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42v to 12v

Discussion in 'General Electronics Chat' started by SimonTHK, Aug 28, 2016.

  1. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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  2. ChrisP58

    ChrisP58 Well-Known Member

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    The gate stopper resistance is needed when the gate is driven from a low impedance source such as a logic device or a gate driver.

    In this circuit it is not necessary, as the two 10K (effectively 5K) resistors already provide more than enough resistance to dampen out any oscillations. And the zener does not matter as long as it's clamping voltage level is far away from the turn on threshold of the mosfet.
     
  3. spec

    spec Well-Known Member Most Helpful Member

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    I am currently basking in the sun by the pool in an upmarket hotel near Barcelona, :cool: and my missus has forbidden me to even think about electronics so you will have a respite from my ramblings for about eight days. :p

    spec
     
  4. dave

    Dave New Member

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  5. SimonTHK

    SimonTHK Member

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    Well I did try my own theory on this :) and it seems to work very well as a pull down, so I need not pull to 0v through latch.

    I also provoked the damn thing as much as possible. Cut the wire to the gate and had constantly 4 times per second for 1 minut, switch it on off. Works fine.
    Also constanty swithed output current on of and made sparks by cutting the wire and connecting them with my hands. For like 3 minutes, and still works.

    Overall it all seems to work perfectly great, and I am very thankfull. I am using 12v zener, though some said I didnt have to.

    Now I need P channel mosfet to drive the charge control. I thought I could use N-channel, but I cant. So I will try and make circuit later with P-channel mosfet aswell :D

    I soon will provide with pictures of the project.
     
  6. SimonTHK

    SimonTHK Member

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    Hello all

    Here are a small pictures of the battery packs. They have installed the 123smartBMS and now also soon my own little board :)

    I did have to use a p channel mosfet aswell, to control the charge input. I tried to figure it out myself, using the stuff from here, gate source voltage etc... and I now made it work aswell.
    So it all seems to work perfectly well :)

    I will test more.

    and I will put of finished schematics etc
     

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  7. SimonTHK

    SimonTHK Member

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    Hello guys

    Can you tell me if this is an ok way to connect to a P-mosfet? Gate to source is maximum 25v.

    The latch relay opens till 0v, and let current run through the voltage divider, so gate voltage is 24v.
    When latch relay doesnt open til 0v, the gate will be pulled up, to 48v, which should open the mosfet, and the gate source voltage should then be 0v.

    Is this right?

    Ty in advance
     

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  8. ChrisP58

    ChrisP58 Well-Known Member

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    It will work, but you're operating the gate right on the edge of failure. Most Mosfets are fully turned on at a GS voltage of only 10V, so there's not much advantage to running the gate higher than that. You can use the two 10K resistors, but I would recommend a 12 to 16 Volt zener across the GS pins. This will also protect the Gate from transient voltage spikes.
     
  9. SimonTHK

    SimonTHK Member

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    Ty I will do that.
     
  10. SimonTHK

    SimonTHK Member

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    Bump
    I need to... This project... :D well the N channel works superior fine, but I have problems with the P- channel. Actually itm works... BUT!
    The 48v to source is not constant, it is only there when the charger is connected, else it is 0v ofcourse. So yes it probably works fine when charger is constantly giving 48v, but it wont allways do that, it needs to have connection through the p mosfet to start deliver 48v.

    So how do I connect this. Source cant be the battery plus pole, because the charger is the battery plus pole. That would make the charger not care about the mosfet. This is difficult for me to figure out :)
    When the source gets to 0v, the gate is "right now" connect through a zener diode, to the battery plus pole, which is actually also connected to drain. Tus source is 0v (charger disconnected) and gate is 12v.

    I am confused by this. There has to be a way.

    Could I place a diode between battery plus (drain) an source, so that constant voltage from the battery plus, is delivered to source, but it cant charge backwards through the diode, because yes... it is one way only :)
    Or would the mosfet in "turned on mode" without a charger, discharge the battery over the mosfet?
    So much... thinking.

    TAKE IN MIND. That earlier in this thread I was using a 36v battery, now I use a 48v battery. That is why I have changed some voltages.
     

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    Last edited: Sep 26, 2016
  11. spec

    spec Well-Known Member Most Helpful Member

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    Don't forget that there is a high current intrinsic diode in a PMOSFET: cathode to source and anode to drain. So the battery charger output will always be at least at the battery voltage minus about 0.6V.

    Some chargers are OK with this and do not drain the battery too much but for other chargers it is a problem (there is always something to catch you out in engineering).:D

    spec
     
  12. SimonTHK

    SimonTHK Member

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    Ok
    So that means, that I dont need the diode, because it is actually allready there inside the p-mosfet?
    Is this why the circuitĀ“s I have made so far, generally seems to work fine? I just had my mind thinking, that I was doing something wrong.

    You do see the problem, that the source cannot be 0v, because it has to be of higher voltage, when the gate and source voltage should be the same, for it to "close" the mosfet.

    I... cant... be.... an engineer, that isnt employed as one.
     
  13. spec

    spec Well-Known Member Most Helpful Member

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    Hi Simon,

    Yes, your circuits will be fine.

    spec
     

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