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Circuit Element for variable current control

Blue Bomber Fan

Aug 23, 2017
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Hi everyone,

I am new to electronics and have little experience. I am working on a prototype and need some advice from you experts. I am needing to incorporate into my circuit an element that controls the current flow in real time. To be more specific, I have a device running with 2 sources in parallel; one source is variable and the other can be constant. Ideally, what I would like is to dial in a desired current into such a circuit element which I would need to constantly flow throughout. So if the variable source dissipates, the element will allow more current to be drawn from the more constant, reliable source to compensate and make that current whole again. What is such an element called, is it easy to buy, how expensive approximately, and do you recommend where I can buy one?

This is all DC btw.

Thank you so much in advance!
 

Alec_t

Jul 7, 2015
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Welcome to EP!
What are the sources? What voltage? What current?
 

Blue Bomber Fan

Aug 23, 2017
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Thanks! This is an LV application, and while I don't know exact details until it's built, I am expecting about 10 V and less than 5 A
 

Minder

Apr 24, 2015
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B.B.F., Must be Winnipeg'er !
For parts I use DigiKey, next day delivery.
M.
 

Minder

Apr 24, 2015
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In the general sense it is the load that decides the current, there is means of current regulators such as configuring a LM317 as a current limiter.
What kind of currents are we talking and the application type?
M.
 

Blue Bomber Fan

Aug 23, 2017
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In the general sense it is the load that decides the current, there is means of current regulators such as configuring a LM317 as a current limiter.
What kind of currents are we talking and the application type?
M.
The load operates at an optimal current, which I have yet to determine until I build it, but it will be probably 2-5 A. I would like the 2 sources I have to provide this current together (so I am connecting them in parallel). The issue is, the current that can be drawn from one of the sources will fluctuate, and so I need the other source to compensate for that to maintain a constant current at all times. So in theory I would connect this hypothetical circuit element in the circuit of the non-fluctuating source, and when the fluctuating source has a dip in current, the element can "open up" a bit more, allowing more current to be drawn from the non-fluctuating source to compensate and keep the current at the optimum level.

I apologize if this is a tad confusing. I would be happy to clarify further if this does not make sense! Thank you!
 

BobK

Jan 5, 2010
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It would clarify things if you would tell us what you are trying to do instead of how you think it should be done.

What are the two source of "current" are they actually current sources or are they voltage sources? (A voltage source tries to maintain it's voltage independent of the load, a current source will ajjust its voltage so that a specific current is drawn.)

You cannot simply parallel two voltage sources. All of the current will come from the higher one, and in fact it will push current in the wrong direction through the other one. I am not sure about paralleling current sources. In theory it should work, but there are probably practical problems.

Bob
 

Blue Bomber Fan

Aug 23, 2017
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It would clarify things if you would tell us what you are trying to do instead of how you think it should be done.

What are the two source of "current" are they actually current sources or are they voltage sources? (A voltage source tries to maintain it's voltage independent of the load, a current source will ajjust its voltage so that a specific current is drawn.)

You cannot simply parallel two voltage sources. All of the current will come from the higher one, and in fact it will push current in the wrong direction through the other one. I am not sure about paralleling current sources. In theory it should work, but there are probably practical problems.

Bob

Bob,

OK, let me try saying it this way: I am trying to run a load at constant current. For this, I have a voltage source (V1) that can provide just short of the current that is needed by said load. To add the current that is needed, I need to connect another voltage source (V2) to the circuit. V1 does not provide a constant current, and in fact I expect it to dissipate over time. Therefore, the current provided by V2 will need to increase so that the total current is exactly what is needed by the load.

Does that help a bit?
 

duke37

Jan 9, 2011
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Put a diode in the output of each voltage source so that the higher voltage one does not bully its little friend. Connect these diodes to a constant current circuit.
 

AnalogKid

Jun 10, 2015
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What are the output voltages of the two sources?

What is the voltage drop across the load when it has all the current it wants?

The difference between the source and load voltages is the headroom athe regulator or control circuit has to work with.

ak
 

kellys_eye

Jun 25, 2010
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Most 'loads' draw the current that they need and no more - paralleling supplies will increase the total current supply capability.

e.g if you have one power pack that delivers 1A and another that delivers 0.5A (both the same voltage) you can parallel them to get a maximum of 1.5A.

If your load requires only 1.2A then neither supply can do this individually but both together CAN but the load will still only 'take' 1.2A even if the source can supply 1.5A. You don't need to regulate the current - the load does that.

If you definitely want to control the current (limit) then you need a power supply that will do this - it achieves this by lowering the voltage as the set current level is exceeded. Such limiters can be made using the LM317.
 

BobK

Jan 5, 2010
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You will not tell us what the voltages are, ypu will not tell us what the current is. You are cagey about this voltage source that deteriorates over time. You will not tell us what the load is. How do you expect us to help you?

Bob
 

Blue Bomber Fan

Aug 23, 2017
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Put a diode in the output of each voltage source so that the higher voltage one does not bully its little friend. Connect these diodes to a constant current circuit.

