Parts availability

Discussion in 'General Electronics Chat' started by jimbarstow, Nov 13, 2011.

  1. jimbarstow

    Thread Starter New Member

    Nov 8, 2011
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    I'm trying to build a water tank depth sensor. I started with the design in the Practical Arduino book but decided to go with a design on the Freescale site.

    I started to put together an order for the parts from digikey but discovered that for some parts they had a minimum order quantity of 2000.
    I tried other sites (mouser, allelectronics) which seem more geared toward non-industrial supply but they didn't have what I needed. (0.2
    mf ceramic capacitor, for example.)

    Any suggestions on where to go for parts? (I'm in California.)
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    please post all the parts that have that minimum order quantity at digikey. preferably the digikey link.

    are you sure you mean 0,2 milli Farad? Why does it need to be ceramic?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    I think it's more likely to be 0.2uF, or 200nF. You might use two 0.1uF/100nF caps in parallel.

    However, you really need to reference exactly WHICH design you went with on the Freescale site:
    1) Document number, such as AN1950.
    2) Page number where parts list is.
    For example:
    AN1950
    http://cache.freescale.com/files/sensors/doc/app_note/AN1950.pdf
    Figure 2, page 4 is where the parts list is located for the circuit in question.

    or:
    AN1950
    http://cache.freescale.com/files/sensors/doc/app_note/AN1516.pdf
    Figure 4, page 4 contains parts list and schematic
     
    Last edited: Nov 13, 2011
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I have seen mF or even MF (LOL) or MFD written for microfarad instead of μF. A long time ago that might even have been normal practice, buy nowadays its better to stick to the international conventions for prefixes.

    As to the capacitors themselves, it might be worth checking whether their value is critical. If not, you may find that you can use an alternative value such as 0.22(μ?)F.
     
  5. jimbarstow

    Thread Starter New Member

    Nov 8, 2011
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    The specific article is:

    http://cache.freescale.com/files/sensors/doc/app_note/AN1324.pdf?fsrch=1&sr=1

    The documents on the freescale site were actually very useful. There was a discussion about different types of amplifiers to use for pressure sensors and they said there was very little difference. The said the precision of the resistors had a much bigger effect than the particular design.

    Should have been clearer about the capacitance and used the right symbol; it is 0.2 μF. I'd figured I could use 2 0.1μF capacitors since they are more available but I didn't want to give up yet.

    The other part which I'm having trouble spec'ing is listed as a "trim resistor". The article doesn't specify directly what the value is but another freescale article that talks about the same circuit says it is "nominally" 39.2K. Does this mean it is the maximum resistance so a 50K trim resistor would be sufficient?
     
    Last edited: Nov 13, 2011
  6. SgtWookie

    Expert

    Jul 17, 2007
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    What I would do is use these 0.1uF caps for both C1 and C2 (read below):
    http://www.mouser.com/ProductDetail...GAEpiMZZMsh%2b1woXyUXjzKM/Br94TWB8oLGKWrIMjg=
    0.1uF caps are probably the most common caps on the planet; they are made in such vast quantities that the cost is very low.
    I suggest buying 100 of them; that represents a $4.00 investment, so your per-cap cost is $0.04/ea. If you are going to build more circuits using ICs, voltage regulators, etc. you will need one of these PER IC, so you will use a lot of them.

    If you REALLY insist on using single 0.2uF caps instead, you can use these:
    http://www.mouser.com/ProductDetail...GAEpiMZZMsh%2b1woXyUXjwarXwENlZE5C2MmnSjM5Hk=
    However, the lead spacing is 10mm instead of 5mm, and they are $1.42/ea.
    So, that's $2.84 for two caps, or $4 for 100 caps. You choose.
    As an alternative, you could use this 0.22uF poly cap:
    http://www.mouser.com/ProductDetail...iMZZMsh%2b1woXyUXjyIv%2bSUPGiajNojKQY3%2bckE=
    They are $0.16/ea.

    HOWEVER!
    C1 and C2 are input and output caps for a 78L08 8v 100mA regulator.
    While no external components are necessarily required for proper operation, unless you have at least 10nF/0.01uF capacitance on the output, it may become unstable (consider that to be very likely) and oscillate at high frequency. So, I suggest that you use 0.1uF/100nF from OUT to ground, and ~0.33uF from IN to ground; the caps placed as close to the regulator as possible. This is consistent with most manufacturer's recommendations for I/O caps for the 78xx series of regulators.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    From page 4:

    39.2k being the nominal value means that it is usually the appropriate value for the circuit.

    If you want a ~±5% range of adjustment for zeroing, you can use a 300k fixed resistor in series with a 1MEG potentiometer that is wired as a rheostat (connect the wiper to one of the ends; thus reducing the pot to only two connections). However, you should use 2.5% or better specification resistors. I suggest that you use metal film resistors, as they are more stable and less noisy than carbon film resistors.

    The other quoted option is not good; you don't want to reduce your CMRR if you can help it.
     
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