Re: Switching transistor question
- From: miso@xxxxxxxxx
- Date: Sun, 01 Jul 2007 14:04:38 -0000
On Jul 1, 5:21 am, default <defa...@xxxxxxxxxxxxx> wrote:
On Sun, 01 Jul 2007 05:45:14 -0000, m...@xxxxxxxxx wrote:
Snip
I'd go for a FET.
You may want to drive the gate with a slowly rising signal rather than
just whacking it. There is always a possibility that the device being
fed by the fet will latch up IF it is normally turned on with a switch
on the device itself. In a sense, you are turning on the device in a
manner the designer didn't expect.
Typically a chip road test will catch such problems, but not every
manufacturer tests their chips in such a manner.
Yes, a mosfet seems tailor made. Most of the ones I have handy
require about 4.5 volts to turn on - which was what was stopping me
from using them. Way lower drop than any bipolar so it is the obvious
winner. But the ones I found were $2.50 each / $9 shipping. I paid
$9 for the cameras I'm using. One of my criteria is to keep the cost
low because there's a good chance I won't recover all the cameras I
deploy.
The Subconscious Engineering Department must have been working on this
problem overnight, because this morning I have a slick elegant
solution that doesn't require buying any parts. The picaxe 08M has a
PWM output function programmed into it, actually has two. I can use
that to toggle a voltage doubler with a few small caps and diodes and
switch the FET on. Eureka!
I'm using only two, out of five, I/O pins. So there's still hope for
using a servo to pan the camera and passive IR to trigger action.
(something I may want to add once I get one or two doing what I want).
I am taking advantage of the fact that the internal processor for the
camera is turning the camera on when batteries are inserted - That way
I can over-ride the 40+ second automatic power down the camera has and
only keep it on for 15 seconds.
As for the idea of ramping up the power to the processor inside the
camera (that what you were saying?) that seems like a bad idea. For
a time - the supply would be sitting on the cusp of good/bad power.
Anyhow it appears as if the camera designers already thought of that -
they monitor battery charge and if the power comes on with a low
battery the camera doesn't come on but just alerts you that the
battery is low. There appears to be about a half second or so when
this is going on during power up - screen takes awhile to initialize
and you can't snap a picture until it is done.
Another thing that makes me skeptical - I used a few surplus remote
controls to provide a signal for break beam infrared detectors -
cheaper and easier than building the signal source - the ones I was
using definitely didn't like coming up slowly - they would latch up.
(I had some idea to provide power during momentary outages via a
"super cap" which, when it charged, kept the supply from coming on
quickly and that caused a latch up)
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Latch-up is more likely from a fast rising voltage. I'm thinking of a
few hundred microseconds, not miliseconds. Yes, part of the road test
for chips is a slow turn on, i.e. a few hundred miliseconds. You
really should be able to apply DC at any voltage and no have the chip
latch or behave strangely. The problem with your design is that it is
probably full of cheap ass Chinese parts, so the parts themselves may
not be very robust.
The power fet turn on is a real life problem. Some designers don't
trust the shutdown pins n chips, and like to waste money by using
series P-fets because it is something they control. ]In all fairness,
perhaps not every chip in the system has a shutdown pin, so they
decide to just power off entire sections of circuitry with the pass
fet.
Latch-up testing, generally only done on the initial design, is done
with the power supply applied. That is, the chip is powered and you
push/pull the pins other than supply and ground. Thus most QA
procedures don't test powering up the chip via a pass fet.
.
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