Electronics > Beginners
limit current of USB power to 2.5A
purezerg:
Hi, ok let me explain.
I have a camera that uses firewire 12v to power. it states that it uses 10w. but the problem is it is 12v.
I am thinking of using USB power to boost from 5v to 12v. I have tried those USB boost modules. it seems fine.
the question is how to build a circuit to limit it's current draw to 2.5-2.8a
I have tried hooking it up to a 5a 5v power supply. it seems for the first few mins it draws as high as 3.2a
I am hoping to power the laptop via the USB powered port.
the problem is the port shuts down at 3.0A
james_s:
If you limit the current you are supplying then you won't have 5V anymore, you don't get something for nothing. You have to get the load to draw less current.
ejeffrey:
There are basically 3 possibilities:
* your input voltage is < 5V. This is actually pretty common, at the very least you get IR drop across the cable wires. So you may have only 4.75 V at the input under full load which will increase the current draw requirements. Measure the voltage at the input to the boost converter and if it is too low try a shorter or heavier gauge USB cable.
* your device power draw is > 10 watt. You can easily measure this with an external 12 V supply.
* Your converter is too inefficient. 10 watt is 2 amp at 5V. If your converter is 90% efficient that would be ~2.22 amp input draw. If 75% it would be more like 2.66A. If the input voltage is 4.7 and the converter efficiency is 70%, then you would need to supply 3.1 A. You should be able to make or buy a boost converter with >85% efficiency, but you could easily be doing a lot worse than that.
Figure out which of the above is your problem or problems and go from there. If the problem is your converter efficiency that is easy to fix. If it is the output voltage of the port or the peak draw of your load, you are probably screwed.
purezerg:
1) i have tried with a 12v 3a power supply. it can draw as much as 17w if there are batteries that are not fully charged inside the battery.
2) without any batteries inside. it will draw 9w
3) with full charged batteries, it will also draw 9w.
4) when the camera is active, it will draw 12-14w
5) i do know that 12v @ 0.5A will also work because most macbook pro firewire provides only 0.5a. the powerspike is handled by the batteries.
6) the droop measures as low as 4.91v at the end of 4.5m cable @ 2.5a
7) i dont have a 12v dummy load to measure the efficiency of the boost module. but i do have 2 modules. the good stuff and the cheap stuff. both exceeds 3.0A
8) i have a LMxxx something module that bucks. i tried a 12v3A to 5v. the good module draws as high as 3.2-3.3a. the cheap module draws 3.6a.
for those wondering. the camera is a phaseone medium format camera.
I am wondering if I do a 1.8ohms 8x 2w parallel circuit. would it work?
purezerg:
Navigation
[0] Message Index
[#] Next page
Go to full version