H bridge works using BJTs but not mosfet

Discussion in 'General Electronics Chat' started by lettcco, Nov 30, 2009.

  1. lettcco

    Thread Starter New Member

    Nov 30, 2009
    Hello, newbie here--to the forum and to circuit building.

    I am using a microcontroller to control the direction of a small DC motor using BJT--2N4401-- in a H bridge configuration, all using the same power source of 3V battery, and it works. I read an article that BJTs is not as efficient as mosfet thus I try to replace the BJTs with IRF510, but I couldn't get to work. Am I missing something requirement wise? thanks
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Unlike a BJT that is current controlled, a FET needs a minimum voltage on the gate to place it into conduction. In the case if an IRF510, the gate must be 10 volts more positive than the source lead for full conduction.

    A closer match to your voltage is a VN10LP. That FET is in TO-92, will pass a bit over 250 ma, and only needs 5 volts on the gate for full conduction. Double the source voltage and you're in business (always get a data sheet by Googling the part number).
  3. cheegii


    Nov 24, 2008
    you could use h-bridge mosfet
  4. lettcco

    Thread Starter New Member

    Nov 30, 2009
    Thanks for the replies! I thought that as long as I exceed the Vgs threshold, the mosfet should operate properly. In the IRF510 datasheet, the Vgs threshold is 2-4V, and since my supply is 3V I thought that would be enough.

    If you guys don't mind can you please tell me if my following assuming is correct or not?

    The Rds listd on VN10P is 5ohm for 10V Vgs...so when I use in a h bridge config I would have 10ohm in series with my load. I measure my motor load is <1ohm, so with the battery supply =3V the current that passes through is only ~ 0.3A.

    Going back to my BJTs H bridge, I would have 2 0.75V drop in series with my load, so from the 3V power supply I have 1.5V left for my load. Assuming a ~1ohm load, my current is 1.5A. For a motor application, I want all the current I can get...so I am better off with my BJTs?

    Thanks in advance
  5. mik3

    Senior Member

    Feb 4, 2008
    In your case it would be better to use your BJTs unless you buy MOSFETs with lower Rds(on). However, MOSFETs will need an H-bridge MOSFET driver IC to work properly.
  6. Audioguru

    New Member

    Dec 20, 2007
    The threshold voltage is when the Mosfet is barely turned on (or is almost turned off) with a current of only 0.25mA. It needs much more gate voltage to be a switch.
  7. SgtWookie


    Jul 17, 2007
    The threshold voltage is where the MOSFET just begins to conduct at a specified low current. For an IRF510 to be fully turned on, it needs Vgs to be 10v.

    You would need a rather hefty very low logic-level MOSFET then.

    You really should post your circuit.
    I see from your 1st post that you are using 2N4401 transistors. Those are rated for 600mA, but the Vce starts getting rather high after about 300mA which is a practical limit.

    You would be better off using PN2222/2N2222 or BC337/BC338 transistors, as their Vce will be lower at higher currents. They are rated for Ic=800mA, but 500mA is a practical limit.

    You didn't say what you are using for the high-side transistors. If you are using NPN transistors as emitter followers, you are losing at least 0.8v across them, and dissipating a good bit of power as heat. You would be better off using PNP transistors such as the PN2907A/2N2907A or BC327/BC328, which are good complements to the PN2222/2N2222 or BC337/BC338 transistors.

    You might find this page helpful:

    When using transistors as saturated switches, the "rule of thumb" is you need 1/10 of the collector current for base current.
    This can be a problem when you are operating from very low voltages.

    As I mentioned before, it would help us help you much faster if you posted a schematic of your circuit as it is now.
    Last edited: Dec 2, 2009