JFET Switch

Discussion in 'The Projects Forum' started by jcbeck84, Nov 11, 2009.

  1. jcbeck84

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    I have a 24V relay that I would like to turn on with a 24V power supply, but I would like to turn it off when another control signal is on. I am trying to use a JFET to accomplish this. My JFET handles the current and turns the relay on quite well without burning out. I have tried two seperate configurations in an attempt to get it to work (see attached schematics) In configuration 1 the gate-source voltage ends up being about -6.6V even with no power from the control signal and it pinches shut and the relay is off when it's supposed to be on. In configuration 2 the gate source voltage never gets high enough to pinch off when control power is applied. I think configuration 1 is the way to go but I'm clearly missing something to pull down the voltage at the source.
     
  2. jcbeck84

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    I should have mentioned that the control signal drops from 4 to zero volts and I would like the relay to be on when the control signal is at 0.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, you have 4v-24v specified as the control voltage range in the schematic.

    Now you're saying that the control signal drops from 4v to zero volts.

    Which is it? Or is the control voltage continuously variable?

    Can the control voltage be at any level from 0v to 24v, or are there three discrete steps; 0v, 4v, and 24v?
     
  4. jcbeck84

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    Sorry that was a little confusing. The control voltage is 0 when it is off (when i want the relay on), but when the control voltage is on it can be anywhere between 4 and 24 volts (when I want the relay off)
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    In that case you need a comparator controlling a transistor or power MOSFET instead of a FET.

    Are the two supplies isolated, or do they share a common ground somewhere?

    Oh, and you're talking 24V DC, right? Not AC?
     
  6. jcbeck84

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    The control signal comes from an actual controller which I have no options with, but it does eventually tie back into the same 24V source driving the relay. I'm only dealing with DC. The only power levels I have to work with is the 24VDC supply and I simply need to sense whether power is being sent to a solenoid (the 4-24V is read across this solenoid) or not.
     
  7. jcbeck84

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    I am unfamiliar with MOSFETS, could you post a schematic of the circuit how it would apply in this situation along with a short description?
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    N-channel MOSFETs generally require a Vgs of 0v for them to be considered fully OFF, and a Vgs of 10v (but not exceeding 20v) to be considered fully ON. Logic-level MOSFETs are considered fully ON when Vgs>=4.5v.

    P-channel MOSFETs are also off when Vgs=0. To turn on, Vgs needs to be -10v, down to -20v

    Most MOSFETs will be destroyed if the Vgs limits are exceeded, which is Vs +/-20v.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What is the current requirement, or coil resistance, of the 24v relay that you need to activate when the signal drops below 4v?

    This will make a difference in the design of the circuit.

    At the moment, I'm considering a comparator circuit with a threshold level of about 2v that will turn on a PNP transistor to source current to your relay's coil.
     
  10. jcbeck84

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    At 24V it has a resistance of 1113 ohm, or just under 22mA of current.
     
  11. jcbeck84

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    I am curious to see the comparator circuit as well as how you will set the comparator threshold to 2 volts since I don't have anything but 0 and 24 available. But then I guess if I already knew how to do it then I wouldn't be asking for help.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    See the attached schematic.

    The LM111/LM311 is a single comparator.

    R6 limits the loading on the input signal.
    Since your solenoid load is inductive, there should be a diode connected across it, cathode towards the more positive side. Without it, there will be a large reverse voltage spike when current to the solenoid is disconnected.

    The cyan plot (A) represents a sinusoidal waveform varying from 0v to 20v; basically your simulated input signal.

    R1/R2 comprise a resistive voltage divider network. R1 is a potentiometer that can be used to "fine tune" the threshold level, anywhere from nearly 0v to nearly 4v. As shown, R1 is set to mid-point, so it has a value of 1k Ohm; 1k/(1k+10K) = 0.0909...; 24v x 0.0909.. = 2.1818...V.

    The reference (threshold) voltage is supplied to the inverting input of the comparator.

    R4 provides hysteresis. This keeps the circuit from oscillating all by itself; it adds stability.

    R3 limits the current sunk from the base of Q1 to around 5mA.

    R5 keeps Q1 turned off when the comparator output is not sinking current.

    Q1 sources current to the relay coil when the output of the comparator is low, which closes the relay contacts.

    D1 is there to suppress the reverse-EMF pulse which occurs when the current through the coil is turned off. Without D1, the transistor would probably be destroyed by this reverse-EMF pulse.

    Rload is an arbitrary value representing your load.
     
  13. jcbeck84

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    Based on what I do know I see nothing wrong with this. Let me build it and get a meter in my hand and we'll take it from there. It is impressive (to me at least). I like how you got the 2V reference, nice and simple.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    One item that I omitted is a capacitor across the power/ground leads of the comparator. It is always good practice to use a 0.1uF capacitor across the power connections of an IC to help remove transient voltages (spikes/dips) from the IC's supply.
     
  15. jcbeck84

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    I should have the circuit built by the end of the day (my parts haven't come in yet). But I was looking over the datasheet for the LM311 (it was easier to get than the LM111) and it says the supply voltage at VCC+ is not supposed to be above 18V, which it is in this case. It also says VCC+ - VCC- can't be above 36V, which is fine in this case. Could you explain to me why having VCC+ of 24V and VCC- of 0V wouldn't damage the chip?
     
  16. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I don't see a limitation of 18v in National Semiconductor's datasheet for the LM111/211/311.

    Absolute maximum across the supply is 36v.
    Maximum input voltages are 30v above the negative supply, down to the negative supply voltage. If dual 15v supplies were being used, it would be -/+15v - but that isn't the case here.

    It will be fine.

    Note that both pins 1 and 4 on the LM311 must be grounded, or you won't get an output.
     
  17. jcbeck84

    Thread Starter Active Member

    Dec 20, 2008
    46
    0
    The circuit worked just as expected, once I figured out that my transistor was fried and conducting regardless of the base being grounded. These things will happen though. I appreciate your help SgtWookie. Got the job done and I learned a few things in the process.
     
Loading...