Driving relay with IRF510

Discussion in 'The Projects Forum' started by nathomas, May 27, 2011.

  1. nathomas

    Thread Starter Member

    Mar 3, 2011
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    I am implementing the circuit shown in the attachment. The problem is that the 4001 is working fine and supplies 6.5 volts to the the IRF510 but the MOSFET is nt switching. I tried applying a voltage from a dc supply unit to the MOSFET and it works fine. Is there a solution to why the 4001 isnt driving the MOSFET. The voltage coming out of the 4001 is 6.5 volts but the voltage at the gate of the MOSFET shows only 0.5V. Thanks in advance for the help.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    The CMOS chip can't pull up the output with the additional load of the LED and resistor. Additionally, the IRF510 is going to be just barely conducting with 6.5 volts on the gate (the threshold voltage is just enough to start the device into conduction, but not enough to cause full conduction).

    FET's want at least 10 volts on the gate, and 12 is even better. CMOS logic is happy with 12 volts Vcc. The 10K gate resistor is only going to slow switching - something more like 100 ohms is plenty to prevent oscillation.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    I believe that you have the 100k (brown,black,yellow) resistor from gate to source and 10k (brown,black,orange) resistor from the 4001 output to the gate swapped (exchanged); so if the output of the 4001 is 6.5v, your Vgs (gate to source voltage) will only be around 0.59v.

    As Beenthere mentioned, changing the resistor from the 4001 output to the gate with a 100 Ohm resistor will correct that problem.

    The circuit is shown connected directly to the battery via a fuse. Be aware that the 78xx series of regulators use between 5mA and 6mA current even when there is no load on the output of the regulator, and that may drain your battery, causing a short life.

    What is this circuit for?
     
    Last edited: May 27, 2011
  4. nathomas

    Thread Starter Member

    Mar 3, 2011
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    I have avoided using the 78xx series regulator. I am using a 12V input directly. I am using this circuit as a temperature control using an RTD. so do you think if i jsut swap out the 10k with a 100 ohm resistor the IRF510 will switch? My relay is a 12V 10A relay.
     
  5. CDRIVE

    Senior Member

    Jul 1, 2008
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    Why not move the LEDs to the drain of your FETs? It will prevent them from loading the gate voltage from the 4001's.
     
  6. nathomas

    Thread Starter Member

    Mar 3, 2011
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    I tried connecting the 100 ohm resistor as you suggested but it still does not switch. The voltage at the 4001 output pin is now 2.5V and at the gate of the MOSFET is 1.5V. I was using a 7402 before the 4001 and it could directly switch the MOSFET. But I have the board already built and it's gonna be difficult to modify from a 4001 to a 7402. any suggestions.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    A 7402 is "original" TTL. and could hardly be more different from 4000 series CMOS. The logic families are not at all compatible.

    That said, if the CMOS Vcc can be increased to 12 volts and you make the other suggested changes, this might work. The gate of the 510 has to be at least 10 volts above the source to completely turn on.

    There seems to be a fascination with newbies involving the threshold voltage for a FET. That is a point where the device just starts to conduct. Used as a switch, the gate has to be well above the threshold - at least 10 volts.
     
  8. nathomas

    Thread Starter Member

    Mar 3, 2011
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    The Vcc of the Cmos is already at 12V and i changed the 10K to a 100ohms. The 100K is still in place but the 510 still doesnt switch. Is there any other way I can get the MOSFET to switch with the 4001. or can i use something other than the 510 to switch the relay with the 4001.
     
  9. CDRIVE

    Senior Member

    Jul 1, 2008
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    You can try a logic level FET or a Darlington BJT. You may be able to use a 2N2222 too. What does your relay coils draw? You can also reduce the load on the 4001 output as I suggested earlier.... Move the LEDs! They're not helping the situation.
     
  10. hh2281

    New Member

    Dec 18, 2008
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    in stead of loading the leds from gates of Mosfets load them from Drains this makes sense
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
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    That makes a vote of two. Do I hear a three from anyone? :rolleyes:
    Really, if you increased the 4001's Vdd to 12V, as you said, and assuming that your FETs are properly oriented, the only thing that could be pulling the outputs down are those LEDs. Capisce? ?
     
    Last edited: May 31, 2011
  12. wayneh

    Expert

    Sep 9, 2010
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    First, for testing, just eliminate the LED and see if it starts working as hoped. If so, go ahead and move it.
     
  13. beenthere

    Retired Moderator

    Apr 20, 2004
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    See post #7 - +4
     
  14. CDRIVE

    Senior Member

    Jul 1, 2008
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    Bada, bada, bada... going once for 4. Do I hear a 5?
     
  15. nathomas

    Thread Starter Member

    Mar 3, 2011
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    I did remove the led..Shows 12V from the 14001 output..Made the necessary connections with the 100 ohm and the 100k. Shows the same voltage and still doesn't switch the MOSFET...
     
  16. beenthere

    Retired Moderator

    Apr 20, 2004
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    Are you certain that the presence of +12 volts on the gate - relative to the source - is not turning on the FET? Is the source grounded and the load in the drain circuit? Are you sure of the pinout?

    How about a meter check of the FET? You should see a PN junction with the cathode to drain and anode to source, with all other checks open.
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    In this thread: http://forum.allaboutcircuits.com/showthread.php?t=52940
    I told you that TTL gates (the 7402) were not rated for that high of a voltage.

    Why did you make the circuit with the 7402 instead of the CMOS part?

    The original 7402 specifications show 4.5v to 5.5v as the allowable Vcc range; 74HC02's can use up to 6v for Vcc. If you try to operate it on 12v, you will be far beyond the design specifications and will most likely burn the IC out in a short period of time - if it even works at all in the beginning.
     
  18. CDRIVE

    Senior Member

    Jul 1, 2008
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    Woah, these terms lost me. Is it a typo? :confused:
     
  19. CDRIVE

    Senior Member

    Jul 1, 2008
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    Last we heard, he was using a 4001.
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    Beenthere is talking about the body diode in an N-ch enhancement mode MOSFET. If you look at the datasheet for an IRF530, you'll see the body diode anode connected to the source terminal and the cathode connected to the drain. It's swapped around for P-ch MOSFETs; anode on source terminal, etc.

    Whoops - I see that now. There was not a resolution to that in the previous thread.

    Some time ago was looking at 4000 series CMOS and loads vs logic levels; seems to me that around 3.6mA was the maximum load without reducing the logic level to the "indeterminate" zone. Anyway, assuming that the LEDs on the gates have a Vf of 2v, changing the 500 & 560 Ohm resistors to 3.3k should still allow the LEDs to illuminate (if dimly) and achieve the desired 10v out. If it still is not getting up there, perhaps that 10k to 100k resistor from gate to source terminal is not the right value, or the MOSFET is defective.
     
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