How do constant current loads work and how to make one

[Alexsieg]

Hi, I'm not the best at understanding electronics, how does a constant current load work and how can i make one, and what makes them have limits before they get fried?

 

[AnalogKid]

There are several ways to make a constant current circuit, and searching for the term 'constant current schematic' will return many schematics. One way is to monitor the voltage across a resistor, and use an opamp to automatically adjust the conduction of a transistor such that the voltage is a constant value. A constant voltage across a fixed resistance means there is a constant current through the resistance.

Depending on the voltage across the resistance vs. the voltage across the transistor, you can distribute the power dissipation between the two components. After that it's an issue of thermal management. If the parts exceed their rated power dissipation, they burn up. If they exceed their rated operating temperature, they burn up. Heatsinks help, but do not guarantee survival.

ak

 

[Alexsieg]

Im not really sure how all that works, all I've really made with electronics is a power bank from a few laptop batteries and just mess around with stuff.

 

[hevans1944]

So, why do you want a "constant current" circuit? What kind of load do you want to attach to it? Do you use any meters or math when you mess around with stuff? Please tell us what you are trying to DO.

 

[Ratch]

Im not really sure how all that works, all I've really made with electronics is a power bank from a few laptop batteries and just mess around with stuff.

If you cannot understand the explanation of how to do it, and have minimum electronic construction skills, how do you expect to achieve the goal of building what you want. Perhaps a study of elementary electrical theory knowledge and hardware basics is in order first.

Ratch

 

[Miguel Lopez]

Depending on the value of constant current that you need, the LM317 is always a good choice (at least to me)

http://www.bristolwatch.com/ccs/LM317.htm

 

[Alexsieg]

So, why do you want a "constant current" circuit? What kind of load do you want to attach to it? Do you use any meters or math when you mess around with stuff? Please tell us what you are trying to DO.

I just want to make a constant current load instead of buying one, and how powerful could i make one without it being ridiculously expensive.

 

[Alexsieg]

If you cannot understand the explanation of how to do it, and have minimum electronic construction skills, how do you expect to achieve the goal of building what you want. Perhaps a study of elementary electrical theory knowledge and hardware basics is in order first.

Ratch

I know most of the basics of electronics i just don't know how to make circuits that to crazy things.

 

[duke37]

At a rough guess you should be able to make a 1000V 1A supply for under $10000. A bargain.

 

[(*steve*)]

Are you wanting to produce a load for testing batteries?

If so, what is the maximum and minimum voltage you would want applied to it, and the maximum and minimum current you'd like to set it to sink?

 

[hevans1944]

I bet I could do something for a dollar. My specs, of course.

 

[Herschel Peeler]

Hi, I'm not the best at understanding electronics, how does a constant current load work and how can i make one, and what makes them have limits before they get fried?

More specifics, please. Vague assistance is all we can give for vague requests.
What schematics have you found so far that might be close to your needs?

 

[Herschel Peeler]

Hi, I'm not the best at understanding electronics, how does a constant current load work and how can i make one, and what makes them have limits before they get fried?

Something like this? Basically you use a transistor as the load. You turn it on and into conduction just enough to be the resistance you need. Instead of driving the load it is the load.

 

[Miguel Lopez]

I bet I could do something for a dollar. My specs, of course.

I can do it cheaper. Of course, my own specs

 

[hevans1944]

I can do it cheaper. Of course, my own specs

Yeah, Miguel, no way I can compete with off-shore labor. I bet someone in Asia can make this for nothing, and make a profit on the volume sold.

 

[Alexsieg]

I only need it to go up to 25v and around 5 amps.

 

[(*steve*)]

Ok, so they are the maximum ratings, what is the minimum voltage across the input that should allow it to operate at maximum current? What are the typical current you'd want it to operate at -- especially the lowest?

 

[Alexsieg]

The lowest voltage i would want it to run would be 3v, and a external power supply to keep it running of course.

 

[(*steve*)]

And the smallest current? Would you want to be able to easily and accurately set it for 1mA, or would a couple of hundred mA be practically the smallest current you'd require?

 

[Alexsieg]

And the smallest current? Would you want to be able to easily and accurately set it for 1mA, or would a couple of hundred mA be practically the smallest current you'd require?

The lowest i think should be 200ma if not then 400ma