2N7000 mosfet?

Discussion in 'General Electronics Chat' started by rougie, Jul 14, 2013.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    Hello,

    I am using some 2N7000 mosfets in my circuits and every time I fiddle or handle them due to rewiring a minor portion of my circuits, I end up damaging one.

    Is it normal that Mosfets get damaged just by touching them or handling them... I mean, its already 3 time this happens!!!

    Are they that static sensitive??

    Oh! by the way, when I want to do quick measurements on the fly, I often unplug and plug these things without shutting the 5 volt power source of my circuit simply because I am to lazy!!! :rolleyes: Can this damage a mosfet??

    thanks all for your feedback
    r
     
  2. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    I am not certain if you are trolling... but yes a Mosfet is static sensitive. You should be using ESD precautions.

    It is bad practice to modify a circuit under power. I am often guilty of that and yes it is a way that you can end up with damaged components.
     
  3. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
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    Hello Potatoe Pudding,

    No troll here ... I'm just some guy trying to do electronics !!!

    In any case .... thanks for your help ...

    really appreciated !
    r
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    The MOSFET gate has a thin oxide for insulation and can be easily damaged by static. Typically the gate can tolerate no more than ±20V gate-source voltage.
     
  5. donpetru

    Active Member

    Nov 14, 2008
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    When removing or fitting a mosfet in the circuit, short terminals with a thin wire.
     
  6. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
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    Hi, guys thanks for your help!!!

    Another thing I am noticing is suppose I have two mosfets (lets call them M1 and M2) with the same part number, when I plug the first one (M1) into my circuit, and for example it's gate voltage would be 2.5V, I then read a current of x through the Drain/source path. Now if I swap the M1 mosfet with M2, which has the same part number, the current throught the drain/source is different!!!! Same circuit two different currents !!!

    Before I get into the specifics of my circuit at hand and its currents/voltages, I was simply wondering if mosfets have a varying gain from one to the other just like transistors do?

    In my circuit I am noticing that when I use M1 to drive an IR led, there is less current, then if I were using M2 with both given the same gate voltage? In my circuit there is two other transistors (2n2222) in series with the mosfet... But this shouldn't affect the current through M1 or M2 as these transistors are fully saturated.

    All to say is my circuit isn't perfect and I know that it's my fault... I will start reading up on mosfets and any ideas would certainly be welcome.

    r
     
  7. Robin Mitchell

    Well-Known Member

    Oct 25, 2009
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    The chances of finding two same transistors (eg BC548) that have the same characteristics is very unlikely. That is why when designing a circuits using those components try to make the operation independent on said characteristics. That's why negative feed back is used :p
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    2N7000 Rds(on) is only guaranteed (spec'ed) for Vgs>4.5V. With Vgs=2.5V, some units might not conduct at all.
    See attachment.
     
  9. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    Hi Ron_H,

    ahhhrrgrg! That's why I was getting some parts working at Vgs of 2.45 and some at to 2.58 and others at 2.8... And mosfet being exponential in nature, these small differences cause significant discrepancies in the current they conduct!

    But how can I resolve this, is there any mosfets similar to the 2N7000 that have a 2.0Vgs? It seems the 2n7xxx all have a Vgs > than 4.5 !!

    Discouraged! :confused:

    But I don't understand that the spec says 4.5VDC, and all the mosfet parts I used all had an Rds of way less than 4.5VDC. Typically when I apply approximately a little over 2.1VDC at gate, these parts usually start to conduct!!!! Mind you, at 2.1 VDC, my current is very small ex: 1, 2 ma... and as I increase Vg to 2.5 or 3.0VDC I tend to draw more current approximately 35ma .... ??

    Anyhow thanks for your help!
    r
     
    Last edited: Jul 15, 2013
  10. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    Post your schematic.
     
  11. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    >>>Post your schematic.

    Yes absolutely... I am drawing it up... please stand by... thanks

    r
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    At Vgs=4.5V, the 2N7000 is guaranteed to have Rds<5.3Ω. The datasheet doesn't say what will happen if Vgs<4.5V. It might work, it might not.
    There are MOSFETs available with Rds(on) guaranteed for Vgs<2V, but they are probably in surface mount packages. Can you handle them?

    Do you understand that "<" means "less than", and ">" means "greater than"?
     
  13. BobTPH

    Active Member

    Jun 5, 2013
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    A MOSFET's gate threshold voltage is where it only begins conducting. Check out the current at the theshold voltage spec, it is typically in microamps. From there to a somewhat higher voltage (about 2V more) it is in the linear region. The current is linear in the gate voltage. At higher voltages, it is saturated and the Rdson is much lower. For switching applications, you want to gate voltage to be out of the linear region.

    Bob
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    They are that easy to damage with static!

    One of the compensations of the mental case in the flat above keeps damaging the water pipes and flooding my flat - its so humid, I could throw P-MOS about like it was DTL!

