The schematic on the left is for an RGB light kit that I want to install in a car to replace the stock LED. As shown the RGB LEDs have common anode that is connected to 3.0 V, and there is a micro-controller that controls three N-channel MOSFET transistors (BL2300) to set the color and brightness. (Correction to the schematic, the red and green leds each has a 15 ohm resistor in series not 150 ohm, while the blue LED has a 10 ohm resistor)
The car runs the stock LED module through 12 V PWM signal to control the brightness and to softly turn on and off the light. My goal is to integrate the car's brightness control with the replacement light to maintain the same effect.
So I am thinking the best way to do that is by adding a transistor to control the 3 V input voltage with the PWM signal from the 12 V line. (Schematic on the right)
Can I use BJT transistor? I have a lot of NPN and PNP waiting to be used, or a MOSFET is needed/better? (The LED is small and don't use much current)
What size and how many resistors do I need? And will the 12 V at the base/gate have an effect on the voltage/current going to the LEDs? Do I need a voltage divider?
Will this added transistor cause voltage drop that will impact the brightness? If yes, by how much?
Update: (I also corrected the schematic since the 4.7k ohm is a pull down resistor and not the gate resistor)
Based on the information given below and my understanding I am going to use a logic level N-Channel transistor with 100 ohm resistor at the gate. My signal is at 12 V and Vds is 3 V which means Vgs will be about 9 V.
The transistor is going to be either AO3400 or RJK0451DPB (since I have them both)
- Has a maximum Vgs rating of 12V (at the limits!)
- Gate Threshold Voltage (maximum) 1.45V
- Ron a bit high (maximum 33 mOhm at Vgs=4.5V)
- Very fast (td(on)=3ns, td(off)=25ns, tr=2.5ns, tf=4ns)
- Has a maximum Vgs rating of 20V which is good.
- Gate Cutoff Voltage (maximum) 2.5V (This is cutoff not threshold)
- Ron is low (maximum 9.6 mOhm at Vgs=4.5V)
- Fast (but slower than AO3400) (td(on)=13ns, td(off)=48ns, tr=4.8ns, tf=6ns)
- Syncing the two PWM is too hard so I will try without.
- Using P-Channel will required inverting the signal, which means more components. N-Channels seems good for my case.
Datasheets for the transistors: