How to design a true constant current source ? - Page 1
Forum section: electronics / projects

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: June 29, 2022, 10:00:43 am
content: 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.

author: perdrix
author_rank: Frequent Contributor
timestamp: June 29, 2022, 10:08:12 am
content: 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.

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: June 29, 2022, 10:17:36 am
content: 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.

author: Codemonkey
author_rank: Regular Contributor
timestamp: June 29, 2022, 10:32:20 am
content: Is your supply voltage high enough ?

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: June 29, 2022, 10:36:54 am
content: I need one to be operated at about 5 volts and one more that need to operate at about 20-40 volts.

author: RogerThat
author_rank: Regular Contributor
timestamp: June 29, 2022, 10:39:39 am
content: This:https://www.researchgate.net/figure/Circuit-diagram-of-the-constant-current-source_fig4_45899961The 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.

author: Codemonkey
author_rank: Regular Contributor
timestamp: June 29, 2022, 10:52:18 am
content: No constant current generator is going to be able to put 200mA through a 100 ohm resistor when fed from 5V. See Ohms law!

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: June 29, 2022, 11:00:55 am
content: so can I have a mechanism that can adjust the voltage automatically if possible to keep the current constant.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 29, 2022, 11:04:08 am
content: 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?

author: ebastler
author_rank: Super Contributor
timestamp: June 29, 2022, 11:06:45 am
content: Quote from: Arsh ahmad on June 29, 2022, 11:00:55 amSure. That‘s whateveryconstant current source does.

author: Zero999
author_rank: Super Contributor
timestamp: June 29, 2022, 11:07:09 am
content: V = IRI = 0.2R = 100V = 0.2*100 = 20VYou 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.

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: June 29, 2022, 11:19:04 am
content: I am using 3A LDO as a 5v supply.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 29, 2022, 11:23:35 am
content: I was curious to find out what a "true" CCS is.Quote from: Arsh ahmad on June 29, 2022, 10:00:43 amI 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.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 29, 2022, 11:40:06 am
content: 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.

author: ebastler
author_rank: Super Contributor
timestamp: June 29, 2022, 12:01:07 pm
content: 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?Anycurrent 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 voltagedownwards, 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?

author: Zero999
author_rank: Super Contributor
timestamp: June 29, 2022, 12:21:14 pm
content: Quote from: Arsh ahmad on June 29, 2022, 11:19:04 amAnd what's powering the LDO regulator?

author: eugene
author_rank: Frequent Contributor
timestamp: June 29, 2022, 01:54:33 pm
content: 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

author: David Hess
author_rank: Super Contributor
timestamp: June 29, 2022, 11:21:10 pm
content: Quote from: Codemonkey on June 29, 2022, 10:52:18 amThere 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.

author: andy3055
author_rank: Super Contributor
timestamp: June 30, 2022, 12:09:04 am
content: 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!

author: IanB
author_rank: Super Contributor
timestamp: June 30, 2022, 12:22:23 am
content: How about placing a 1 MV supply voltage in series with a 5 MΩ resistance?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.

author: bdunham7
author_rank: Super Contributor
timestamp: June 30, 2022, 12:35:35 am
content: Quote from: Arsh ahmad on June 29, 2022, 10:00:43 amHow 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.

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: June 30, 2022, 04:49:38 am
content: Yes, Think I can use a boost convertor to boost that 5v rail high enough so my current does not change.

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: June 30, 2022, 04:50:35 am
content: yes, The +- 1mA tolerance will be fine

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: June 30, 2022, 05:10:17 am
content: Quote from: Terry Bites on June 29, 2022, 11:40:06 amThanks, I added an activation circuit to activate it through a pulse and it works well

author: Zero999
author_rank: Super Contributor
timestamp: June 30, 2022, 10:35:35 am
content: Quote from: Arsh ahmad on June 30, 2022, 04:49:38 amYou didn't answer myprevious 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.