Thank you, and yes, I was also thinking I will need to add a diode so that the sources do not work against each other and only contribute current to the load. However, neither voltage sources I am using are capable of delivering constant current, hence why I need a circuit element that will allow the total current across the load to be constant despite variable output from the two sources.

What are the output voltages of the two sources?

What is the voltage drop across the load when it has all the current it wants?

The difference between the source and load voltages is the headroom athe regulator or control circuit has to work with.

ak

Thank you for your response! As I mentioned, I am not exactly sure of the values of each component because they have not been built yet. However, I estimate that the voltage drop of the load will be around 10 V. "V1" (the fluctuating source) will also be around 10 V (but this will dissipate through the course of operation, as mentioned). "V2" is a standard variable DC power source capped at 20 VDC max.

I will need something that will increase the current draw from V2 to exactly compensate when V1 decreases, and decrease it when V1 increases, automatically (i.e. without having to adjust the knob on V2). I want an "active, automatic adjustor", if you will.

Most 'loads' draw the current that they need and no more - paralleling supplies will increase the total current supply capability.

e.g if you have one power pack that delivers 1A and another that delivers 0.5A (both the same voltage) you can parallel them to get a maximum of 1.5A.

If your load requires only 1.2A then neither supply can do this individually but both together CAN but the load will still only 'take' 1.2A even if the source can supply 1.5A. You don't need to regulate the current - the load does that.

If you definitely want to control the current (limit) then you need a power supply that will do this - it achieves this by lowering the voltage as the set current level is exceeded. Such limiters can be made using the LM317.

Thanks for your response! Ay, there's the rub, though! My load is not "constant" either. Too little voltage supplied and the current through it is not optimal, and too much and it degrades. I wish it was a little more simple :p

I will look more into LM317 and get back if I have questions. You said that it lowers the voltage if the current is exceeded, but does it also work the other way, automatically and in real time? Thanks!

You will not tell us what the voltages are, ypu will not tell us what the current is. You are cagey about this voltage source that deteriorates over time. You will not tell us what the load is. How do you expect us to help you?

Bob

Apologies for the vagueness, but I would rather not disclose exact details at this time, and if that sabotages my ability to get help on this board, so be it. However, I did disclose approximate voltages (~10 V) and current (<5 A) in the OP. If the type of information I am revealing is insufficient to get the help I need from you, I fully understand. Thank you for your replies, regardless.
 

kellys_eye

Jun 25, 2010
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If you have a single load and two supplies the current drawn depends on the load, not the supplies.

But if one supply is incapable of delivering the required current then two supplies (provided the total current supply by both, i.e. the sum) will 'self regulate' to provide the required current. Series diodes will prevent one supply from 'driving' the other.

This is akin to your car battery/alternator arrangement.

I think you may have a misunderstanding of the processes of supply/demand.
 

Blue Bomber Fan

Aug 23, 2017
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Something like this maybe?
View attachment 35834

Cool scheme, thank you! I must admit, I am a novice and have no training as an electrical engineer, so I am unable to read the "harder" parts of that. In general, the left side of it is what I am describing: 2 voltage source in parallel with a load (or in your scheme, a few loads), with diodes to direct the current flow to the load(s). The other parts I am unable to interpret.

If you have a single load and two supplies the current drawn depends on the load, not the supplies.

But if one supply is incapable of delivering the required current then two supplies (provided the total current supply by both, i.e. the sum) will 'self regulate' to provide the required current. Series diodes will prevent one supply from 'driving' the other.

This is akin to your car battery/alternator arrangement.

I think you may have a misunderstanding of the processes of supply/demand.

It is indeed possible that I have a misunderstanding! That's why I am consulting more knowledgeable people like you :)

I certainly understand and agree with the 2 sources with diodes/car charging analogy. It is the "self regulating" part that I am either confused about, or think that my load is a special case. For the load in my project, you can apply "any" total voltage and a resulting current will pass. There is a spectrum of current that the load will handle, from microamps to amps. However, I need it to pass a rather specific value of current (likely 2- 5 A, I would guess at this point), and I would like this value to be constant throughout operation despite the voltage in V1 dissipating over time, and thus needing V2 to be stepped up to exactly compensate.
 

Minder

Apr 24, 2015
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It would help to know the various supply source details as well as the nature of the actual loads you intend using.

(BTW I am in viewing distance of the Stadium!)
M.
 

Blue Bomber Fan

Aug 23, 2017
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It would help to know the various supply source details as well as the nature of the actual loads you intend using.
M.

I understand that I am (intentionally) vague about what exactly the circuit elements are, and I do apologize that it might make things more complicated to answer. I fully understand.

What I will say is that V1 is just a regular DC power supply, nothing special. I have incorporated it into the design simply as supplemental source to offset the power dissipated of V2 overtime. V2 is not "reliable", but in an ideal world, V2 would exactly power the load (as that's what I design it to do). 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. I have an optimal current I need it to run at, and for this I would to have a circuit element that assures the combined power from V1 and V2 is the exact I will need to achieve this. Does that make sense?

(BTW I am in viewing distance of the Stadium!)
M.

Cool!! I was in grad school at U of M while it was being built and had a perfect view from my desk of it as it happened!
 

Alec_t

Jul 7, 2015
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The other parts I am unable to interpret.
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.
 
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