Electronics > Beginners

Need help with NPN and unknown load

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colt:
Hello! I am trying to reuse an old digital camera to make a long duration time-lapse rig. I was able to add some components to the camera so that when power is applied it automatically takes a picture. However, when I try to use an NPN bjt to switch the power the camera does nothing.  My circuit is pretty simple but I need help interpreting some unfamiliar waveforms that I'm getting on the scope. Here is my circuit.



When sw2 is pressed I expect the camera to turn on as the bjt would now create a path to ground. The camera does not turn on but instead at test point ch2 there is a voltage spike above the npn. I did not take a screenshot, but increasing the value of the gate resistor above 1.1k causes the frequency of the spikes to decrease.

sw2 100 OHM


Zoomed out


If I hold sw1 then I get the following waveform

sw1 1.1k


I am applying DC voltage where the old lipo connected to the camera. Could it be the charging circuitry that is causing the voltage spike?
Do I need to switch the power above the camera with a pnp?
Thanks!

magic:
My guess: the camera turns on, dumps a lot of current on Q1, voltage across Q1 rises, voltage across the camera drops, the camera turns off and tries again.
You need a bigger Q1, something like BD433 at least, or more base current, or both.
It's not unusual for compact digicams to take up to 1A peak.

Even better solution is to use a MOSFET. The ubiquitous IRFZ44N might do if its "typical" performance curves are anything to go by, if not, I'm not sure what popular equivalent "logic level" switching MOSFET might be, maybe somebody else will know.
MOSFET has the advantage of much lower voltage drop and no base drive, so higher overall efficiency. You may care about such things if it's battery powered.

colt:
Thanks for the explanation. I had a IRFZ40 laying around and that did the trick.

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