Booster circuit USB

Hi,

I seek a device/circuit on the market that can boost 5Volts from an USB port to 20V DC.

Please advice if there infomation missing in my description. Thank you.
csharp_learnerAsked:
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SteveConnect With a Mentor Commented:
As advised in the first post, it's not gonna happen. Only way would be to combine the output of several usb ports like some cd writers do.
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SteveCommented:
boosting the voltage will greatly reduce the current supplied and put a strain on the USB power system.
I've had a rummage around and cannot locate one online and I'm not sure you will.

USB power is quite low and not designed for anything using a lot of current. Any increase in voltage comes at the cost of a decrease in current further meaning the supply will be quite weak.

What do you need to power?
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csharp_learnerAuthor Commented:
i need a 5V from an USB port and boost it to Max 20V then go into my circuit with a variable resistor to control the output voltage with 500mA.

Is this possible?
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☠ MASQ ☠Commented:
The single USB 2.0 standard port can deliver a maximum of 500mA and is rated at 5V (+/- 0.25V) DC - the system monitors the current used to prevent damage to the hub so no you wouldn't be able to transform the voltage up to 20V DC usefully.
This might help you get close though but whehther the power output is actually useful after stepping up the voltage is questionable.
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csharp_learnerAuthor Commented:
is there is no such product in the market now?
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☠ MASQ ☠Commented:
Not really for the reason in my last post.
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DavidPresidentCommented:
just google DC to DC converter.  this is a simple off the shelf thing.  you may have to add variable output.  probably only a few bucks in volume
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wyliecoyoteukCommented:
as stated above, no, you can't do this.
5v @500mA would give you a maximum of 125mA @20V, and even then, you might actually blow the USB controller.
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DavidPresidentCommented:
Right wylie - but only defense is that I was responding from tiny cellphone screen and didnt read anything about output current requirements.  

csharp -- if you can figure out how to step up voltage, but keep output current the same, then you'll become a gazillionaire.  
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dbruntonCommented:
Not possible if I understand what you want.

You want the output from a USB port of 5 volt 500 mA to be boosted to 20 volt 500 mA.  That's not possible as MASQUERAID has stated.

You are asking for 4 times the power output from the USB socket without allowing for circuit inefficiencies.

It also seems you want to alter the voltage from 5 v to 20 v.

Usually in a circuit like that you'll start with a higher voltage say 30 volts and use the variable resistor to cut it down to the voltage required.

For example here is one http://www.electronics-project-design.com/voltageregulator.html
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evilrixSenior Software Engineer (Avast)Commented:
The formula for power is P = IV, there for V = P/I and I = P/V. In other words (as you've already been told) since voltage is inversely proportional to amps for a constant power rating you absolutely cannot increase both without an additional power source.

http://www.kpsec.freeuk.com/power.htm

This is where powered USB comes in.

http://en.wikipedia.org/wiki/Powered_USB
http://poweredusb.org/whatispusb.html
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