Calculating resistors....
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Tibia
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Calculating resistors....
I'm making a small three or four USB port charger. I've got a 7805 from something, how much resistance do I need to knock it down to 500 mA for the USB ports? Also, is it better to use one, straight on the 7805, or three, one on each USB port?
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Adding a resistor will knock the voltage, not the current. You don't need to limit the current, because electronics only draw what they need.
If you have 3 devices running at the same time, each drawing 500mA, then that would be 1.5A. I'm pretty sure a 7805 can supply that much, so you should be fine with one for all three.
If you have 3 devices running at the same time, each drawing 500mA, then that would be 1.5A. I'm pretty sure a 7805 can supply that much, so you should be fine with one for all three.
Umm, noMario wrote:Adding a resistor will knock the voltage, not the current. You don't need to limit the current, because electronics only draw what they need.
If you have 3 devices running at the same time, each drawing 500mA, then that would be 1.5A. I'm pretty sure a 7805 can supply that much, so you should be fine with one for all three.
A resistor will do both at the same time. the resistors RESIST current at low resistance and at high resistance they restrict voltage and current. It is a good idea to limit the current though, if something goes wrong, things are less likely to blow up.
Most 7805's are designed for 1 amp.
USB charger = something hat an charge USB connect mobile phones etc, right?
You don't need to "knock down" the current with a resistor - just make sure the circuit is properly designed to handle the currents you want it to handle.
The USB standard dictates that every USB port must be able to supply 500 mA if it's a USB 1.x port and 2 A if it's a USB 2.0 port. Not all utilities will draw that muh current, but something that's charging a battery will likely nearly max out the port.
As Gordon1 points out, most 7805 are speced at 1A (Possibly more with a proper heatsink) Thus I recommend you to use one 7805 for each port you want to add. But you'll probably hit the limit for the PSU you're using before you hit the limit for 3-4 ports.
You don't need to "knock down" the current with a resistor - just make sure the circuit is properly designed to handle the currents you want it to handle.
The USB standard dictates that every USB port must be able to supply 500 mA if it's a USB 1.x port and 2 A if it's a USB 2.0 port. Not all utilities will draw that muh current, but something that's charging a battery will likely nearly max out the port.
As Gordon1 points out, most 7805 are speced at 1A (Possibly more with a proper heatsink) Thus I recommend you to use one 7805 for each port you want to add. But you'll probably hit the limit for the PSU you're using before you hit the limit for 3-4 ports.
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Tibia
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Yeah, it's so I can charge my MP3 player without a computer. I plan on just pulling the data pins right out of the USB ports I use.
I've got an 18V wall wart for use in the home and a 12V car adapter I'll be using. I just want to be damn sure I don't fry anything in the event of a surge. I've also decided to just use two USB ports to keep it small. Should I still use two 7805's?
I've got an 18V wall wart for use in the home and a 12V car adapter I'll be using. I just want to be damn sure I don't fry anything in the event of a surge. I've also decided to just use two USB ports to keep it small. Should I still use two 7805's?
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No! That's not how it works. The device you're charging won't magically feel the current limit which the 7805 can handle and adapt to it. It will draw the current it wants, and if the 7805 catches fire, so be it.Mario wrote:You can use 2 7805s if you like. If you use one, you'll just get slower charging times.
Also you can't use two 7805's in parallel for a single device. Because of how it works, only one of them will actually be used. (You can still use one for each port without problem.)
Another consideration is that a 7805 will generate more heat, the higher the input voltage. While a 7805 can usually handle 1 A if the input voltage is 9V, it might get a little hot at 12 V, but if you try use 18 V as input voltage, it will likely catch fire if you draw 1 A from it. That's nthe downside of a linear regulator.
I recommend the following: http://www.dimensionengineering.com/DE-SW050.htm
It's a replacement for the 7805, but it's a switching mode regulator rather than a linear one. This means that it doesn't have the heat problem I described above, and it can be put in parallel.
Buy one or maybe two of those and put in parallel. Then feed your three USB ports with that.
Good luck!
This is a pretty common problem.
My bad I was actually thinking of LM317's
http://cache.national.com/ds/LM/LM117.pdf
refer to page 22 where they use 3 LM317's in parallel with 0.2ohm resistors on the outputs.
And then look at all the other circuits... Some interesting plans for LM317 based switching regulators.
http://cache.national.com/ds/LM/LM117.pdf
refer to page 22 where they use 3 LM317's in parallel with 0.2ohm resistors on the outputs.
And then look at all the other circuits... Some interesting plans for LM317 based switching regulators.
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