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How con i limite a 20A of current to get 1.5A to power a circuit.

Silence19

New Member
If i have a battery of 3.7v 20000mah and i want to power a circuit with an ic with max current o 2.5A.
How can i limit the current to get 1.5A to power the ic with out frying it.
 

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danadak

Well-Known Member
Most Helpful Member
1669671072368.png

The 2N2905 not adequate Ic so select a larger device with some margin. The transistor
operated in linear region so a couple of watts, needs a heatsink. Pick one that not only
handles the current but has decent beta and leave enough Vce on the transistor to
support the beta.

This approach yields good compliance range for your relatively low battery V
application.

Different part :



Regards, Dana.
 
Last edited:

Diver300

Well-Known Member
Most Helpful Member
Also, it's a 20 Ah battery. Ah, or Amp-hour, is a measure of capacity, not maximum current, and there is no direct relation between the Ah rating of a battery and the maximum current, although higher Ah rated batteries often have higher maximum current ratings.

If you take 1.5 A from the battery, it should last for 20 / 1.5 = 13.33 hours.

It could be a good idea to put a suitable fuse, say 2 A, in series with the +ve of the battery. If all goes well, the fuse will do nothing. If things go wrong, the fuse could prevent further damage.
 

danadak

Well-Known Member
Most Helpful Member
Something simpler :

1669676483872.png


The Zener is represented by V2, could not find a 1.2V spice model, although
zeners that low exist.

Note Pdiss of power transistor, definite heat sink needed.

Lastly compliance of current source down to 1.5V so not too bad.


Regards, Dana.
 
Last edited:

Silence19

New Member
As long as the voltage is correct you don't need to worry that the power supply is capable of supplying more than 1.5 amps. The load will only take the current it requires providing that the power supply is capable of supplying 1.5 amps or more.

Les.
U mean, no matter the current supplied the circuit will draw the current it needs to power up only.
So what makes circuit or an ic burn out?
 

Pommie

Well-Known Member
Most Helpful Member
So what makes circuit or an ic burn out?
Too much voltage which will cause more current. If the voltage is correct then the device will take the current it needs. Like Dick said, a 10A socket (or whatever it is in your country) will power an LED lamp that needs just 0.03A to power it.

Mike.
Edit, check what voltage your circuit/IC needs.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
U mean, no matter the current supplied the circuit will draw the current it needs to power up only.
So what makes circuit or an ic burn out?
Yes; with the appropriate supply voltage, the device will be fine as long as it's not overloaded.
It's current available, rather than it being forced in to whatever is connected.
Voltage is like "pressure" that can force too high power, it's that which is critical.

Failures are typically from too high a voltage so it breaks down internally, or drawing too much current from (through) the device; either a large overload that blows the internals directly, or with less extreme overloads, heat building up until the device fails from excess temperature.
 

Silence19

New Member
Too much voltage which will cause more current. If the voltage is correct then the device will take the current it needs. Like Dick said, a 10A socket (or whatever it is in your country) will power an LED lamp that needs just 0.03A to power it.

Mike.
Edit, check what voltage your circuit/IC needs.
Thanks man
I understand it now, i am a beginner in electronics.
 

Silence19

New Member
Yes; with the appropriate supply voltage, the device will be fine as long as it's not overloaded.
It's current available, rather than it being forced in to whatever is connected.
Voltage is like "pressure" that can force too high power, it's that which is critical.

Failures are typically from too high a voltage so it breaks down internally, or drawing too much current from (through) the device; either a large overload that blows the internals directly, or with less extreme overloads, heat building up until the device fails from excess temperature.
Ok u mean if i drive 10A 10v through a circuit with a max of 2.5A 5v it will burn out.
But if i do drive it with 10A 4.5v through it, nothing will go wrong with the circuit right.
( should i say the current doesn't really matter, its the voltage going through it that matters right)
Like the more the voltage the more it push current through the circuit right?
And the less the voltage the less it drives current though the circuit.
Am i right or am still missing something?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Am i right or am still missing something?
Pretty much right.

The current rating of the supply does not matter, as long as it is high enough to run whatever is connected.

On the other side of things, if you eg. used an IC or transistor with a maximum rating of 2.5A and tried to use that to control a load (such as a motor or lamp) that takes 5A at the working voltage, then the IC or transistor is probably going to start leaking it's magic smoke!
 

Pommie

Well-Known Member
Most Helpful Member
To use the water analogy, voltage is pressure, current is how much can be supplied or is supplied. Turn a tap on enough and all the pressure will be gone (at the tap inlet) and lots of water will flow. As you turn it off, the pressure increases as the flow decreases. The tap is the same as a variable resistor or, in your case, your circuit (which is partially open). Not a very good analogy but hopefully you get the idea.

Mike.
 

Les Jones

Well-Known Member
Most Helpful Member
If you think about it another way then if there was nothing connected to the output of the power supply and it was still outputting 10 amps it would have to supply tens of thousands of volts so it could arc across it's output terminals as the only way to provide a path for the current.

Les.
 

danadak

Well-Known Member
Most Helpful Member
As long as the voltage is correct you don't need to worry that the power supply is capable of supplying more than 1.5 amps. The load will only take the current it requires providing that the power supply is capable of supplying 1.5 amps or more.

Les.

Current limits can protect both upstream circuits and downstream. A circuit can fail drawing
excess current or a short or an open or anything in between. A current limit can leave a
load still active while limiting its max allowed current, or it can make the downstream
load fail to operate due to voltage collapse. You can have fixed current limits like being
discussed (graph on left) in this thread, or even foldback current (graph on right) limits :
1669719298716.png


Graph on left notice when in current limit the load voltage can range from
Vconst to 0, depends on what current limit circuit and load are doing. The
current limit V vertical portion of graph can also have more of a slope to
it, depends on how its implemented.


Regards, Dana.
 

Silence19

New Member
Pretty much right.

The current rating of the supply does not matter, as long as it is high enough to run whatever is connected.

On the other side of things, if you eg. used an IC or transistor with a maximum rating of 2.5A and tried to use that to control a load (such as a motor or lamp) that takes 5A at the working voltage, then the IC or transistor is probably going to start leaking it's magic smoke!
Clearly understood it now.
Thanks man.❤
 

Silence19

New Member
To use the water analogy, voltage is pressure, current is how much can be supplied or is supplied. Turn a tap on enough and all the pressure will be gone (at the tap inlet) and lots of water will flow. As you turn it off, the pressure increases as the flow decreases. The tap is the same as a variable resistor or, in your case, your circuit (which is partially open). Not a very good analogy but hopefully you get the idea.

Mike.
Yea man just a beginner
I just tried to lay out the idea i had in mind.
But thanks man
 

Silence19

New Member
If you think about it another way then if there was nothing connected to the output of the power supply and it was still outputting 10 amps it would have to supply tens of thousands of volts so it could arc across it's output terminals as the only way to provide a path for the current.

Les.
Thanks man
 

Silence19

New Member
Current limits can protect both upstream circuits and downstream. A circuit can fail drawing
excess current or a short or an open or anything in between. A current limit can leave a
load still active while limiting its max allowed current, or it can make the downstream
load fail to operate due to voltage collapse. You can have fixed current limits like being
discussed (graph on left) in this thread, or even foldback current (graph on right) limits :
View attachment 139475

Graph on left notice when in current limit the load voltage can range from
Vconst to 0, depends on what current limit circuit and load are doing. The
current limit V vertical portion of graph can also have more of a slope to
it, depends on how its implemented.


Regards, Dana.
Thanks dana✌
 

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