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+5V from +35V

Discussion in 'Electronic Design' started by Marco Trapanese, May 9, 2008.

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  1. Hello,

    I want to obtain supply logic (+5V) from a main power supply (+24V -
    +35V DC). The line is quite dirty because it's used by several stepper

    I'm looking for a cheap and very low-count components way to regulate
    the logic supply (Iout < 50 mA).

    I tried with a resistor divider but the power rating needed is very high
    (> 2W) for the series resistor. A DCDC converter would be nice but they
    are expensive if no external components are needed, or cheap but they
    will require several components.

    There are a smarter way to do this? Also a filter against spikes will be
    very appreciated... :)

    Thanks in advance!
    Marco / iw2nzm
  2. Jacques

    Jacques Guest

    TO-220 package with a small heatsink
  3. Jacques ha scritto:

    Are you sure?
    If I have 35V on the input and 5V on the output the 7805 will have 30V
    across both terminals. With an output current of 50 mA it should
    dissipate 15W. I'm afraid the small heatsink can't help enough...

    About the motor spikes? Would you connect directly the regulator to the
    main power supply?

    Marco / iw2nzm
  4. Guest

    Don't top post. And the 7805 isn't a good choice to hook up to a dirty
    supply voltage that can run up to 35V and spike quite a lot higher.

    The adjustable LM317HV can take a 56V volt differential, as can a
    number of three-lead 5V linear regulators designed for the automotive
    market. Dropping 50mA into a 5V rail from a 35V rail implies
    dissipating 1.5W in the regulator. A 750mW 300R series resistor could
    halve that, while still leaving 4V of headroom when the supply was
    down to 24V.
  5. Marco Trapanese ha scritto:
    Pardon, it's 1.5W indeed.

    Marco / iw2nzm
  6. 30 x .05 = 1.5
    7805 is specified at 35 V in max, for 5 V out.
    Else add a LM317 as pre-regulator to say 15 V, it will take the spikes.

    35 V - LM317 - 15 V - 7805 - 5 V
    Fewer watts and fewer volts per IC.
    Or 2 x LM317.
  7. Phil Allison

    Phil Allison Guest

    "Marco Trapanese"

    ** Use a damn 16volt, 3W zener to fed the 7805.

    Cheaper than chips.....

    ....... Phil
  8. Hi Marco,
    ask google about roman black converter. I guess this will meet your needs.

  9. Marte Schwarz ha scritto:

    Thanks to you too!
    Marco / iw2nzm
  10. Spehro Pefhany ha scritto:

    Marco / iw2nzm
  11. mpm

    mpm Guest

    Spehro beat me to it. The National part would have been my
    recommendation as well.
    You might even get the 2575 for the same price? Little higher current
    I have seen them very competitively priced in the past.

    For the TO-220 5-pin package, I've found the Avid 7136 heat sink is a
    really nice compact fit.
  12. mpm ha scritto:

    Thanks for the tips!
    Marco / iw2nzm
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