MosFET Location in circut?

Discussion in 'General Electronics Chat' started by LeRadelle, Sep 26, 2014.

  1. LeRadelle

    Thread Starter New Member

    Sep 9, 2014
    21
    0
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,676
    899
    Between the LED's and ground would be the most common. However, you could put the mosfet in the positive supply line, but then you would need a different driver. I hope this drawing is clear. I am not very adept with Paint.

    Capture.PNG

    John
     
  3. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,004
    1,525
    Wouldn't that in reality be in position "B"? Since the 'black' wire is shown as the common/ground, from the constant current/constant voltage source, as shown. I'm assuming that is what is meant by "cc/cv" in the original picture.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    Yeah, the line to a separate "ground" should just be eliminated. The return to the power supply is enough, and most would refer to it as the ground of this circuit.
     
  5. LeRadelle

    Thread Starter New Member

    Sep 9, 2014
    21
    0
    Yes RED wire is + and BLACK is ground, CC is constant current mode and CV is constant voltage this circuit will work in CC mode since we are talking about LED driver so you think that place B is the most common place to put an N-ch MosFET to act as a switch?
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,234
    You need to be careful if you put it at B. When it is off the output of the CC source will go to maximum voltage. When the switch is closed it may take some time to reduce that voltage damaging the LEDs.
     
    LeRadelle and wayneh like this.
  7. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    Yup, the standard MOSFET switch is an N-channel controlling the return to ground. It's usually the last thing in the trip to ground, so that the voltage on the gate is as high as possible above the source (which is normally at ground). For instance if there was a 3V LED after it, then the gate-source voltage would be reduced by that 3V. That wouldn't always matter, but it's bad form and would definitely fail at lower supply voltages.

    ronv raises an important point. What is your constant current supply?
     
  8. LeRadelle

    Thread Starter New Member

    Sep 9, 2014
    21
    0
    That was my thought too, but i need someone to confirm it for me, thank you! :)

    I will be using power supply 15 or 18 volts, 10 amps,as MosFET i will be using 13 x IRL510 controlled by ATmega16 that will power 7 to 15 strings of 6LEDs in series, each mosfet will drive currents from 140-500 mA depends on how many strings of LEDs are connected to that mosfet.LEDs are red and blue, forward voltage 2.1 to 2.35V for red and 3.2 to 3.5 for blue, blue leds will be connected 4 in series but that is not important now.CC/CV i will buy from ebay for 5$ each specs say it buck cc/cv here is the link so you can see.

    So place A(between power source and CC module) is more safe for mosfet to be placed?
     
    Last edited: Sep 26, 2014
  9. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,234
    Yes. Towards the end of the "how to" it says "if output is unloaded do not connect LED."
     
  10. LeRadelle

    Thread Starter New Member

    Sep 9, 2014
    21
    0
    I dont see the point in "output is unloaded do not connect LED". I think its just for step 1 & 2 till i configure the CC module?!?
     
  11. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,234
    Yes, you are right. But still a bad idea to switch the output.
     
  12. LeRadelle

    Thread Starter New Member

    Sep 9, 2014
    21
    0
    Any solutions? Would constant voltage change the situation? I mean if CC is not CC jet its replaced with CV of constant value of 12.7V for whole system...typ forward voltage is 2.1V for current 20mA so 6x2.1V is 12.6V for red(Nichia NSPR510GS) LEDs and 4x3.2V is 12.8V for blue(Nichia NSPB500AS) LEDs so i put 12.7V as middle for both colors?How does that sound, ive been watching the current vs temperature factor for both type of leds its safe to drive them on 20-25mA till 65-70 °C i will have maximum of 50-60°C....will it make difference for mosfet if i use CV instead CC? for CV i would use something like this.
     
    Last edited: Sep 28, 2014
  13. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,234
    Switch the input to the CC source (B). Then the voltage will come up slowly.
     
  14. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    Do we know what the switching frequency is? I'm assuming infrequent, not more than 1Hz?

    I'm wondering if the CC source can tolerate the switching and can keep up.
     
  15. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,234
    Oh, I see your concern. Maybe he is trying to make a dimmer?
     
  16. LeRadelle

    Thread Starter New Member

    Sep 9, 2014
    21
    0
    Switching frequency will be 1Hz or less.
     
  17. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,234
    Yes, so basically an on / off switch. So put it at A.
     
  18. LeRadelle

    Thread Starter New Member

    Sep 9, 2014
    21
    0
    Yes it will act as on/off switch...will that position of mosfet make problems for CC module or it should work fine with frequency 1Hz or less?
     
  19. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,234
    It should work fine. Just like turning it on and off.
     
  20. LeRadelle

    Thread Starter New Member

    Sep 9, 2014
    21
    0
    Since i will use 13 CC modules connected to power supply, is it good to put between power source and CC module something like this to control voltage and current?
     
Loading...