Cannot get this 12 V to 30 V step up working!!

Discussion in 'The Projects Forum' started by hrstar24, Nov 16, 2011.

  1. hrstar24

    Thread Starter Active Member

    Apr 7, 2009
    32
    0
    So here is the data sheet for the step up converter I am using:

    http://www.sparkfun.com/datasheets/IC/MC34063A.pdf

    The circuit I am trying to build is on page 5. I have double and triple checked to make sure everything is connected correctly. For some reason the diagram doesn't have units on some of the components, so here is what I plugged in:

    .22 ohm for Rsc. For this, I placed four 1 ohm resistors in parallel. Each resistor has a 1/4 W power rating, is this a problem here? Do I need a higher power rating?

    180 ohm for R at pin 8

    330 pF for Co

    100 pF for cap coming off Vin (12 V) at pin 6.

    For the diode, it looks like a zener but the number code is for a Shottkey diode. I am using a Shottkey diode that has a higher voltage and current rating than the one shown.

    Also, I am using a 180 uH inductor, not a 170.

    First of all, am I assuming the right values for the caps and Rsc? Really wish they listed the units.

    Secondly, here is what happened when I turned on the circuit. Vout started at 12 V, and slowly dropped to 0, and the chip started getting really hot. I have plenty of chips to spare, so I am not worried if I burn a couple out. I have replaced the chip, still nothing. Any suggestions?
     
    Last edited: Nov 16, 2011
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    330 pF for Co

    should be 330 uF

    100 pF for cap coming off Vin (12 V) at pin 6.

    should be 100uF

    Cap type used should be solid Tantalum, low ESR designed for switcher use. There are also some aluminum electrolytics usable at 100 kHz switch frequency, do NOT use generic aluminum caps they will not work.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Co is supposed to be 330uF. That's 330,000nF, and 330,000,000pF. Just a bit off.

    The "100" cap at pin 6 is 100uF, or 100,000,000pF.
    Both of these caps should be a low-ESR type.

    It's "Schottky" - it's actually named after German physicist Walter H. Schottky (b.1886-d.1976).

    Can you tell us what your diode's part number is?

    180uH is OK, but do you have a part number for your inductor? If it's about the size of a resistor, then it's really not designed to be used as a power inductor.

    If units are not shown for a resistor, assume that the value is in Ohms.
    If units are not shown for a capacitor, assume the value is in uF (microfarads).
    If units are not shown for inductance, I'd use uH - but one seldom sees inductors without the unit specified.
    [eta]
    Bountyhunter beat me to it - but this is his specialty ;)
     
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