Electronics > Beginners
Think I figured out why I couldn't power a camera
lordvader88:
I have a AX500 (iirc) camera and it eats batteries. I tried to power it off a 12V 2A wallpack, that runs an LM317 adjustable regulator. So I feed it 3V, and it would only powerup for a moment, then shut off. I thought maybe door clip bridging the 2xAAs might have been hooked to something that needed 1.5V, so I tried that to no avail.
Then I measured the current draw, and what happens it the SMPS in the cam, that charges up a bank of caps to 320V, was drawing over 1.5A, and the LM317 is only rated for 1.5A. I didn't notice before that it was getting super hot in the few seconds the cam tried to start. So it's hitting it's thermal limit or current limit and shutting down.
So I will have to make a better 3V source and try again. I'll also charge some AAs, and measure the current on them. To me thats a crazy amount to draw from batteries, but it would explain why they die so quick.
I really want to look at its PCB. So far I took the case apart, but thats hard to do. I don't want break it tho just to map out the PCB for fun.
Brumby:
If your camera only draws 1.5A, then that won't be the limiting factor. The LM317 datasheet specifies 1.5A as the minimum, with a typical maximum current of 2.2A (for Vin<15V).
At 12V in, 3V out, the LM317 will have to drop 9V. At 1.5A that works out at 13.5W. While that is under the Pmax of 20W, you have another problem.
With a junction to case thermal resistance of 5ºC/W, you are going to have to have quite a decent heatsink to keep the junction temperature below the 180º limit where it will shut down. For an ambient of 25ºC, this means a figure of 6ºC/W or better for the heatsink and thermal paste. That 6ºC/W is a maximum and it still has the regulator running way too hot for my liking.
A better approach, IMHO, is to reduce the voltage drop across the LM317 by powering it with a 9V - or even better still, a 6V - supply. This will reduce the power dissipation to 9W or 4.5W respectively. You will still need a heatsink, but not as beefy as you would for 12V.
lordvader88:
I don't remember the number, maybe 1.6A, but it's enough to shut it down, yeah it's not heatsinked. I'll check again, this was July/August .
IDK what the voltage did, I'll check more, I have an LED the goes out under short-circuit, it stays on so I didn't suspect heat/current problem. But I also didn't remember that its a major boost -converter of some type in the camera, 3V to 320V, thats huge.
ArthurDent:
Watch this video.
MK14:
Terrible video.
You have to use the right battery type for a camera (and set the battery type in the menus if applicable and make sure the battery is not too run down, else it won't supply enough current). Some don't supply enough current, so the camera can't work properly.
This is a somewhat known problem, and why some sources recommend rechargeable battery types. As these usually have no problem supplying the necessary current.
It seems, the LM317 is (as you suspect), is just being overwhelmed by excessive current requirements, at times, and hence is not working properly.
You could try a higher current version of the LM317, such as the LM350T, with around a 3amp max output current limit.
Navigation
[0] Message Index
[#] Next page
Go to full version