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USB charger

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Zanders

New Member
Hello everyone, first of all I have to apologize because of my English, it's still my second language.

Because i have to travel a lot i could use a USB charger, to charge my mp4 player. Because USB output voltage is 5V, i had an idea to use 7805A semiconductor.

I would use 9V battery as a voltage source and 7805A circuit as a voltage converter, Do you think i need heat sink ?

And also what do you think about this project ?
 

MikeMl

Well-Known Member
Most Helpful Member
Depending on your level of experience, a buck switching regulator is a better choice. A good switcher is >90% efficient, and will work even as the battery discharges to just above 5V. A SEPIC switcher will deliver 5V even as the battery sags below 5V.

The linear 7805 regulator will quit working when the battery reaches the drop-out voltage of about 6.6V. With a fresh battery, 40% of the battery power is being dissipated as heat.
 
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Sceadwian

Banned
How much current are you drawing from the the 5V?
 

Zanders

New Member
MikeMl: could you give me any part numbers ?

Sceadwian: USB current is 100mW-500mW, i have to take measurements.
 

Sceadwian

Banned
By strict USB specs you can't guarantee to get this on every machine. USB is only 'required' to give 100ma's per port in hardware. Technically the USB device needs to request for more power beyond that.
 

kchriste

New Member
Forum Supporter
It would be smarter just to use 4 AA, C or D cells. The charger will probably work fine on 6V. If not, put a diode in series.
 

Sceadwian

Banned
It MAY work fine on 6 volts. USB chargers tend to feed single lipoly cells. If a charger uses a linear regulator from the incoming to the charge circuit you could overheat the regulator. Many modern devices uses SMPS IC's though, and bucking 5 vs 6 volts shouldn't hurt.
 

MikeMl

Well-Known Member
Most Helpful Member

Sceadwian

Banned
I bookmarked that little gem =)
 
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