mosfet with solenoid load

Discussion in 'General Electronics Chat' started by swatchit, Nov 27, 2006.

  1. swatchit

    Thread Starter New Member

    Sep 30, 2006
    4
    0
    I was trying out a mosfet (IRF530) with a solenoid load (and back-emf diode bypass in place) connected to an 8v supply. After triggering the gate, the solenoid energised, but remained energised even with no gate voltage. Like a thyristor, I thought. I found that a high value resistor between source and gate allowed the mosfet to turn off when gate voltage removed.
    I didn't think such a resistor was necessary. What do you have to say about it?. ~~Sammy ~~
     
  2. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    If by no gate voltage you mean that you left the gate floating, then, yes it would most probably stay conducting.

    The reasons are, first, when you applied a voltage at the gate, the gate capacitance is charged up to a certain voltage and it would need a path to discharge this capacitance when you removed the voltage. Since there is no path (except the leakage current) when you left the gate floating, then the MOSFET would conduct until the capacitance is sufficiently discharged (might be a relatively long time).

    Second, there might be a small leakage current flowing from the drain to the gate. This leakage current will charge the gate capacitance to a sufficient voltage to cause the MOSFET to conduct.

    These are the reasons why the gate-source bypass resistor is necessary, especially when the gate is left floating.
     
  3. swatchit

    Thread Starter New Member

    Sep 30, 2006
    4
    0
    Thanks for your reply.
    What I'm doing is feeding an output from a picaxe to the mosfet. The picaxe circuit examples page shows an output pin connected directly to the gate of a mosfet. So, I expect no discharge resistor is needed in that situation. None shown. What I had been describing in my query was with the gate floating. ~~ Sammy ~~
     
  4. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    If the output of the PICAXE is high and low (not high and floating), then there is no need for a GS bypass resistor. I, however, would put a series resistor to limit the maximum gate charging current to a suitable value.
     
  5. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    If its a PICAXE 8 or 8M then the outputs are also inputs, so make sure you set that pin as an Output, using the OUTPUT statement, or use the LO statement to force the pin into a low output state when not switched on.
    If the pin is inadvertantly switched to an Input, then the pin will be floating.... a feature I have used in the past to simulate tri state outputs......
     
Loading...