Just a quicke.

Discussion in 'The Projects Forum' started by Bazzer Englander, Jun 12, 2012.

  1. Bazzer Englander

    Thread Starter New Member

    Apr 18, 2011
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    I have a 2N7000 Transistor,does the N mean it's a NPN type? also what would happen if it was put in round the otherway? Any help would be gratefully accepted.

    Bazzer.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    Take a look.

    By "the other way", do you mean substituted for a PNP, or mis-wired?
     
  3. panic mode

    Senior Member

    Oct 10, 2011
    1,321
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    2N7000 is a N-channel mosfet. NPN or PNP indicated BJT type transistor.
    unlike BJTs mosfets are symmetrical devices and S and D is actually defined by connection of the bulk. in case of 3-pin packaged mosfets, you no longer have control of how the bulk is connected so make sure to verify pinout using correct datasheet.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,452
    3,371
    The N has nothing to do with the polarity of the transistor.
    For example, 2N3906 is a PNP BJT.
    No, you cannot interchange the Emitter and Collector and expect the circuit to work. There is a rare occasion that it still works.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Here is Fairchild Semiconductor's datasheet for the 2N7000: http://www.fairchildsemi.com/ds/2N/2N7000.pdf
    2N only means that the component has three terminals. It might be a BJT transistor, a MOSFET, a JFET, SCR, TRIAC, or other three-terminal device.
    1N means that the component has two terminals; usually diodes.
    3N means that the component has four terminals.

    As you'll note on the 1st page of the datasheet, the gate is the middle pin, 2. The source terminal is the leftmost pin, 1, if the flat on the side of the transistor was facing you with the terminals pointing downwards.

    If you accidentally installed it backwards, the body diode would conduct at roughly 0.6v, and increase with drain current; you wouldn't be able to turn it off, but you might be able to vary the current flow somewhat.
     
  6. Bazzer Englander

    Thread Starter New Member

    Apr 18, 2011
    18
    2
    On the diagram it shows BC & BE,on both legs it's shown as a diode, so BC is going away from B to C & the same with B to E, going away from B to E, so if the transistor were reversed,ie rotated around B could it make any difference, On my PCB the positions of three legged transistors is printed on the board, on the one board that I've been using as a pattern the differance is opposite to the diagram on the board, I will try to download some pictures of the PCB, a completed board & a component list, the item Q1 is the one I'm concerned about, I don't know if you can see from this why I'm getting feedback to LED (D4), The way this should work is if the power supply is the correct way round then the green LED (D5) should be illuminated, If the polarity is reversed, the red LED (D4) & (D5) will be illuminated, the red to show that polarity is reversed and green to show that it's been rectified, if there is a earth/ground fault an orange LED (D8) will illuminate.
    I've had dozens of these that have worked correctly it's only the last one's I'm having trouble with, before anyone says " look at one that works" all the one's that work are sealed into their containers with potting compound, the few the I have tried to remove destroys the components,(which is what it was meant to do) so I have none to compare to.
     
  7. WBahn

    Moderator

    Mar 31, 2012
    17,788
    4,808
    The schematic symbol for the NFET actually shows what will happen if you reverse the part. Notice that the bulk pin (the one between the source and drain pins) looks like it has a diode in it. That's because it does. That represents the channel-to-bulk parasitic diode. If you reverse the part (flip it upside down in the schematic) you will have a path from the top pin down to the bulk pin and then through the parasitic diode to the bottom pin. It will be like putting a diode across the FET with the cathode on the ground side. Thus even with no gate drive, you will still have a one-way conducting path through the parasitive diode.
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,156
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    I can't see Q1 in your pictures. No matter. The issue is whether it is installed correctly? It matters.

    Current through the body diode may be enough to activate the relay coil. Whether it activates or not, there should be a measurable voltage drop across the relay's coil if Q1 is installed backwards, because there's nothing to stop current flowing through it. And there will be a drop of 0.7V or so across Q1's source and drain pins, again because a current is flowing through that body diode.

    The source pin should be grounded and the drain pin connected to the relay. You should be able to determine all this from the solder side of the board, even if the top is potted. Maybe the bottom is too? In that case you might be able to crack open the relay to probe the terminals from the inside.
     
  9. Bazzer Englander

    Thread Starter New Member

    Apr 18, 2011
    18
    2
    Hi Waynen,
    Q1 is top left corner above C6, any way I've decided to put another one together and turn the transistor round & see what happens, if you don't see any more posts from me you know it failed,:D:D:D

    Bazzer
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Maybe you have some counterfeit 2N7000 Mosfets that have their pins backwards.
     
  11. Bazzer Englander

    Thread Starter New Member

    Apr 18, 2011
    18
    2
    Well I didnt blow myself up, reversing transistor C6 made no difference at all, the only thing is I'm back to having an earth fault light on, as I've said before I'm not an electrical engineer,I was a design engineer so I can read and understand drawings, I can copy an assembly, which is how I have put two of these switches together,The pictures that I used to work from it's worth noting are of one of the early ones that worked ok, so I'm at a loss as to why I'm having so mutch trouble.

    Bazzer
     
  12. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    By "reversing", do you just mean turning it around 180°? If the hypothesis is that the failure is due to incorrect assembly, I guess that test makes sense. Another hypothesis (a bit of a reach but not impossible) is that the pinout is different on the parts you have than it was for the original. Turning it around is unlikely to fix that. Do you have any documentation with that part?

    One thing that might help you is a way to test your MOSFETs. For quick and dirty testing I use a 9v battery and an LED with a 1kΩ (any value 470 to 2.2K will work) resistor attached. Use alligator clips on test leads to connect the MOSFET source pin to battery ground and its drain pin to the LED-resistor. Touch the other end of the LED to the battery positive. It should not light (be sure it is oriented so that it DOES light if the MOSFET is shorted across).

    If it does light (bad sign), tap the gate pin onto battery ground. If this doesn't turn off the LED, your MOSFET is failed to a short.

    If the LED did not light (good sign), tap the gate pin onto the battery + terminal. The LED should light (great sign). If so, it appears to be good and functioning as a switch. If not, your MOSFET is failed open.

    If you don't know any of the pins, use your DMM's diode tester to find the one combination that gives a reading. That's the body diode conducting from source to drain, the other pin is the gate.
     
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