Using 2 Mosfet to Switch 2 Different voltage

Discussion in 'General Electronics Chat' started by ghild, Aug 20, 2014.

  1. ghild

    Thread Starter New Member

    Aug 20, 2014
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    Guys I really need your help,
    I am New herem and I am from Indonesia

    I want supply Stepper Motor Driver with 12v and 24v , because there is case when I want to use 12v and there is other case when I want to use 24v.

    I want using 2 mosfet as switch,
    http://www.rohm.com/web/global/products/-/product/RSR030N06

    1st mosfet as 12v switch
    2nd mosfet as 24v switch

    I really confuse about this, I have tried simulation a lot in Proteus, but not working properly.

    please help me for properly circuit
    thank you in advance
     
  2. ghild

    Thread Starter New Member

    Aug 20, 2014
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    here I attach the schematic
     
  3. ScottWang

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    Aug 23, 2012
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    The Zd1 was used to limiting the Vg, R2, D1 was used to turn off Q2, when /Q1-1 is high, R7, D2 was used to turn off Q4, when Q1-1 is high, they were designed to avoid /Q1-1 and Q1-1 become high at the same time.

    If you don't want this function, then you just take them away and using 4.7K for R1 and R6.

    [​IMG]
     
    Last edited: Aug 20, 2014
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Correct me if I'm wrong, but:
    1) When FET1 is on, won't 24V at VM1 drive current through the body diode of FET2 and fry it?
    2) If Q1-1 and /Q1-1 are both high then Q2 and Q4 are both on and both FETs are on, shorting the 12V and 24V supplies :(.
     
  5. ScottWang

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    Aug 23, 2012
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    Thank you Alec_t.
    You are right, I forgot to add two Schottky diodes, if you find some other errors, please let me know, thanks.

    [​IMG]
     
  6. ghild

    Thread Starter New Member

    Aug 20, 2014
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    Thank you ScottWang :)
    you are so kind..

    I think Q1-1 and /Q1-1 never at the same logic :)
     
  7. ghild

    Thread Starter New Member

    Aug 20, 2014
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    ScottWang, So I think not need R2, D2, D1, R7...
    but could you tell me why u use two transistors each mosfet ? do you have any source to learn about it ?

    thank you
     
  8. ScottWang

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    Aug 23, 2012
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    R2, D2, D1, R7...these are the protection components, maybe some day you will know it's very useful.

    Why I used two bjts, actually there is nothing special about this circuit, these functions just according to the basic theories of mosfet and bjt, and plus some tricks from my experience, I can't creat a new theory, so, only study the theories form other theory creator, doing the experiments more and more, also they are all the basics.

    Knowing the basic knowledge, they just like you got the blocks of lego toys, when you knew every functions of each piece, then you can create the modeling more and more and that depends on your ideas.

    The Q1 was a switch for Vg of mosfet, and the Zd1 was to limited the Vg, R5 was provided a low level voltage when Q1 was turn off, given a low level voltage as 0V will make mosfet turn off, Q2 was another switch for the Q1, provided the Ib for Q1, and the switch was controlled by Q1-1, R4 was to limited the Ib of Q1, R3 was to turn off the Q2 quickly when the Q1-1 wasn't input a high level voltage.

    Same function can be use many different ways to reach, so you knew the theories then you can do your own tricks.
     
  9. ghild

    Thread Starter New Member

    Aug 20, 2014
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    Thank you Scott for your explanation,

    yeah I am just lack of experience, so I need to simulation that before going further, but when I simulation at Proteus sometimes the circuit don't work properly (I try circuit from "working board") so that make me afraid to going further.

    do you have any way to make sure your circuit works properly ?
    or just trial and error to earn experience ?
     
  10. ghild

    Thread Starter New Member

    Aug 20, 2014
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    it is not about "12v and 24v switch using mosfet" circuit :p
     
  11. ScottWang

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    Aug 23, 2012
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    12v and 24v switch using mosfet , it just a part of your functions, so it's ok for that.

    I'm not sure this circuit will work and complete match your needs or not, but it looks ok, the next step is your turn to simulate or using breadboard to do the test.

    Sometime when I attach the circuit, if any errors the other members will correct it, or doubts it, and then same situation that I will also correct some others if any errors, so that's why we can learn from each other.
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    For the FET shown the gate threshold voltage could be as high as 2.5V according to the datasheet, so a Vgs voltage of at least 5V should be used to ensure the FET is turned fully on. This means if the source voltage is 12V or 24V as required then the gate voltage must be > 17V or 29V respectively.
    In theory, they shouldn't be; but Scott wanted to make sure :). In practice it depends whether there is any overlap between the Q and /Q waveforms. Unfortunately, simulation shows the posted circuit (old or new) doesn't prevent both FETs being on at the same time. The Schottky diodes do, however, now prevent the two supplies being shorted when that happens.
     
  13. ScottWang

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    Aug 23, 2012
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    Thanks Alec_t.
    In the beginning that I want to use the P-ch to design, I'm not sure which one is better(now is still the same), so I used the N-ch to did it, but that 's a bad idea, I change to the P-ch, if you can provide some infos of mosfet, that's good for this circuit, the same, if you find any errors please point out, sometimes that's my blind or concern not enough cause that, thanks.
     
  14. Alec_t

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    Choice of FET would depend on the peak current drawn by the stepper motors.
     
  15. ScottWang

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    Here are the p-ch mosfet 30V 4A, it's similar as original N-ch, this is one of them.
     
  16. Alec_t

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    Gchild, do you want one control voltage to switch the 12V supply and a second control voltage to switch the 24V supply (so that both can be off when required), or will just one control voltage suffice to select between the two supplies (so that one or the other is always on)?
     
  17. ghild

    Thread Starter New Member

    Aug 20, 2014
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    Hai Scott, I simulate it to Proteus, the Zener Diode makes Output voltage from 24V become 10V , when I remove the Zener diode , the output become 23V
     
  18. ScottWang

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    Aug 23, 2012
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    Normally, you need the P-ch to be the switches is better, but I didn't give you that suggestion, still tried to followed what you described, and Alec_t already mentioned that can not be done.

    The following is using p-ch mosfet to be the switches, you can simulate first, and the p-ch mosfet that i already mentioned on #15.

    [​IMG]
     
  19. ghild

    Thread Starter New Member

    Aug 20, 2014
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    the input of monostable is one, and the output are Q and /Q .
    so when Q is high , it is switch to 24 V .
    and when /Q is high , it is switch to 12 V.

    because I just need supply stepper motor only around 45 seconds,
    when I have done with it , I want switch it to 12V , because if I supply it too long , the Motor getting hot.

    I drive 6 stepper motors,
    and I use it at 24v one by one.

    why I using 24V , because the motor will instantly STOP when I command it to STOP. when using 12v there is delay until the motor STOP perfectly
     
  20. ghild

    Thread Starter New Member

    Aug 20, 2014
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    Scott the voltage on Gate is Okay ?
    the datasheet tell the Gate-source voltage is around 12V.

    And I think the diode at drain is reverse.
     
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