# Circuit Element for variable current control

#### kellys_eye

Jun 25, 2010
6,502
As I mentioned, the load in this case will not dictate the current needed (I don't think?) because the load will accept any power and just run at a lower current.
Oh yes it will. Unless you deliberately limit the current then the load will dictate what the current is.

#### Blue Bomber Fan

Aug 23, 2017
13
M1 is a MOSFET. R2 is a low value resistor in series with M1 and the Load (here 1Ω) and develops a voltage proportional to the load current (which is common to all three). This voltage is compared, by opamp U1, to an adjustable reference voltage which is tapped off by potentiometer U3 across a 2.5V fixed reference voltage generator U2. The opamp output drives the gate of M1 and so adjusts the load current such that the two voltage inputs to the opamp are equal. Since the reference voltage is constant that means the load-current-derived voltage is also constant. M1 is your gizmo which allows more or less current flow as required to keep the load current constant.

Thanks Alec, that's a very helpful description (although humbly, still a tad over my head). I will certainly keep it for reference! Much appreciated!

Oh yes it will. Unless you deliberately limit the current then the load will dictate what the current is.

I understand where you're coming from. A voltage source can in theory pass "infinite" current with an infinitely low-R load (right?). I am definitely having some confusion for my application in particular, though.

To elaborate, the I-V relationship for my load is S-shaped. It passes negligible current at low applied voltages, then follows ohm's law in its "operating" zone, before leveling off at a max current irrespective of voltage. I want to operate at the top of the ohm's law region, and therefore must apply the corresponding voltage at this current. Therefore, I design my source ("V2") to do this. Picture V2 as a battery, where the voltage slowly decays as it is discharged. This must be compensated with a supplemental power source.

Perhaps I just answered my own question, and it is actually the voltage I need to maintain? So if that's the case, is there such a circuit element (device) that can maintain constant voltage? And in this case, perhaps I should be connecting these sources in series?

I am sorry that if I am not explaining or interpreting things correctly. I am learning.

#### Alec_t

Jul 7, 2015
3,584
Now that we have more info about the load it does indeed seem that a voltage regulator may be better suited to this project.

#### Blue Bomber Fan

Aug 23, 2017
13
Oh yes it will. Unless you deliberately limit the current then the load will dictate what the current is.

I thought more about your comment since my previous post, and I think it is now making more sense to me.

One point that may or may not be important that I want to mention is that I do not want to consume any more power than is required (as energy efficient as possible). So in my previous post, I mentioned how the I-V profile of my load is S-shaped, right? Therefore, if I were to set V1 (the wall-outlet source) to a very high value so that I can always max out on the current regardless of what V2 is doing, I would be consuming more power (the top of the "S", so I'm using a higher voltage and not getting any returns on current, P = IxV, so P is higher than needed). Whereas ideally, this voltage would adjust in real time so the total voltage delivered to the load from V1 and V2 is the voltage @ desired current.

Now that we have more info about the load it does indeed seem that a voltage regulator may be better suited to this project.

So assuming all I said is right above, yes, I would need a voltage regulator. What type of device does this, and where can I buy one? And is it "variable", i.e. can I "dial in" the voltage I'd like to be passing? Is it just a variable resistor I need?

Thanks again all!

#### kellys_eye

Jun 25, 2010
6,502
if I were to set V1 (the wall-outlet source) to a very high value so that I can always max out on the current regardless of what V2 is doing, I would be consuming more power (the top of the "S", so I'm using a higher voltage and not getting any returns on current, P = IxV, so P is higher than needed).
No, the power you consume is whatever the load is drawing - not what you set your power supply to. Where else would the power go?

I've read many posts from people who think they will blow their equipment up because they are using a 3A power supply when their equipment is stated to require only 2A. You can use a 1000A power supply and it won't do any damage - the equipment will take what it needs, no more, no less (unless there is a fault situation of course).

#### Blue Bomber Fan

Aug 23, 2017
13
No, the power you consume is whatever the load is drawing - not what you set your power supply to. Where else would the power go?

I've read many posts from people who think they will blow their equipment up because they are using a 3A power supply when their equipment is stated to require only 2A. You can use a 1000A power supply and it won't do any damage - the equipment will take what it needs, no more, no less (unless there is a fault situation of course).

