Transistor Troubles

Discussion in 'The Projects Forum' started by JNBUK, Jul 20, 2012.

  1. JNBUK

    Thread Starter New Member

    Jun 4, 2012
    I'm having some trouble with a HGate, I'm not sure if I have fried my transistors or if I am missing something.

    I'm using IRF540 MOSFETS.

    The behavior I am observing is confusing me, between "on" and "off" I am seeing a drop of ~0.8v over the HGate load. Since that is very close to diode bias I'm wondering if I have managed to toast the MOSFETS somehow?

    My gates are hooked up to a 5v source (arduino) and the source and drains are hooked up to a variable voltage power supply. Both share a common ground.

    Essentially, when the MOSFETS are supposed to be "off" they still pass current and show a voltage.

    Any input welcomed, consider me a rank amateur.

    EDIT: Just adding a little extra info and ill add a circuit diagram.

    I have tried adding a pull down (7.5k) between the gate and ground as i was wondering if there was some residual voltage holding the MOSFET somewhat active. Seemed to make no difference.

    Apologies for poor diag!
    Last edited: Jul 20, 2012
  2. WBahn


    Mar 31, 2012
    Have you considered using PFETs for the top transistors?

    Are you sure that you have the drains of M1 and M2 tied to the high side of the supply and the sources of M3 and M4 tied to ground?

    If the 0.8V is the voltage across the load, then if that is related to a forward-biased diode it would almost have to be something within the load.

    How are you driving the gates of the four FETs? Be sure not to let them float.
  3. wayneh


    Sep 9, 2010
    Any chance the MOSFETs are turned around, so that you are running current through the body diodes from source to drain?
  4. SgtWookie


    Jul 17, 2007
    You're going to have trouble with the upper MOSFETs whether you use N-ch or P-ch.
    IRF540 MOSFETs need Vgs to be 10v to be fully turned on, and 0v to be fully turned off. If you're driving the gates with an Arduino, the gates won't get more than 5v over ground, which won't turn them fully on.

    So, if you wish to drive them from an Arduino, you will need logic-level MOSFETs; those are specified for operation with Vgs at 4.5v or more.

    Your upper MOSFETs are another story. In order to turn logic-level n-ch MOSFETs on, you will need the gate voltage at 4.5v when referenced to that MOSFET's source terminal. That means you will need to bring the gate at least 4.5v higher than your variable voltage supply.

    As WBahn mentioned, the 0.8v drop is a giveaway that you have one or more MOSFETs in backwards. There is an internal body diode connected from the drain to the source terminal, cathode to the drain. If you install them backwards, the diode will be forward-biased and will conduct.