# Current limit problem of battery charger.

#### Aplicant

Joined Feb 23, 2017
32
Hello to all
I am new here
I made smps battery charger based on the tl494
Output voltage:14.4
Max output power:200watt

Problem: When i put lower load than 5amps .charger works fine
But when i put higher load than 5amps (for example: load with 7amps)charger sounds noise chrrrrrrrr
Problem :when i put charger to 12v lead acid battery which has 70% charge , charger work fine because it gives lower current to battery
When i put charger to battery with 25-30% charge
It sounds chrrrrrrr
How to solve this problem
I want to charge 30% battery with 5a only
I dont need more current than 5amps

#### Dodgydave

Joined Jun 22, 2012
11,236
You need to lower the voltage on pin 15 of the internal comparator, your Ref voltage is set to 1v, which means you will be passing 10Amps before it limits, so lower it to 500mV with the resistors, that will give a maximum of 5amps.

I dont see your circuit using pins 1,2 to regulate the 14V output???

PS can you post a better circuit with component values...

#### Aplicant

Joined Feb 23, 2017
32
Thanks Dodgydave
My goal is current limit so i can give another circuit
This is

This circuit can give 5v/10a
Input :32v/10a
My question:
In figure 9 5v/10a=0.5ohm
If we put a load with 0.3ohm to this circuit,what happens? 5v/0.3ohm=16.66a
Can This circuit always give 10a to this load and does circuit work fine?
Although the load is higher or lower resistance it can give 10a?
My goal is :
This circuit should give 10a to lower resistive load
Despite load varying ,10a must be the same

#### Dodgydave

Joined Jun 22, 2012
11,236
No, your current limit is set by R13, and the Ref voltage on pin 15,

so your ref voltage is at 1V on pin15, which means 1V has to drop across R13 which is 10Amp, you need to put a variable resistor in place of R1, to lower the ref voltage, this will lower the current limit.

If you try to draw more than the limit set on pin15, the circuit will shut down.

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#### Aplicant

Joined Feb 23, 2017
32
No, your current limit is set by R13, and the Ref voltage on pin 15,

so your ref voltage is at 1V on pin15, which means 1V has to drop across R13 which is 10Amp, you need to put a variable resistor in place of R1, to lower the ref voltage, this will lower the current limit.

If you try to draw more than the limit set on pin15, the circuit will shut down.
Thanks for you useful infomation
You said My circuit is based on 10a current limit
Then my goal is constant current set

#### Dodgydave

Joined Jun 22, 2012
11,236
Then you need to redesign your circuit, you cannont have a Constant current source with a Fixed voltage!!

#### Aplicant

Joined Feb 23, 2017
32
How to design them: constant current
Help me

#### Dodgydave

Joined Jun 22, 2012
11,236
You will be better using a single op amp and pass transistor for this design, like this example, the Ref voltage fed in needs to be stable.

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#### Aplicant

Joined Feb 23, 2017
32
Thanks a lot
I will of course make as you said

#### Aplicant

Joined Feb 23, 2017
32

This is battery charger
Is there constant current system here?

#### Aplicant

Joined Feb 23, 2017
32
What system from Rsense up to feedback ic15pin

#### Dodgydave

Joined Jun 22, 2012
11,236
You wont be able to make a Constant current circuit using a TL494, its a switchmode psu chip.

Search for Constant current op amp circuits, using a mosfet transistor.

Why dont you start by telling us what type of battery want to charge...

#### Aplicant

Joined Feb 23, 2017
32

#### Dodgydave

Joined Jun 22, 2012
11,236
Lead acid batteries need a Constant Voltage of 14V, so your first circuit is fine, just use the current limit at 10amps or less.

#### Aplicant

Joined Feb 23, 2017
32
Thanks
Give me current limit circuit

#### Dodgydave

Joined Jun 22, 2012
11,236
Look at your post#3, and make R1 a variable 1k resistor...that will set the Current.

Make R9 a 4.7K variable resistor to set the Voltage.

#### Aplicant

Joined Feb 23, 2017
32
I=V/R
12A=12V/1R
Problem:how to give only 5a to 1ohm load in 12v
May be transistor system or other system in series to load
But i dont want series resistor to give 5a to 1 ohm load because it drops voltage

#### wayneh

Joined Sep 9, 2010
17,493
Not possible. If you place anything other than 5V across a 1Ω load, you will not get 5A. You started with Ohm's law – do you not believe it? Once you fix two of the parameters, the third is predetermined.

If you mean that you want to deliver 5V and therefore 5A, starting with a 12V supply, then what you want is a 12V-5V DC/DC adapter rated to 5A. Devices to use in a car to provide USB power are cheap and plentiful but I'm not so sure about getting 5A from most of them.

On eBay you can find inexpensive DC/DC buck converters. Search for one rated to well over 5A.

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#### bertus

Joined Apr 5, 2008
22,264
Hello,

5 Amps in a 1 Ohms load will be 5 A X 1 Ω = 5 Volts.
If you want it to be connected to 12 Volts, you will have to drop 7 Volts.
This can be done with a resistor or a transistor as current source.
In both cases there will be 7 watts dissipated in the voltage dropper.

Bertus

#### LesJones

Joined Jan 8, 2017
4,172
You have no option but to drop the voltage to 5 volts if you want 5 amps through a 1 ohm resistor. You could get the same average current or the same average power by using pulse width modulation. (Note the duty cycle would not be the same for the two conditions as the peak current would be 12 amps.) You could also use a switch mode regulator to drop the 12 volts down to 5 volts. We may be able to suggest a better solution if you tell us what you are trying to achieve.

Les.