I draw the circuit for driving Strings of LEDs. I have regulated DC supply of 17.5V @ 1A. I didn't choose constant current supply because it was adding extra cost to the whole project, therefore for driving LEDs I am using Constant Current source driver based on N channel MOSFET and NPN Transistor with some resistors, it's cheap and cost effective solution, as I have to keep the cost of project as low as possible.
I visited many website and tutorials for how it works and have this circuit made, I am still not sure about the circuit :-
Here is the Circuit to Drive 20 LEDs using ESP8266 SoC Module.
Component Specifications :-
White LED(SMD) - Package 2835
- Vf = 3.1-3.2V
- If = 60mA(Typ), 90mA(Max), Pulse current = (150mA)
Q1 (N-channel MOSFET) - Package SOT-23
- IDS: 5.8A
- VGS: ±12V
- VDS: 30V
- VGS(th) = 1.05V
- RDS(ON) (at VGS=10V) < 28mΩ
- RDS(ON) (at VGS = 4.5V) < 33mΩ
RDS(ON) (at VGS = 2.5V) < 52mΩ
Some Other info :-
Link to Datasheet :- http://www.aosmd.com/pdfs/datasheet/ao3400.pdf
MMBT3904 NPN Transistor
Link to Datasheet :- http://www.onsemi.com/pub/Collateral/MMBT3904LT1-D.PDF
Some Calculations :-
- The Needed Current to fully glow all 5Series 4Parallel LEDs =
- Total voltage drop across LEDs =
- Input Voltage =
17.5V @ 1A
- The Vbe of NPN =
The Current though MOSFET and LED will be defined by R5 and R6 resistors. Therefore
R5 = R6 = 0.7/If(LED) R5 = R6 = 0.7/0.45 ohm. (450 mA taken, extra 100 mA as buffer) R5 = R6 = 1.5 ohm
Power Dissipation at R5 and R6 :-
Ps = 0.49/1.5 W Ps = 326 mW
Voltage Drop Across MOSFET:-
Vm = Vs - Vf(LED) - Vbe Vm = 17.5 - 16 - 0.7 Vm = 0.8 V
Power Dissipation on the MOSFET :-
Pm = Vm * If (LED) Pm = 0.8 * 0.45 Pm = 360 mW
Reference PCB board with Aluminium Heatsink on the backside :-
- Is circuit appropriate to drive all 20 Leds with above calculations?
- Any thermal run away issue with the circuit?
- Do I need limiting resistance on the LED side?
- What does Pulse current refer to in LEDs?
- What is the use of Resistance R21 & R7 in circuit?
The Vf vs Junction temperature Graph :-
The Vf vs If Graph :-