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controlling 12 Volt loads via Propeller — Parallax Forums

controlling 12 Volt loads via Propeller

TinkersALotTinkersALot Posts: 535
edited 2010-10-02 00:57 in Propeller 1
Can driving the base of a TIP29 provide control of 12 volt loads in the C-E side of the transistor?

I have put a "sketch" of the circuit I have in mind in the attached file.
txt
txt
ascii.sketch.txt
790B

Comments

  • LeonLeon Posts: 7,620
    edited 2010-09-26 10:48
    Load current?
  • TinkersALotTinkersALot Posts: 535
    edited 2010-09-26 10:55
    Curiously enough, the current requirement is not stated, but I would like to switch power to these:

    http://www.parallax.com/StoreSearchResults/tabid/768/txtSearch/750-00060/List/0/SortField/4/ProductID/633/Default.aspx
  • LeonLeon Posts: 7,620
    edited 2010-09-26 11:16
    It'll be the current taken by the tubes, plus a bit more.
  • TinkersALotTinkersALot Posts: 535
    edited 2010-09-26 11:32
    Hi Leon,

    In this document:

    http://www.parallax.com/Portals/0/Downloads/docs/prod/audiovis/CCFLTubes12-v1.0.pdf

    I find this:

    12V / 300 mA power consumption ( I'm going to presume this is the current draw from a single tube based on the nature of this document).

    Since each of the inverters can power 2x tubes, the current output from the inverter looks to be 600 mA. Now if I assume a 25% "efficiency loss" (don't know if I stated this correctly) then by the back of my napkin we're in the neighborhood of approx. 750 - 800 mA.

    Does that sound about right?
  • JonnyMacJonnyMac Posts: 9,236
    edited 2010-09-26 11:41
    The attached circuit is something I've used in a few projects, including a recent prop lighting project for Hollywood FX wizard, Steve Wang (who created the Predator and many other really cool movie monsters).

    The reason for the high-side circuit is that it allows me to control common-cathode RGB LED modules. The other pic is a 12-output board using this circuit that I put together for my Nuts & Volts column (November, 2010).
    high_side_driver.jpg
    640 x 348 - 29K
    power_12.jpg
    640 x 480 - 130K
  • jeff-ojeff-o Posts: 181
    edited 2010-09-26 12:32
    Just use a logic-level MOSFET. Want to switch 10 amps? No problem!
  • TinkersALotTinkersALot Posts: 535
    edited 2010-09-26 14:30
    @JonnyMac -- thank you for the reference design. That is a nice board. Is that going to be offered for sale at all?

    @jeff-o -- thank you for the suggestion, but I don't think I need that much "head room" on the switching current for this project. Looks like mosfets, while beefy, are about 20 cents more apiece than regular-old-bipolars. I'll have to consider the trade off between "capacity" vs. cost.
  • Dr_AculaDr_Acula Posts: 5,484
    edited 2010-09-26 17:31
    Gain of that transistor is 40-75, so say it is 50, then 20mA into the base will be switching 1Amp so it will be in the right ballpark. Max current is 1A (3A peak) so again you are in the right ballpark.
  • JonnyMacJonnyMac Posts: 9,236
    edited 2010-09-27 05:44
    TinkersALot wrote: »
    @JonnyMac -- thank you for the reference design. That is a nice board. Is that going to be offered for sale at all?

    GadgetGangster.com usually creates kits of my N&V projects. As with my others, the output board sits on top of a standard Propeller Platform (I also designed).

    http://www.gadgetgangster.com/find-a-project/56?projectnum=168
  • jeff-ojeff-o Posts: 181
    edited 2010-09-27 16:40
    TinkersALot wrote: »
    @JonnyMac -- thank you for the reference design. That is a nice board. Is that going to be offered for sale at all?

    @jeff-o -- thank you for the suggestion, but I don't think I need that much "head room" on the switching current for this project. Looks like mosfets, while beefy, are about 20 cents more apiece than regular-old-bipolars. I'll have to consider the trade off between "capacity" vs. cost.

    Ah, but you need resistors in there as well to support BJTs. No resistors needed for the MOSFET! Also, you don't have to use a 10 amp device if you don't need that much power.
  • LeonLeon Posts: 7,620
    edited 2010-09-28 03:16
    A pull-down resistor on the gate is a good idea!
  • jeff-ojeff-o Posts: 181
    edited 2010-09-28 04:39
    Leon wrote: »
    A pull-down resistor on the gate is a good idea!

    Aw, fine, you caught me. Yeah, a pulldown is a good idea. I still prefer using mosfets to bjts for things like this, even if they are a whopping 20 cents more expensive!
  • Martin HodgeMartin Hodge Posts: 1,246
    edited 2010-09-28 07:29
    I agree. I tried BJTs to switch these very lamps and nothing worked as well as MOSFETS.
  • Clock LoopClock Loop Posts: 2,069
    edited 2010-09-29 11:57
    I have used mosfets for control of 12v inverters for el wire, (basically the same as ccfl)

    I found that it was great and all, but shaping the waveform of the ac supply gives us so much more control.

    Anyone have a easy way to try to not only shape the ac waveform but also do it with with the 3.3v output of the prop?


    H-bridge combined with a rectifying circuit to get the high voltage from the ccfl inverter?

    H-bridge control would need opto isolation... meh.. no real easy way..

    I like using the inverter as a supply for my ccfl tube h-bridge.(rectified its really dc, a few diodes and a cap to hold the dc for the h-bridge?

    Then one can see what different hz rates will do....
  • youngbillyoungbill Posts: 54
    edited 2010-10-02 00:57
    Martin Hodge wrote: »
    I agree. I tried BJTs to switch these very lamps and nothing worked as well as MOSFETS.
    do you need a resistor between the mosfet and the prop?
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