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# what is the easiest way to do this?

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#### david90

I need to design a circuit that can output 12V @ 3A max. The current can be controlled with a PWM signal. For example, 100% duty = 3 A and 50% duty cycle = 1.5 A.

Any the help would be appreciated!

Impossible without input voltage.

input voltage =12V

Do you need current regulator or voltage regulator? What is the range of input voltages or is it 12V regulated?

I think, David wants to design a constant current driver. If that's the case, then the following circuit should be able to work as a PWM-controlled constant current sink. It’s possible to make a similar circuit that will work as a constant current source.

By theway, David, what is this circuit for?

Kender

### david90

Points: 2
yes ,this is a good idea to fulfil your goal
two points you may take care of :
1) use R3/R4 as a voltage divider at the postive input of the OP
2) since the load current is 3A , so you can use a sense mosfet instead of a resistor directly sense the load current. the sense mosfet can use 1:N to save the power disspation

kender said:
I think, David wants to design a constant current driver. If that's the case, then the following circuit should be able to work as a PWM-controlled constant current sink. It’s possible to make a similar circuit that will work as a constant current source.

By theway, David, what is this circuit for?

Kender

This circuit will not work for me. I should of been more clear about my design question.

My uC circuit output a pwm signal with a duty cycle that is proportional to the RPM of a car. Currently I output the pwm signal to the gate of a mosfet and connect an array of LEDs in parallel (LOAD) between 12V and mosfet drain like in your schematic above.

What I would like to do is limit the current to load (~3A) for short circuit protection and also to get rid of the high wattage resistor that I currently have to limit current to my led array. Correct me if i'm wrong but the circuit above will not dim or brighten my LED array to the RPM signal.

I'm new to analog circuitry so any help would be great. Thanks!

+1 helped.[/code]

hello ,Kender. i have two questions:

first, how can it out 12V voltage, there is a positive Vds drop except Native MOS device .

second, i still can not understand the PWM controll, why it can give 3A when 100% duty but 1.5A when 50% duty, is there a very big capacitor at the output? otherwise when the gate voltage of power device is high ,there is no current pass the power device, who drive a 1.5A output current?

kender said:
I think, David wants to design a constant current driver. If that's the case, then the following circuit should be able to work as a PWM-controlled constant current sink. It’s possible to make a similar circuit that will work as a constant current source.

By theway, David, what is this circuit for?

Kender

david90 said:
What I would like to do is limit the current to load (~3A) for short circuit protection and also to get rid of the high wattage resistor that I currently have to limit current to my led array.
Using a fuse (resetable or single-blow) tends to be a standard approach to a short-circuit problem.

david90 said:
Correct me if i'm wrong but the circuit above will not dim or brighten my LED array to the RPM signal.
The circuit that I’ve posted will dim the LEDs to PWM duty cycle. First the RC averages the PWM and thus converts it to DC, whose votage is proportional to PWD duty cycle. The OpAmp, the current sense resistor and MOSFET are wired as a constant current source, whose current is proportional to the voltage on the positive input of the OpAmp.

PWMing the LEDs directly should do the job. 80/20 hindsight - you don’t need the circuit I posted.

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