Shop OBEX P1 Docs P2 Docs Learn Events
3.3V logic mosfet? — Parallax Forums

3.3V logic mosfet?

lardomlardom Posts: 1,659
edited 2014-05-16 12:07 in General Discussion
I want to build a circuit that can handle at least 10A at 12V. I'll go higher after I gain experience in controlling electrolysis current. I know I'll fry a couple propellers during the learning period. At the moment I can only dim LEDs with a pot.
Is there a mosfet designed for 3.3V logic? FETs seem to be a good way to isolate circuits. I could drive several darlingtons or ULN2803's in parallel but that's a guess. I think it's smarter just to ask for advice.

Comments

  • T ChapT Chap Posts: 4,223
    edited 2014-05-12 09:47
    I have used this part on projects driving direct from the Prop.

    http://pdf.datasheetcatalog.com/datasheet/on_semiconductor/MTP75N03HDL-D.PDF

    http://www.allelectronics.com/make-a-store/item/mtp75n03hdl/power-mosfet-mtp75n03hdl/1.html
  • lardomlardom Posts: 1,659
    edited 2014-05-12 10:02
    T Chap, Thanks. It says typical gate voltage is 2.7V and max is 4V. I'm going to get several. At the moment I'll be switching @ 2KHZ. It looks like a great fit. Do you have to use a resistor with a mosfet?
  • T ChapT Chap Posts: 4,223
    edited 2014-05-12 10:44
    As in a gate resistor? I don't know the math on that but I would put something between the Prop and the gate. If it catches on fire then you know you screwed up.
  • Mark_TMark_T Posts: 1,981
    edited 2014-05-12 13:02
    What's the gate capacitance? If its quite high I'd add a resistor to limit the current
    spikes on switching.
  • T ChapT Chap Posts: 4,223
    edited 2014-05-12 13:11
    Also if you are concerned about boot up then use a 10k to gnd at the gate pin. This will prevent craziness while the prop is in input
  • lardomlardom Posts: 1,659
    edited 2014-05-12 21:37
    I'll use a 10K pull-down and a current limiting gate resistor. I'll use low voltage to start. I couldn't find the same part at Digikey or Mouser so I better stock up! Thanks for the link.
  • LawsonLawson Posts: 870
    edited 2014-05-12 23:29
    I've recently used a STP22NF03L direct driven from a Prop pin. (with a 100K pulldown ofc.) It's solidly on at 3.3 volts, and the gate charge is low enough that it should switch in 10-20nS. While it's rated for 22A, it looks like the 50mili-ohm on resistance will have it dissipating 5 watts with a 10A current. Probably better to find a lower resistance, higher gate charge part. Search for a threshold voltage of about 1 volt to find FETs that are solidly on at 3.3 volts.

    Marty
  • max72max72 Posts: 1,155
    edited 2014-05-12 23:56
    IRF3708 has been suggested a lot in the past

    http://it.mouser.com/Search/Refine.aspx?Keyword=irf3708

    Massimo
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2014-05-13 06:27
    Lawson wrote: »
    the 50mili-ohm on resistance will have it dissipating 5 watts with a 10A current.

    Oh?
  • LawsonLawson Posts: 870
    edited 2014-05-13 07:13
    PJ Allen wrote: »
    Oh?
    Yea, ignore that suggestion for the STP22... while it's gate charge is small enough to switch fast with a prop-pin, 10 amps is WAY more current than it can conduct with a 3.3v gate voltage. Reading the datasheet more carefully, it looks like it will current limit at about 4 amps and get hot as heck. T Chap's suggestions are much better.

    Marty
  • lardomlardom Posts: 1,659
    edited 2014-05-13 12:34
    I just ordered 10 75N03HDL mosfets to get the price break. Being shipped from CA to NY so I'll do some research while I wait. I especially want to know how to switch full on and off and how to prevent 'shoot through'.
  • T ChapT Chap Posts: 4,223
    edited 2014-05-13 14:41
    Can you describe the circuit you have in mind an the application you nee to switch?
  • lardomlardom Posts: 1,659
    edited 2014-05-13 15:48
    My circuit controls LED brightness with a pot @2Khz PWM. I built it to control current to electrodes for electrolysis. I'm experimenting with HHO gas. I'm powering a set of stainless steel plates submersed in electrolyte with a car battery charger. It consumes a lot of current.
    I've found that the more plates I use the more current it consumes. I'm hoping I can get away without using a gate driver for now. I don't have a scope so I have to be cautious and seek a lot of advice.
  • Mark_TMark_T Posts: 1,981
    edited 2014-05-13 16:08
    lardom wrote: »
    I'm powering a set of stainless steel plates submersed in electrolyte with a car battery charger. It consumes a lot of current.
    I've found that the more plates I use the more current it consumes.

    27Ah per mole of electrons, ie per 18g of water electrolysed assuming perfect efficiency, which is unlikely.
  • jmgjmg Posts: 15,173
    edited 2014-05-13 21:40
    lardom wrote: »
    I just ordered 10 75N03HDL mosfets to get the price break.

    Ideally, you should select FETS that actually Spec a Value at the Vgs you intend to use, or lower.
    eg The OnSemi ATP218 does, as does AOS AON7520
  • lardomlardom Posts: 1,659
    edited 2014-05-14 07:59
    jmg wrote: »
    Ideally, you should select FETS that actually Spec a Value at the Vgs you intend to use, or lower.
    eg The OnSemi ATP218 does, as does AOS AON7520
    That's good advice. I'll try using a transistor to raise the gate voltage if necessary. I've heard that static electricity can destroy a mosfet so I still have a lot to learn.
  • Ron CzapalaRon Czapala Posts: 2,418
    edited 2014-05-16 10:16
    Just got a Sparkfun email featuring a P-Channel MOSFET - NDP6020P

    corrected

    -24 A, -20 V

    RDS(ON) = 0.05 ohms @ VGS= -4.5 V.
    RDS(ON) = 0.07 ohms @ VGS= -2.7 V.
    RDS(ON) = 0.075 ohms @ VGS= -2.5 V

    Any comments?
  • RDL2004RDL2004 Posts: 2,554
    edited 2014-05-16 11:48
    Don't know what Sparkfun told you, but just for the record, those are ohms, not watts. Maybe the symbol got mangled in translation somewhere. It does look like a nice mosfet though.
  • Ron CzapalaRon Czapala Posts: 2,418
    edited 2014-05-16 12:07
    RDL2004 wrote: »
    Don't know what Sparkfun told you, but just for the record, those are ohms, not watts. Maybe the symbol got mangled in translation somewhere. It does look like a nice mosfet though.

    I copied it from the datasheet and pasted into notepad and the ohm char got changed to a "W" - weird.

    I can't find whether the mounting tab is connected to the drain or source...
Sign In or Register to comment.