Re: Problem applying a mosfet

default wrote:
mosfet in the circuit. Mosfet is IRLZ24 type, N channel in
the ground or minus leg of the camera.

The little darling in question is a Saitek "Pocket Cam X" I opened
it up soldered wires to the battery tabs and shutter button and ran
them out the case to a connector.

Intent is to use a pair of D batteries external to the camera. No
external power jack except it will accept 5 V via the USB cable -
but that wastes a lot of power.

No, this baby and their megacam power up and initializes when
batteries are put in - the reason there's so much radio control,
aerial photo, stuff on the web for hacking into it - and the low
price.

I'm running a battery life test on it now it has taken 257 pictures
and still counting over 2 days on 3 AA NiMH cells. Camera takes ~7.5
seconds to power up and get straight, shutter actuates, 6.5 seconds
for pix to go into memory, then every thing goes to sleep until the
controller turns the mosfet on again.

Goal is 900-1200 pictures over a week during daylight for time lapse.
Time delay per picture adjustable over the range of 1 minute to 8
minutes per pix.
--

That's the Aiptek PocketCam X, right? Very interesting
behavior. What are a few good websites to learn more
about it? It sounds quite useful for making a smart
uP-controlled time-lapse camera, or a trail camera for
shooting wild animals. We have a coyote family active
in our backyard that we'd like to know more about. I'd
like to understand a bit more about what they're up to.

Here's a table of some of the logic MOSFETs in my stock
drawers, with your part at the top for comparison.
(Id* is the maximum mosfet current for Id=100C.)

Id* Rds @ Vgs
part Vds A m-ohm
------- --- --- --- ---
irlz24 55 13 105 4.0
fqa33n10L 100 23 43 5.0
irlz44n 55 33 35 4.0
irl3715L 20 38 15 4.5
irl2505 55 74 13 4.0
fdp7045L 30 75 4.8 4.5
fdp8030L 30 - 3.6 4.5

As you can see, there are far better parts to choose from,
than the one you picked. Sorry! However, none of these is
specified for operation at 3.0V gate voltage, let alone 2.0V,
etc., end-of-life battery voltage. Looking at the fdp7045L's
data*** plots, we can estimate Ron might increase to about
10 m-ohms at Vgs = 3V, and perhaps 20 or 25 m-ohms at Vgs =
2.5V, but below that the on-resistance climbs dramatically.

One good solution for you is to create a small gate-voltage
power source, which would say double the battery voltage,
for the low-power purpose of running the mosfet's gate, and
perhaps a few other things that need more voltage. A part
like the 7660 can accomplish that task in one step, from
voltages as low as 1.5 volts. From $1.30 at DigiKey.
.

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