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Propeller Project Board USB - power from 24vdc using the optional LDO? — Parallax Forums

Propeller Project Board USB - power from 24vdc using the optional LDO?

GdungeGdunge Posts: 3
in Propeller 1
I see that the PPBUSB can handle up to 16v. I also see that there's a place on the back to solder down your own TO-263 LDO.

I have a project that needs to run on a battery pack that can get up to 28v or maybe higher depending on how it gets charged. It includes a laser range finder that uses 9-24vdc @ < 5W. (Call it 5W.)

My first thought is to find an LDO with an input range that includes the battery, and an output range that includes the Project Board and the laser.

Like this one:
http://www.mouser.com/ProductDetail/Infineon-Technologies/IFX25001TC-V10/?qs=wK%2boHS4yu54Oxpbr8cfNnA==#.Vg1ewCnJkAA.email

It would be nice to solder an appropriate LDO to the optional LDO pads and run the Propeller and the laser from it, but I'm guessing that there will be... trouble... because the LDO pad's input is connected to the board's VIN, which would apply 28v to some of the rest of the board (such as the onboard voltage regs).

I think I'm asking the question "What are the constraints when using the optional LDO?" The documentation doesn't seem to specifically address this, beyond saying that the LDO's input hooks to VIN, and that the board's maximum VIN is 16v. (I also note with interest that there's an overvoltage LED.... what other overvoltage provisions might be present?)

Failing this, I could add some power terminals and an appropriate LDO to the proto board space, then connect the output to a handy VIN terminal.

One like this, perhaps:
http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/KA78R15CTU/?qs=jgLoCfU%2bJVmIQ1IEpU3PmQ==#.Vg1c3FV6Ptw.email

Any gotchas with this approach that I should be aware of?

Any and all comments and suggestions welcomed.

TIA,

Doug

Comments

  • Cluso99Cluso99 Posts: 18,069
    Without reference to the actual board you are using I make these comments.

    You should use a switching power regulator circuit to drop your voltage down to a usable input voltage for Vin. One such regulator comes in a 3 terminal package and 78xx pin out and does not require a heat sink. IIRC it's a Recon? 78E05 5V 0.5A output. Without checking the circuit I cannot say if this input voltage is adequate for your board. Of course it's not going to power your laser rangefinder.

    An LDO dissipates a lot of heat when regulating a high voltage down to a low voltage. For example, presume the regulator has an input of 10V and outputs 5V. Presume a current of 100mA (0.1A). The regulator will actually need to drop 5V at 0.1A which is 0.5W. That's easily done with a decent sized regulator such as the TO252 package. However, if your current is 0.5A it will be 2.5W. Now if your input voltage is 15V and 0.5A that's 5W and some considerable heat would be generated!

    A switcher doesn't waste the power in heat. Hence you will use less total input power, and your batteries will last considerably longer.
  • Mike GreenMike Green Posts: 23,101
    edited 2015-10-01 17:13
    Do not apply more than 16V to Vin! At the very least you'll blow out the input filter capacitor and perhaps damage the switching regulator. You could add the power terminals and a 16V LDO regulator to the proto space, but you'll waste a lot of your battery's power (28V - 16V = 12V x [power draw] all turned into heat). At the very least, get a switching regulator that's rated for well over 28V input and puts out maybe 12V. The rangefinder and the protoboard will run happily off that.

    Here's one datasheet for a switching regulator that can handle up to 40V input and outputs 12V at 3A:
    from DigiKey
  • GdungeGdunge Posts: 3
    Please excuse my very late reply to this thread.

    Thanks for the advice - I will refrain from applying 24V to Vin, and I will use a switching regulator.

    It turns out I've used switching regulators before. Here's the one I'm familiar with:

    http://www.mouser.com/ProductDetail/RECOM-Power/R-78C12-10/?qs=waQl70lBfV2GPfbg5I2b2A==

  • Cluso99Cluso99 Posts: 18,069
    @Gdunge
    There are other versions of these that are only 0.5A and lower output voltages. I use a 5V version.
  • GdungeGdunge Posts: 3
    Cluso99 wrote: »
    @Gdunge
    There are other versions of these that are only 0.5A and lower output voltages. I use a 5V version.

    Do you need to take any measures to reduce EMI (on input or output) for the RECOM switchers? They sure look tempting! TRACO and CUI also make similar parts but they need filtering. RECOM claims you usually don't need it.

    Thanks again!
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