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Transistor with resistance controlled by the amount of input power

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Abin76

New Member
Hi, I am modding my computer motherboard and I have a RGB led strip header. But the fact is it can only supply 1 Amp of current through the 4 pin header. I want to be able to use a variable resistor to control a 12v line straight form my power supply that can provide power for a bigger LED that uses more than 2 amps . But i want this variable resistor to be controlled by the amount of power supplied by the pins on the motherboard and not by dialing a knob manually.

The end result i want is that if I put an effect where the LED fades a red color back and forth, I want to be able to control a 12v line straight from my power supply connected to a 2 amp LED, but want it to have that effect which is controlled by the motherboard.

(This is for one color but i will duplicate it 3 times for each color so i only need help with one color.)

PLS help
 

MikeMl

Well-Known Member
Most Helpful Member
Welcome.

Would need to know the characteristics of the signal that comes out of the motherboard?
Is it PWM (pulse width modulated) or is it a steady voltage?
What peak voltage?
How much current can that signal drive?

Does your LED(s) have a constant current driver attached or internal, or is the current through the LED set by a resistor?
 

Abin76

New Member
Im almost certain that it is a steady DC source. Peak voltage is 12v as the pins are (12v)(G)(R)(B), the 4 pin header can drive 1 Amp max.
 

MikeMl

Well-Known Member
Most Helpful Member
So do you want to drive just one 2A LED, or three of them?
Can you you just parallel the RGB outputs (by setting each one to full output) to drive a single color LED?>
You didn't answer the question about the 2A LED. Does it have internal current-limiting?
 

Abin76

New Member
i want to run 3 leds of different color according to the pin color. yes i want to run 3 leds but the Leds are different color and i want the pins to match the color. I want the red pin to run the red 2A led. No i dont think i can parallel them. because the header supplies a maximum of 1 Amp and i think it only uses that much power when the leds are turned to white and set at full brightness.

No i would need to add a resistor to limit the current of the led.
 

MikeMl

Well-Known Member
Most Helpful Member
There is something wrong with your assumption that the motherboard output is a linear voltage source that ranges from 0V to 12V @ 1A maximum. Say you had a high-power LED that has a Vf of ~3.3V @ 1A (quite common). You would have to put a 7.5Ω 10W power resistor in series with the LED in order to limit the current to 1A when the motherboard output voltage is near 12V.

Look at this: it shows the V(f) across the LED, the current through the resistor and LED, and the power dissipated in the resistor as a function of increasing V2 (the assumed motherboard output voltage). Is that how a 1A LED would be connected (including the 7.5 Ohm 10W power resistor)?

d22.png

I am fairly sure that the RGB outputs from the motherboard are in fact PWM.
 

Abin76

New Member
Sorry i didnt understand what you meant really. I cant really read that graph as it is way too complicated for me, i am a beginner. I just wanted to know how i can get an led to behave the way it would if connected directly to the motherboard. but i want the power source to be from the power supply instead of the usual motherboard header all while being able to make the led do effects that a led connected to the motherboard could do normally.
 

ronsimpson

Well-Known Member
Most Helpful Member
I cant really read that graph
Bottom of graph is input voltage. 0 to 12V
Green trace shows voltage on the LED. From 0 to 2.8V there is no current. LED light at 2.8V and little current.
Red trace is current in R2, which is the same as LED current. 0 to 1.1A
Blue trace is power in LED. watts.

The graph shows when the supply is low there is no current.
The LED needs 2.8V to have any current and 3.8V at 1.1A.
 

MikeMl

Well-Known Member
Most Helpful Member
Abin76 ,
It is not possible to suggest how to do what you are asking without knowing exactly what the mother-board is putting out on the R,G, and B outputs. It will take looking at those signals with an oscilloscope to find out, or possibly looking at the schematic of the mother-board.
 

Dr_Doggy

Well-Known Member
I believe what you are looking for is called optical-isolation,
also a relay would work if it doesnt need to toggle on and off very much/fast

... Our question for you is what do these normal leds normally do, are they power and HD lights, do they flicker and flash?
 

