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Old 04-03-2013, 07:53 PM #1
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Default USB Powered 5mW Green Laser Pointer

Hi, I'm currently doing a project to power my 5mW green laser pointer using USB. I know that USB supplies 5V and maximum current of 500mA. My laser requires 2 AAA batteries to be powered so it gives a total of 3V. So, I decided to make a simple voltage regulator using LM317 circuit to step down the voltage from 5V into 3V. It works just fine. However, when I use the USB port from my school computer, it gives me warning message of "USB Power Surge on Hub" because it says I'm drawing current more than 500mA from it. 500mA is too much and it does not make any sense to me that I'm drawing current more than that. But, if it's true that I am drawing more than that, can someone tell me where is my mistake in my method? Thanks.


p/s: This is the laser I'm using. Amazon.com: 5mw 532nm Astronomy Powerful Green Laser Pointer - Black: Electronics
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Old 04-03-2013, 11:43 PM #2
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Default Re: USB Powered 5mW Green Laser Pointer

Everything looks fine to me. Does it still fail with that switch open? Can you measure the current, or does it shut off too fast?
The USB power rail might be limited to 200mA per channel, and 500mA total perhaps.
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Old 04-03-2013, 11:46 PM #3
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Default Re: USB Powered 5mW Green Laser Pointer

Any other devices on that hub?
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Old 04-04-2013, 12:42 AM #4
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Default Re: USB Powered 5mW Green Laser Pointer

the 317 disipates excess power as heat. you get 2.5w from usb and need no more than 40mw for the worst possible driver circuit in the pointer.
check out some of the maxim chips that use switch mode to drop 5v to 3 volt.
maxim do all sorts of power ic's to step voltage up or down with brilliant efficiency. they also kindly send you free samples from the website.
if you dont want to wait, grap a 555 timer and make an astable occilator with adjustable duty cycle. 1khz at 60% duty and a 1nf capacitor on the o/p
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Old 04-04-2013, 03:07 AM #5
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Default Re: USB Powered 5mW Green Laser Pointer

Quote:
Originally Posted by drspastic View Post
the 317 disipates excess power as heat.
You mean voltage. Voltage != power.

Quote:
Originally Posted by drspastic View Post
need no more than 40mw for the worst possible driver circuit in the pointer.
How did you arrive at that number?

Quote:
Originally Posted by drspastic View Post
grap a 555 timer and make an astable occilator with adjustable duty cycle. 1khz at 60% duty and a 1nf capacitor on the o/p
There is already a driver in the circuit. That may be successful at lowering the average power, but the max current will still be the same, and it may still trip with over-current.
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Old 04-04-2013, 05:15 AM #6
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Default Re: USB Powered 5mW Green Laser Pointer

USB devices (1.1 anyway) are supposed to only take 100mA of power or less, with 500mA allowed for the entire hub of 5-devices. If the hub is designed around this limitation, and doesn't allow one device to such all 500mA, it may be triggering that overload fault.

The ports on your home computer may be designed to provide more power without problem, and that might be why you don't see these problems at home.
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Old 04-04-2013, 08:12 AM #7
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Default Re: USB Powered 5mW Green Laser Pointer

I don't think it's a problem that it doesn't work at school, highschool or below that is. In college no one really cares, but in HS teachers will suspend kids for cheep red pointers, or take it away at the very least.

Just watch what your doing, k?
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Old 04-04-2013, 06:36 PM #8
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Default Re: USB Powered 5mW Green Laser Pointer

My laser works fine and seems perfect when it is connected to the USB hub. There are no other USB device used except for my laser. The problem is that the computer keeps saying I'm drawing too much current and it might harm the port. I wish I can measure which part of my circuit is actually using that much current, but I don't know how.
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Old 04-04-2013, 06:52 PM #9
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Default Re: USB Powered 5mW Green Laser Pointer

multimeter set to the DC milla amps section?
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Old 04-04-2013, 07:05 PM #10
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Default Re: USB Powered 5mW Green Laser Pointer

Quote:
Originally Posted by Cyparagon View Post
You mean voltage. Voltage != power.



How did you arrive at that number?



There is already a driver in the circuit. That may be successful at lowering the average power, but the max current will still be the same, and it may still trip with over-current.
1: we both know ohms law
2: your lucky to get 30% efficiency in the little pointers i think hes ripping apart, plus the cheap crappy driver circuit (or resistor) built in to the pointer no doubt ***** the rest of the forward voltage in excess from a 3v (no load) battery as heat. shuffle it about in your head you might get a similar figure.

maybe the op can post the current drawn from the pointer with 3v using a multitester. then we dont have to guess.

1khz smpsu will be smoothed to the mean set by the duty cycle once it hits the usb inteligent power monitor by the capacitor at source and the local cap plus the inductance of the lead.

my figures are arbitrary but im a good guesser.

truly the op should pull the diode driver and make a high efficiency smpsu with feedback controlling current, optical power, and thermal overload.
when i was at HS i didnt want to make any more work for myself than needed.
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Old 04-14-2013, 03:41 AM #11
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Default Re: USB Powered 5mW Green Laser Pointer

Quote:
Originally Posted by drspastic View Post
your lucky to get 30% efficiency in the little pointers i think hes ripping apart, plus the cheap crappy driver circuit (or resistor) built in to the pointer no doubt ***** the rest of the forward voltage in excess from a 3v (no load) battery as heat. shuffle it about in your head you might get a similar figure.
Practically all green pens <100mW will draw between 200 and 500mA. That works out to be a bit higher than 40mW
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