    Keep static sensitive parts in their antistatic packaging until you're ready to mount it on the board - if you breath on it before, and from time to time during handling it, the moisture in your breath helps dissipate stray charges.

    The UK is relatively humid at the best of times, so a lot of people just get away with it a lot of the time. I've read that some parts of the US get seasonal very dry winds that make anti-static handling a monumental hassle!!!
     
  15. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
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    hello guys,

    Ron_H, I see what you are saying.... 4.5VGS gets us a guaranteed rds of 5.3
    ohms. But I designed my circuit with the intention of using it within the linear region somewhere starting at 2.0Vgs up to 4.5Vgs. And most
    Mosfets work okay in this range.... but as I am finding out
    that the IDS is not the exact value from one Mosfet to another
    when the same VGS is applied... for example at VGS = 2.5Vdc, some
    Mosfets allow 5 ma and others will allow 7.5 ma through their SD junction
    which powers an IR led ... if my led is to provide a certain quantity of ir light
    when bouncing back to the IR receiver, then I need a constant
    current all the time from any MOSFET!!! Either 5 ma or 7.5ma !!!!

    Bof these surface mount parts are difficult to handle especially
    when testing preliminary circuits like I am doing.

    Hi Bob,
    >>> For switching applications, you want to gate voltage to be out of the linear region.
    Well that's just it, I am not using the MOSFET for switching applications
    I am using it to ramp up the current to an IR led. My VGS starts
    ramping from Aprox 2.0Vdc to about 5 Vdc.

    I use the IR led to determine distance of an object; so you can
    imagine if every MOSFET doesn't ramp up exactly the same
    way as Vgs is applied, I get discrepancies in the distances !!!
    Ex: One MOSFET allows enough IR led current to read about 9"...
    if I replace the MOSFET with another one, I read about 11"...
    This is unacceptable. Was wondering if much more precise
    mosfets exist that have their linear region very identical to
    each other.

    Thanks all
    r
     
    Last edited: Jul 16, 2013
  16. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    We get cases like this all the time. We waste a page or two of posts concentrating on the wrong question, because the OP has already gone partway down a dead-end path.
    You can't do linear control of LEDs with open loop MOSFETs. You need to sense the current, convert that to a voltage, and compare that voltage to your control voltage in a feedback loop. Alternately, you could use pulse width modulation. Tell us more about what you are trying to do, starting with measuring distances:
    Minimum to maximum current, available supply voltages, etc. The more specific you are, the better we can help you.
     
  17. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    Have you considered using a Sharp Infrared Range Module instead?
     
  18. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    That sounds like a better idea than mine.:D
     
  19. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello guys,

    <You can't do linear control of LEDs with open loop MOSFETs. You need to sense the current, convert that to a voltage, and compare that voltage to your control voltage in a feedback loop. Alternately, you could use pulse width modulation. Tell us more about what you are trying to do, starting with measuring distances:>

    This is interesting... Ron_H. For a guy like me to do this, It would be wise for me to start off with some guidance as to which circuit strategy I should use. I could try myself, by doing experiments using transistors, opamps and probably again ending up down the death-end path .... I would really like some help to start me off... if that's okay with you. I was just wondering, can we do that without using an MCU, like for example sensing my current and converting it to a precise voltage in respect to what the distance corresponds to and then modifying my control voltage accordingly... by using using a few external components such as an op- amp, transistors etc... !!!

    I would try this by myself but I am concerned as to weather I will use the wrong parts ... :cool: although I am very tempted to try :p

    I will focus on the problem itself... The attachment shows what my circuit looks like and what I am trying to do. We have a MOSFET (which as you say is likely not the correct part to use for linear operations) that is driving a led. We need the same current through the led for any given mosfet when VGS is at a particular voltage (For example's sake VGS = 2.5VDC)! Right now a DAC is controling the mosfet's gate... The DAC cannot be changed as it is non relative to my question. There you have it, the core problem is when I change mosfets (same part numbers 2N7000) while keeping the same VGS, my IR led slightly varies in current hence making my circuit lack stability.

    I apologize as I unfortunately will be out of my office for the rest of the day ... and I will be back tomorrow.

    As for the sharp sensor, from what I can remember, these things go for approximately 10-13$ each and I need 3 sensors... which is not in my target price and therefore I need to build it from scratch. And besides, it will give me an opportunity to learn something new :D

    Ron_H, sorry for wasting two pages of posts, this really wasn't my intention....
    I sometimes leave out important details and I can understand how it can get the fellows
    here angry... Again, I apologize !!!

    thanks all
    r
     
    Last edited: Jul 16, 2013
  20. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I don't think anyone is actually angry.
    You need to answer my previous questions:
    Your schematic has a +5V supply. Is that the only one you have available?
     
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