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How to split high current low voltage? Voltage regulators , current regulators(?)

I am using a 20v dc laptop charger that supplies 4 amps. I need 5v dc 4 amps. When I use a linear voltage regulator it’s too much current to pass through safely.. 15 amps change and 4 amps, so I use a buck converter. After converting with buck converter I have a whopping 16 amps available.. and for one thing my buck converter cannot sustain this voltage and I’d also like to reduce the current to what I need, 4-5 amps. From where I’ve read there are many solutions available for splitting voltage and regulating voltage, is there a solution for high current?

Also is it more practical to limit the current before the voltage is converted? How can that be done?

I suppose if I limited the current so that when it changes to 5v it could be the perfect amperage, that’s works better.
I’ve seen current limiters for small current values and also using resistors to limit current or voltage but not on this scale.

EDIT:

this post covers most of the information that may be needed to answer this question Choosing power supply, how to get the voltage and current ratings?? as suggested by @The Photon

the remaining issue i have here is with this example in mind , explained by @stevenvh “Olin explained with fruit, I’ll try to explain with another flow: water. If my faucet can fill a bucket in a minute that’s its maximum flow, or current. That’s the 1000 mA the wall wart specifies. Yet I can open the faucet partly to fill my glass, and then I’m the 500 mA device. The faucet still can supply 10 liter per minute, but will supply less if I ask for less. I can ask for anything as long as it isn’t more than the 10 liter per minute. If I want only a few drops to fill a thimble I’ll be a 1 mA device. From a 1000 mA supply.”

with my setup my faucet(the buck converter) is rated for 10 liters per minute and my pump(laptop power supply) can do 16 liters per minute.. so i am seeking a way to limit the amount of flow, also similar to how a circuit breaker functions. (probably using a fuse)
so the idea is get a bigger faucet or a smaller pump or limit it in between.
i can make it so that my valve only open to 5A

To explain this further i am looking to limit the amount of current that is available so it doesn’t reach 16A output. With a 2A wallwart that plugs into a 15A wall socket , the current limiting is set within the wallwart rather than within the wall socket. The wall socket can provide 15A so that it’s available to more devices and a wallwart is designed so it has current protections limiting it to a 2A draw.

How can i limit the current requested by the buck converter to 5A?

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