input voltage downwards, so you need to supply an input voltage U_min which is high enough: U_min >= I_target * R_max.
  • If your original voltage supply (battery pack, or whatever) does not supply at least the U_min voltage, a step-up (boost) converter is required between the supply and the constant current source.
Do you have a goal for the required precision? By how much can the actual current change? E.g. is 200 mA +- 1 mA acceptable, or what other tolerance do you have?
 
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Offline Zero999

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Re: How to design a true constant current source ?
« Reply #15 on: June 29, 2022, 12:21:14 pm »
I am using 3A LDO as a 5v supply.
And what's powering the LDO regulator?
 

Offline eugene

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Re: How to design a true constant current source ?
« Reply #16 on: June 29, 2022, 01:54:33 pm »
I don't want to be rude, but I think the OP needs to take a little time to learn the basics of how CCS's work, what different types exist, etc. Then they might be able to ask meaningful questions (and understand meaningful answers) about how to design one just for their purpose.

Here's as good a place as any to start: https://circuitdigest.com/tutorial/current-regulators-construction-working-and-design-types
90% of quoted statistics are fictional
 
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Offline David Hess

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Re: How to design a true constant current source ?
« Reply #17 on: June 29, 2022, 11:21:10 pm »
No constant current generator is going to be able to put 200mA through a 100 ohm resistor when fed from 5V. See Ohms law!

There are constant output current versions of all of the constant output voltage switching regulator topologies.  Inductors are natural constant output current devices.  So it is possible with a switching topology.

Switching LED drivers are a modern example and might be a good place to look for ideas if high output voltage compliance is required from a lower voltage.


 

Offline andy3055

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Re: How to design a true constant current source ?
« Reply #18 on: June 30, 2022, 12:09:04 am »
The current through the load will depend on its resistance and the voltage that it sees.  The source cannot "force" whatever you want through the load. Ohm's law trumps your wants and needs!
 

Online IanB

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Re: How to design a true constant current source ?
« Reply #19 on: June 30, 2022, 12:22:23 am »
How about placing a 1 MV supply voltage in series with a 5 MΩ resistance?  >:D

That will deliver a constant 200 mA into most small load resistances, and will work fine in LTspice as long as you don't actually have to build it. The question only asked how to design one, not how to make one.
 
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Offline bdunham7

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Re: How to design a true constant current source ?
« Reply #20 on: June 30, 2022, 12:35:35 am »
I want to design a current source in which even if the load resistance goes up the current remains constant, I need an accurate load current of 200mA. The source should maintain the same current level even if the load resistance is increased.

How accurate and what compliance voltage?  The compliance voltage is your maximum resistance multiplied by your current, so if you need 200mA for any resistance up to 100 ohms, you need a compliance voltage of 20 volts. 

Assuming you don't need an extremely accurate current, a simple LM317 regulator plus a 6.25 ohm resistor powered by a 25VDC source would work.  Just realize that you need a good heat sink for the LM317 because it will dissipate some power with low resistance loads.

A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 
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Offline Arsh ahmad

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Re: How to design a true constant current source ?
« Reply #21 on: June 30, 2022, 04:49:38 am »
Yes, Think I can use a boost convertor to boost that 5v rail high enough so my current does not change.
 

Offline Arsh ahmad

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Re: How to design a true constant current source ?
« Reply #22 on: June 30, 2022, 04:50:35 am »
yes, The +- 1mA tolerance will be fine
 

Offline Arsh ahmad

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Re: How to design a true constant current source ?
« Reply #23 on: June 30, 2022, 05:10:17 am »
so you want a precise current source- well nothings perfect. What kind of precision do you want. See attached. You need only 3 precision resistors and low bias precsion Rail to Rail input opamps.

Or see how CERN do it, a modified version of the same. Note the use of a dc-dc converter to create the top supply rail.

Thanks, I added an activation circuit to activate it through a pulse and it works well
 

Offline Zero999

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Re: How to design a true constant current source ?
« Reply #24 on: June 30, 2022, 10:35:35 am »
Yes, Think I can use a boost convertor to boost that 5v rail high enough so my current does not change.
You didn't answer my previous question. The low drop-out regulator must be powered off a higher voltage. It makes more sense to boost that, rather than the 5V output.
 


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