Re: Where to get SD Card reader

On Jan 1, 5:09 pm, "R.Wieser" <addr...@xxxxxxxxxxxxx> wrote:
Hello rickman,

No, in fact something is very wrong.  When you say that you
enlarged the bias crrent and added another diode "to account
for the lesser voltage drop per diode", that is a bit like a friend
who would say "I cut this board off three times and it's *still*
too short!"

Hmm ... I can only tell you what I observed.  When using a larger current
the voltage over the diodes dropped.  Same as with the parallel
stabilisation I also tried.

When you say larger current, was this the current through the diodes
or in parallel with the diodes? If it was through the diodes, maybe
you are using a tunnel diode with negative resistance...


On a system which defines its supply-voltage in tenths of a volt (the
MicroSD card) a drop of 0.05 volts is a lot, and 0.1 volts a full step on
its scale.

What is "a lot" depends on the spec. What is the voltage tolerance
stated in the spec? If you are talking about a 3.3 volt supply, they
are typically 5% and sometimes 10%. That would be 0.15 volts + or -.
Anything within this range is completely acceptable.


The voltage-change I observed was in that order.  Enough to make the card
switch itself off.

Exactly what voltage did you measure? Even if you are outside of the
spec'd voltage range, devices like this seldom cut off right at the
stated limit.

I am not recommending diodes to drop the supply voltage for a logic
device, but they should work in many applications. However, there is
nothing wrong with using a simple three terminal regulator. They are
cheap, small and simple.

One place where a diode is perfect for power supply voltage adjustment
is when using a CMOS switch as a voltage shifter. The supply voltage
on the switch chip needs to be above 3.3 volts and 5 volts is a bit
too much. 4.3 volts is about perfect to allow the switch to pass 3.3
volt signals while limiting the output from rising much above 3.3
volts. However, the chips I use have the diode on the inside.


Allso, the card itself draws less than a single mA when the memory is not
accessed, but can draw upto 50 mA when doing so (probably write-mode, the
data-sheets do not specify)

Are you familiar with zener regulator circuits?

I think so.

Oh, I read back and see a zener is what you used.

P.s.
Looking tru my stored info regarding the specifics to the current-draw I
noticed your name on a post from back in 25-11-2006 (3.3v <> 5v interfacing
@ 15Mhz) :-)

Yes, is that post relevant?

Rick
.

Quantcast