The circuit was working as logical voltage, only the low and high, and the Q2 as a switch not a resistor to do the voltage divider.

The circuit was used for the current source component, if the power is providing a voltage source, it could be damage the LEDs, you can see the circuit and tell me how do you decide the +V power and the Vf of LEDs?

Thank you. Thank you very much. You explain very clearly^^. I use a 1W led with 3.1~3.4V forward voltage so I use 5V supply

 

Thank you. Thank you very much. You explain very clearly^^. I use a 1W led with 3.1~3.4V forward voltage so I use 5V supply

That is not for you, you need some as these:
R_limit
= (5V - V_R2 - Vds - V_led) / I_led*80%
= (5V - 0.7V - 0.0945V - 3.1V) / 280mA
= 1.1V / 280mA
= 3.93Ω
If you can buy a 4Ω/5W then you can use it, otherwise You can using two 2Ω/3W or 5W in series.

The limiting resistor in series between s of mosfet and b of Q2 and R2.

The Imax of led is 350mA, for the led life, we just use 80% of 350mA that is equal to 280mA, all the parts on the circuit is not enough from the calculation, so we need to in series with a resistor to protect the LED, and the Vf of led are 3.1~3.4V, we choose 3.1V that it can be get more protection when the Vf=3.4V then it just reduce the current of led and it's no harm.

 

I do not understand this calculation. You would have 1.1V on the base of Q2. It will pull the MOSFET gate low long before reaching 1.1V, more like 0.7V.

I believe the calculation is much simpler. Using 0.7V for Vbe of Q2:

0.7 = 0.35 R2

R2 = 2Ohms to get 350mA.

I simulated this with LTSPICE and got 323mA with a 2 Ohm resistor, and 164mA with a 4 Ohm resistor. I also used a 10K resistor to the gate instead of 1M.

Bob

 

I do not understand this calculation. You would have 1.1V on the base of Q2. It will pull the MOSFET gate low long before reaching 1.1V, more like 0.7V.

I believe the calculation is much simpler. Using 0.7V for Vbe of Q2:

0.7 = 0.35 R2

R2 = 2Ohms to get 350mA.

I simulated this with LTSPICE and got 323mA with a 2 Ohm resistor, and 164mA with a 4 Ohm resistor. I also used a 10K resistor to the gate instead of 1M.

Bob

Sorry, but the 1.1V is not on the base, the limiting resistor should be in series with the LED.

 

R_limit
= (5V - V_R2 - Vds - V_led) / I_led*80%
= (5V - 0.7V - 0.0756V - 3.1V) / 280mA
= 1.1244V / 280mA
= 3.97Ω
If you can buy a 4Ω/5W then you can use it, otherwise You can using two 2Ω/3W or 5W in series.

The limiting resistor in series the LED.

 

Then you are not referring to the schematic in post #20?


Bob

 

Then you are not referring to the schematic in post #20?
Bob

The circuit on post #20 was shows that I had a different opinion with to3metalcan, that just according to the circuit to ask to3metalcan.

I said that circuit should be in series with a limiting resistor is better, but to3metalcan said that is ok, so I analyze the current and voltage to describe it, that why has the 1.1V.

 

Hi all,
I made the circuit using LM317 but there is a problem. When I try to connect the gate of the MOSFET to the ground, LED don't off, it remains dim. I use IRF840 instead IRF510. I read in the datasheet shows it's on characteristic, VGS from 2 to 4 VDC.
This is it's datasheet

http://pdf1.alldatasheet.com/datasheet-pdf/view/3045/MOTOROLA/IRF840.html

Can you help me? Thank you

 

Recheck your circuit and post the circuit you used.

Used IRF510 or 840 is not a good choice for a low V and I , because they are used for the high voltage devices or components.

NTE2987_Nch_100V20A_90Ω_Vgs5V_LogicLevel.
IRL540_Nch_100V28A_Vgs4V77mΩ.
IRL520N_Nch_100V10A_Vgs4V180mΩ.

 

Recheck your circuit and post the circuit you used.

Used IRF510 or 840 is not a good choice for a low V and I , because they are used for the high voltage devices or components.

NTE2987_Nch_100V20A_90Ω_Vgs5V_LogicLevel.
IRL540_Nch_100V28A_Vgs4V77mΩ.
IRL520N_Nch_100V10A_Vgs4V180mΩ.

My circuit is totally like this

the I checked the circuit many times. When I apply 5VDC through resistor to the gate of the mosfet, LED is very bright, the current is stable.
For you, which mosfet I should use? Thank you

 

My circuit is totally like this

the I checked the circuit many times. When I apply 5VDC through resistor to the gate of the mosfet, LED is very bright, the current is stable.
For you, which mosfet I should use? Thank you

I can't see your circuit.
For your application, you just choose the price of parts and which one you can get, lower Rds, the Id at least 2.5 times for power supply, over 5 times of current you need, Vg for logical level 4.5V.

 

I can't see your circuit.
For your application, you just choose the price of parts and which one you can get, lower Rds, the Id at least 2.5 times for power supply, over 5 times of current you need, Vg for logical level 4.5V.

Thank for your help^^

 

Hi all,
I get another problem. LM317 is very very very hot even when I used heatsink. I try decrease input voltage to 9V but it still very hot. Can you help me solve this problem? Or can you suggest for me a circuit to driver 1W led (350mA; Vf=3.2V)?
Thank you.

 

Hi all,
I get another problem. LM317 is very very very hot even when I used heatsink. I try decrease input voltage to 9V but it still very hot. Can you help me solve this problem? Or can you suggest for me a circuit to driver 1W led (350mA; Vf=3.2V)?
Thank you.

Measure the current draw - maybe you have a wiring mistake or a short circuit.

 

Hi all,
I get another problem. LM317 is very very very hot even when I used heatsink. I try decrease input voltage to 9V but it still very hot. Can you help me solve this problem? Or can you suggest for me a circuit to driver 1W led (350mA; Vf=3.2V)?
Thank you.

Does the LED has any heat sink?

 

I checked very carefully. I even made another circuit but result still same. I don't use heatsink for LED. Is it affect to LM317?

 

How many LED's are you driving?

 

Hi all,
I get another problem. LM317 is very very very hot even when I used heatsink. I try decrease input voltage to 9V but it still very hot. Can you help me solve this problem? Or can you suggest for me a circuit to driver 1W led (350mA; Vf=3.2V)?
Thank you.

Lowest dissipated power would be 2v above Vfwd so around 9v with the two lamps shown.

I think that LM317 is going to keep trying to provide the current even though you are trying to switch the load from the PWM so it's constantly swinging to VCC until it's cutoff by the IRF520.

 

I driver one Led. When I replace 9VDC and driver two LED, they dim, not enought 80% max current. I have a 12VDC source and I want to use it. Can you help me a satisfying circuit? Thank you

 

Attach your PCB or boreadboard.