I see what you're saying. However, even if the load is defining and drawing a set current regardless of the magnitude of the power supply, if the voltage is larger than required, the power consumed is higher than required because P = IxV (current isn't increasing, but voltage is). That is why I would like precise control of voltage.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
I am constantly amazed that beginners think they have stumbled onto an idea so special that they can't talk about it, that nobody else has ever thought of, all the while having an apparent lack of understanding of the most basic concepts.

You need to realise that you can only trust that any answers you are given are as good as the information you provide. If you are vague and evasive, the "solutions" might be completely wrong.

It's not rare that we close threads like this. It's a bit like a game of 20 questions where we ask the questions and you don't answer them.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510

Please be as detailed as possible.

#### Blue Bomber Fan

Aug 23, 2017
13
I am constantly amazed that beginners think they have stumbled onto an idea so special that they can't talk about it, that nobody else has ever thought of, all the while having an apparent lack of understanding of the most basic concepts.

You need to realise that you can only trust that any answers you are given are as good as the information you provide. If you are vague and evasive, the "solutions" might be completely wrong.

It's not rare that we close threads like this. It's a bit like a game of 20 questions where we ask the questions and you don't answer them.

Hi Steve,

I apologize if this thread is being a distraction to your contributors. I have been quite clear that I will not be clear on details, though, and I will maintain that. I was hoping perhaps my question could be treated like a "word problem" (like on a test) where specifics are often omitted because the scenario is portrayed as hypothetical, yet it is solvable. I know you might say that in order to answer many word problems, more details are needed, and I suppose so. I also suspected that the answer I needed would have been simple but I lacked the background knowledge and could not find it online, so I asked here. That is perhaps not the case.

If my conditions do not meet the terms of your website, by all means, close the thread. I understand.

I would like to add that while I am a "beginner" when it comes to electronics and engineering, the project I am pursuing requires this only as an auxiliary item, and the primary workings relate to a field in which I am an expert. Therefore, I actually have a high level of understanding of the novelty and potential impact.

Thanks everyone for your help! I can turn elsewhere.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
And your response is also common.

"I'm an expert in field A, so I must be trusted to understand the electrical/electronic requirements of some device needed/used in this domain. I will be intentionally vague and it's your fault if you can't be specific. My idea is so special and secret that there will be people lining up to steal it (but not special enough for me to pay someone to sign an NDA and propose a real solution)."
I realised that you're different (is that what you'll say?) but threads of this nature are all too common.

#### Blue Bomber Fan

Aug 23, 2017
13
No need to be cheeky. I was never dishonest or deceiving. I also never claimed to think I am special, but I did take exception to you suggesting that I think my idea is special in spite of being a beginner, and so I defended that.

Bravado over the internet is also all-too common.

I will leave your forum alone now.

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Not to be "cheeky" but I do believe that the natural order of life on Earth does not include the student dictating the rules to the teacher. Perhaps an illogical relationship like that can be found on Instructables.com. That place is loaded with extraterrestrials and chuck full of "vague", so Earthly logic doesn't apply.

Chris

#### kellys_eye

Jun 25, 2010
6,502
if the voltage is larger than required, the power consumed is higher than required because P = IxV (current isn't increasing, but voltage is). That is why I would like precise control of voltage.
You cannot 'add' two voltage sources (supplies) though. One or the other will take control. Fitting two diodes to prevent 'feedback' will only make one or the other supply 'pass'.

You can add current but only if both supplies are equal in voltage - again, if the supplies aren't equal, then the higher of the two will take control.

I'm with Steve on this now - you obviously think you've stumbled onto the holy grail of 'something' and are reluctant to reveal what this is. But if your understanding of basic volts/amps is so limited then you cannot, by simple definition, have the comprehension to create that holy grail in the first place.

If you're not prepared to reveal your idea so we can offer some constructive advice then I'm all for the thread being closed as a waste of time.

#### Alec_t

Jul 7, 2015
3,584
If your load is variable and not under your control then you can make a constant current flow through it OR you can apply a constant voltage across it. You can't do both at the same time.

#### BobK

Jan 5, 2010
7,682
My guess is an HHO generator. The people who promote this fantasy are often confused about the relationsip of voltage, current, and power.

Bob

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