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Author Topic: sparkfun microsd shield needs usb power, won't work with 9v ac-dc  (Read 476 times)
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Hello,

I am having trouble writing to a microsd card using the sparkfun microsd shield wheen using 9v ac-dc adapter. The rated current of the adapter is 650mA. When I use power from the usb cable though, it seems to write. Any ideas?

Thanks

Wallace
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measure 5.0 V and 3.3 V under both usb power and 9v ac-dc. do comparison.
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With usb:
5v supply is 4.63 (and yet it works)
3.3v supply is 3.32
With 9v ac-dc supply:
5v supply is 4.99
3.3v supply is 3.32
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One of important rating of DC supply is ripple rejection rate.
With 9v ac-dc supply might has low ripple rejection rate, put big cap between +9 V and GND might help to filter out ripple.
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Quote
One of important rating of DC supply is ripple rejection rate.
A far more important rating is current. Perhaps your power supply simply can't output enough current. Of course, you've actually said nothing about the power supply except that it takes AC and outputs 9V. You haven't said how that is being used to power the Arduino or the SD shield.
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How much current does the sparkfun microsd shield require to write to the card I wonder? Once again, the supply is rated at 650mA. I would be surprised if that much is required.
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Once again, the supply is rated at 650mA. I would be surprised if that much is required.

That doesn't mean that the supply can provide that much current without problems. For a start, the headline number you see is just something printed on a label and there's no guarantee that the device does actually supply that much current. Secondly, you have no idea how the voltage level and smoothness varies as the supplied current varies. That 650mA could be the maximum current when the deice is short-circuited, or the point where it starts overheating. It's not safe to assume that it will supply the peak rated current while maintaining a smooth regulated supply close to the nominal voltage.
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