Vdrop when Using mosfet for PWM from PIC

Discussion in 'The Projects Forum' started by urb-nurd, Sep 5, 2014.

  1. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    Hey guys, im currently trying to use pwm with a power mosfet to control a heater.
    However the maximum voltage out i get from the pic (seen on the mosfet) is only 4.5v as opposed to 5v.
    I am using a 270 ohm resistor in series with the gate of the mosfet in conjunction with a 10k pull down to ground.

    I also get about 1 volt less through the mosfet than without it at max duty cycle.

    Any hints?

    Thanks!
     
  2. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    In fact, i just checked. Vcc is the same as my max pwm output voltage.
    So the issue isnt there. Vcc to ground gives me 4.6V.

    The problem i am having is i expected to see full voltage at my load from the mosfet, when infact i was getting about 1.5v less than expected.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Sounds like you have the load in the source, not the drain?

    Post a schematic
     
  4. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    load was/is definitely in drain with source connected to the gnd of my pic board.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Choose a logic level MOSFET. A normal MOSFET is not fully "on" at 4.5V gate-source.
     
  6. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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  7. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Well, one tooth at a time.:D
    At 4.5 volts on the gate the FET will have .24 ohms max resistance so you can use ohms law to see what the drop should be.
    How are you measuring the voltage? Is it the same at 100% duty cycle? (4.6v on the gate)
     
  8. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    drop shouldn't be more than a volt.
    measuring the voltage with DMM DC voltage mode.
    Voltage goes from 0-4.5 as duty cycle increases.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    What is the resistance of your heater?
    What is the length of your wiring from the MOSFET to the heater?
     
  10. wayneh

    Expert

    Sep 9, 2010
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    So what exactly is the problem? Your measurement is fouled up because you are measuring an AC voltage with a DC meter. What do you see across the MOSFET at 100% duty (i.e. a DC signal)?
     
  11. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    initially i thought that the 4.5v output from my pic board was due to my wiring, however its VCC pins are at the same voltage.
    So i can't change the 4.5V on the gate for now.
    I think my mosfet is dropping too much voltage, i am waiting for my batteries to charge before i test again.
    I was measuring at 100% duty cycle when in DC mode.
     
  12. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I'm a little confused by these 2 statements.:rolleyes:

    In fact, i just checked. Vcc is the same as my max pwm output voltage.
    So the issue isnt there. Vcc to ground gives me 4.6V.

    Voltage goes from 0-4.5 as duty cycle increases.
    This is only 0.1 volts lower than Vcc.

    If the voltage at the drain is really 3.5 volts at 100% duty cycle there are only a couple of things that can do that.

    1- The duty cycle is not really 100% - You can check this by removing the gate from the micro and hooking it directly to Vcc.
    2 The load is really in the source so it is the threshold voltage lower than the gate.
    3-The load is high. What is the resistance of the load?
     
  13. crutschow

    Expert

    Mar 14, 2008
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    Please state the load resistor and/or the load current. Without that we can't determine what the transistor voltage drop should be.
     
  14. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    The Pwm output is fine. my VCC was less than the expected 5v
    Resistance is 1.8 ohms

    Im running at 7v
     
  15. ronv

    AAC Fanatic!

    Nov 12, 2008
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    So at 7 volts your load would like 3.88 amps. The Rds on at 4.5 volts gate drive is around .24 ohms at 25C - higher as it heats up. So you might expect a 1 volt drop or so. Bet it's also pretty hot. Pick a FET with a lower Rds on value.
     
  16. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    thanks!
    Some clarification also.
    I tested the voltage across drain and source when on and off.
    When off i get 8.5v, when on i got about .5 to .6 volts however my voltage is still only 6.5V across the load.
    also, when i power my board with an external dc power supply i get the full 5 volts on VCC and subsequently on my PWM output at full duty.
     
  17. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    In-fact. when under load, the cells drop from 8.4 to 7.4V
    This coupled with a 0.6V loss in the mosfet seems acceptable to me.
    I end up with about 6.8 across my load.
    I wonder if i can get some better cells.
     
  18. ronv

    AAC Fanatic!

    Nov 12, 2008
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    It sounds like your power supply "sags" under load. If the voltage is 8.5 volts with no load - FET off - and 7 volts with the FET on it is loosing 1.5 volts. It all depends on the specs of your power supply. If you are using a voltage regulator off the 8.5/7 volt supply 7 volts may not be enough for it to regulate. This would explain the low 5 volts. Post the schematic. Use bigger batteries.
     
  19. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    my power source for the heater is two 25500 cells. they were rated for max continuous current draw of 10A
    For the board there is a voltage regulator. i was using usb power for the board before switching to a dc wall wort.
    Im using the mikroe ready for pic board.
     
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