# Constant 5v 1A charger

#### stempanerd

Joined Nov 28, 2015
3
Soooo... my professor wants us construct a charger that will output a constant 5v and 1A current no matter the load which I think is impossible considering Ohm's Law right?

so anyway, we tried to make the circuit based from the "Current Limited 6v Charger" from this the data sheet at page 26

We removed the 1 ohm from the circuit. Did we just technically just made the transistor useless?

When we tested the circuit to charge phone, the output current was just at 500mA. Any way to increase the current draw? We're thinking of placing a constant current configuration after the voltage regulator. Will it work and solve our problems?

Thanks!!

EDIT: We replaced the 1.1K ohm with 810 ohm to get around 5.6v Vout

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#### mcgyvr

Joined Oct 15, 2009
5,394
The 1 ohm resistor sets the current limit..
It has a function and you eliminated it..

#### stempanerd

Joined Nov 28, 2015
3
That's what I thought.. Well, we probably dont need the limit if the draw doesnt even reach 1A...

#### Alec_t

Joined Sep 17, 2013
11,817
my professor wants us construct a charger that will output a constant 5v and 1A current no matter the load which I think is impossible considering Ohm's Law right?
Right! Go back to your prof and ask if he means a 1A current limit.

#### Dodgydave

Joined Jun 22, 2012
9,876
You can have EITHER constant Voltage or constant Current, NOT BOTH at the same time.. !!!

Find out what your teacher wants....

#### dl324

Joined Mar 30, 2015
12,275
Soooo... my professor wants us construct a charger that will output a constant 5v and 1A current no matter the load which I think is impossible considering Ohm's Law right?
If this is homework, you're in the wrong forum.

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#### stempanerd

Joined Nov 28, 2015
3
Right! Go back to your prof and ask if he means a 1A current limit.
Just asked my prof and others, he meant it as the current limit. Though he did gave us a range for the output current of 500mA to 1000mA when charging phones. He wanted to have the charging current to be stabilized within the current range. So far, we achieved a charging current of around 500mA.

Currently, our circuit looks like attached. Any suggestions how to increase the charging current?
Also, The transistor is useless now right?

If this is homework, you're in the wrong forum.
Well, this is our project for our course so i thought it fits in this forum.. if not can anyone help me to move the thread to the right forum? Also, It's a good thing you removed your comment though I still read it. That kind of comment is extremely discouraging and it hurts to anyone CURRENTLY studying for that "field of study" which I think is the point of learning. I hope you won't say that to anyone else that needs help and wants to learn.

EDIT: Forgot to change Q1. Q1 is 2n2222

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#### dl324

Joined Mar 30, 2015
12,275
Well, this is our project for our course so i thought it fits in this forum.. if not can anyone help me to move the thread to the right forum?
I treat school projects that you're doing for a grade as being so similar to homework that there's no meaningful distinction. I know if I was an instructor, I'd want to grade my students on their work. Of course, it's always possible that you're one of the few students with integrity and would give proper attribution...
Also, It's a good thing you removed your comment though I still read it. That kind of comment is extremely discouraging and it hurts to anyone CURRENTLY studying for that "field of study" which I think is the point of learning. I hope you won't say that to anyone else that needs help and wants to learn.
After I re-read your original post, I decided it was probably an inability of your instructor to communicate expectations and you not knowing enough to ask a question; so I removed my somewhat snarky comment...

Take a closer look at how you have the transistor connected.

#### MrAl

Joined Jun 17, 2014
7,895
Hi,

You forgot the current sense resistor. That samples the current and turns the transistor on if needed to limit the current. It connects between the right side of the 100 ohm resistor and ground, where you have a short now.
Does that make sense to you? You still have to pick the resistor value and power rating, do you know how to do that?

Also note that just because your battery does not draw more than 500ma NOW, that does not mean that if it becomes run down more it will not draw the full 1 amp or more. The current limit is there for when the battery is very much discharged, so it does not draw a current which would make it get hot.

#### Colin55

Joined Aug 27, 2015
519
It all depends on two factors:

Can the transformer deliver more than 1 amp?

Will the regulator shut down if the current reaches about 1 amp due to thermal heating.