Abin76

New Member
No they are normal 5050 led strips used for decoration or visual aesthetic. The motherboard has a connector, to connect a strip for lighting. The lights have effects that can chosen in the ASUS Aura software, like breathing, static, strobing, color shift....etc. But the header only supports up to 1 Amp of output power and it says that is about 2 meters of strip. I want to connect more so im thinking of a transistor as a switch idea so i can use a different power source that can supply more power. But then transistors are on or off, so the breathing effects and effects that fade into each other wont work. I want it so that it will work
 

dougy83

Well-Known Member
It would make the most sense if the output from the motherboard is PWM. To boost the current, you can just connect the motherboard output to the gate of a power N-channel MOSFET, 0V to its Source leg, and the LED strip negative to the MOSFET's drain pin. The positive terminal of the LED strip is then connected to 12V.
 

spec

Well-Known Member
Most Helpful Member
Hi Abin76,

As the other members have implied, we need to know how your computer presently controls the LEDS: linear or pulse width modulated (PWM), before we can provide a design for you to drive additional LEDs from an external power supply.

I guess you don't have an oscilloscope, which would be the best instrument to monitor the output drive from your computer so, instead, could you connect up the simple waveform sniffer shown in the schematic below. The circle with a 'V' in it is a voltmeter which can be a multimeter set to a voltage range above 15V but as close to 15V as possible.

Can you then do the following:
(1) ensure that your computer is switched off.
(2) connect the sniffer to the output terminals of your computer that drives the existing LEDs
(3) turn the computer on.
(4) set the computer to drive the exiting LEDS at a fixed brightness but not full brightness (half brightness would be good).
(5) Note any reading on the Voltmeter
(6) Vary the computer control of the LEDs and note any reading on the voltmeter

spec

2017_02_02_ISS1_ETO_WAVEFORM_SNIFFER_V1.png
 
Last edited:

audioguru

Well-Known Member
Most Helpful Member
5050 RGB LED strips have resistors on the strip to limit the current so that 12V can be applied. Some strips have the LEDs controlled by ICs mounted on the strips then the ICs are fed data from a computer and others have separate RGB plus +12V connections.
Which version do you have? You should provide a link to its datasheet.
 

Abin76

New Member
Sorry for the late responses and yes i found out that the 4 pin rgb header is pwm. so what shall i do now.
 

MikeMl

Well-Known Member
Most Helpful Member
Sorry for the late responses and yes i found out that the 4 pin rgb header is pwm. so what shall i do now.
Do you want to drive a new >2A LED in addition to the existing 1A led, or drive the big one instead of the smaller one?
Do you know if the existing LED has its own built-in current limiting?
Can you post a link to the high-power LEDs you want to drive?
We really, really need to know the pulse rate of the PWM signal?
 

Abin76

New Member
Sorry for the confusion but what I'm running is led strips. So my motherboard could only run 1A worth of LED on the header provided. That header can be assigned effects (fading, flashing, breathing) from software on the computer. I want to be able to run more led strips but there is that 1A limit on the motherboard. I want to be able to connect more led strips (basic 5050 RGB LED strips 12v) but still be able to have those lighting effects that the motherboard header provides. (I can connect the strips straight to power supply but then I won't be able to have or control the lighting like would be able to if I connected to motherboard header)
 

dougy83

Well-Known Member
Use a transistor to buffer the current, driven from the header. LED strips have built in current limiting (via series resistors), so it's simple. I detailed the connections for a mosfet in post #12, above.
 

Abin76

New Member
It's it possible that you can make a circuit diagram for me because I am a bit confused by the wording in number 12. I know what it means but the description of the connections are a bit confusing.
 

dougy83

Well-Known Member
I checked online for the pinout of the LED control headers, and it appears that they have 12V/G/R/B, and not what I was assuming prior. So the output is probably open-drain from the motherboard, so you can wire up a P-channel MOSFET as shown in the attached diagram
led buffer.jpg
 
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