trying to find out if either design is correct or better than the other.
included eagle schematic
the gate is controlled differently on MOSFET between the 2 diagrams
Where did you get these schematics ?
What are you trying to accomplish ?
The one on the left will not work, it just has a gate and base connected together, that is going to do nothing.
The one on the right is attempting to do a constant current drive and the FET will get very hot.
Both circuits have lots of raw LEDs in parallel. That is not a good idea at all.
Put another way, whoever drew these schematics needs to do more research.
Case in point.
10 leds in parallel with Vcc of +12Vdc and a single current limiting resistor of 4.7 ohms (3W) : I = 12/4.7=2.55A total
2.55A total / 10 leds = 0.255A PER LED
0.255A PER LED ==> EACH OF THE "20mA" LEDS WILL DRAW 255ma @ 12v WITH ONLY 4.7 ohm current limiting resistor.
If you take ten 220 ohm resistors (one for each led) and put ALL TEN 220 ohm RESISTORS IN PARALLEL just like the LEDS. the total resistance would be 22 ohms , almost 5 times the 4.7 ohms shown in the schematic.
How long do you think a 20 mA led would last with 255mA going through it ?
Raschemmel,
I think you are not understanding that circuit. The 4R7 resistor is not a current limiting resistor but a current sensing resistor. The current limiting is done by the FET running in a linear mode.
How do you calculate the total constant current?
unfortunately, i got from various web searches for constant current drivers and laid-out in eagle so i could start making a couple boards if they worked.
i am trying to design strip lights for various function control and multiple strips for another project im wanting to start. i want paralleled so if one led burns, they all don't stop working.
the strips may have more leds per strip(s) also, so i know i will need to adjust resistor value and wattage.
well, i have tried both circuits last night but the right side with 5V input, from buck/boost board, worked with leds in parallel but not series and a 100K pot for dimming. so i guess i will modify as needed.
10 led used - 3.09v across leds with .27mA draw. no heat off either tip122 or irfz44n.
do i need to re-configure the right design for better connection and/or performance. i will be trying to use tiny85 for dimming, pulsing and other functions. do i need an addition resistor between emitter and base for assured "off" state?
thank you all for comments and input.
thank you in advance.
Wiring LEDs in seriese requires a voltage in excess of the total of all the forward voltages. Wiring them in parallel requires a resistor on each LED even if you have a constant current source.
However normal LED strips have resistors built in and run from a constant 12V.
thank you for input.
i tried circuit figuring "what the hell".
i used the right circuit with each components listed minus the pot. i am using a 328P PWM to control circuit.
i had total of 60 led's in parallel. measurements with a FLUKE DMM, not a cheap one.
total Vf across led bank = 3.12V
total If = .34mA
i ran a LM386 input for 2 hours in garage on work table constantly checking for heat. i did not even get warm on any parts.
not saying that it will always work but i think 2 hours is a good test run, especially in a garage in summer weather in Houston, TX.
i attached a picture of the PCB i made and the eagle files if anyone is interested in what i did.
constant current driver 2.sch (239 KB)
constant current driver 2.brd (63 KB)
total If = .34mA
I think you mean 340mA or 0.34A, You cannot possibly mean 340uA for 60 leds.
340mA for 60 leds is only 5.7mA each which is very small.
I measured the same amount of current (5.6mA) using a 560 ohm resistor with a red led running on 5V.
I thought it would be dim but actually it was quite a normal brightness. Of course it was not as bright as all the other leds that had 220 ohm resistors but it was certainly bright enough that if no one told you , you wouldn't notice anything unusual about it .
How did you measure the total current ?
my bad, it is .34A. thanx for correction. (that's why i respect you guys so much)
i used my fluke wired in series with red lead connected to output from board and black lead to led positive feed.
Maybe I am misunderstanding you but if all 60 leds were in parallel , to measure the total current, you would have to put your meter in series with the power source, not a led , right ?(meaning in series with the power source and the 60 leds)
thanx for correction. (that's why i respect you guys so much)
Thank you . It's nice to be appreciated. Some people don't like us correcting them . They think we are "splitting hairs", but obviously you appreciate attention to detail.