# 5V USB to 12V DC-DC conversion?

Discussion in 'The Projects Forum' started by khmtambi, Jul 27, 2010.

1. ### khmtambi Thread Starter New Member

Apr 7, 2010
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hi all,

guyz i was wondering to make a 12v supply out of 5v 1amp usb. i know this can be achieved by using a voltage doubler or tripler. but the problem is of current. all i want is to power two 0.22amp 12v fans. which makes 0.44 amperes. will this effect my USB port? if it does what should i do to avoid the damage? or is there any perticular circuit using an IC?

2. ### mcgyvr AAC Fanatic!

Oct 15, 2009
5,063
1,099
Why not just use and extra 12V plug from your computers power supply? Computers usually have a few extra 12V plugs

3. ### tom66 Senior Member

May 9, 2009
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Generally, you cannot damage a USB port by overcurrent, due to the built-in overcurrent limiting of most USB ports. This usually consists of measuring the overall bus power and seeing if there is an overcurrent condition.

Let's do some ballpark math. A 5V USB 1.1 output can supply ~5 watts total power (it's a bit more for USB 2 & 3). A fan 12V @ 0.22A uses = 2.64 watts. That is fine then. If you use an average efficiency of 80% for your converter then you consume 3.3 watts.

You cannot use a voltage doubler or tripler. Those circuits are for AC only. Instead, consider a circuit using say the MC34063A DC-DC controller. Here is a tool which will help you design for it: http://www.nomad.ee/micros/mc34063a/index.shtml

Enter 5V input, 12V output, and 250mA (go up a bit from your rating) to get the desired output. Usually you don't need to worry about ripple, enter 100mV(pp), and for Fmin I recommend 30kHz as that is inaudible.

You could also consider other ICs as at 250mA the chip is reaching its peak limit of 1500mA (remember when switching the current is about 5x higher in pulses.) You could also use external transistors.

May 11, 2010
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I believe that USB standard is to provide 100 mA unless the port user negotiates for a higher current by some kind of handshaking. That's off the top of my head and I never used it. Can anyone else confirm it? Although many systems may be overbuilt and simply handle up to 1A all the time anyway.

You don't have enough power anyway for 2 fans on the same USB because they'd be more than 5W, which is the power available at 5V 1A, let alone conversion loss.

But anyway, the short answer is simply that if you have 5V 1A, you can pretty easily convert that to 12V using a boost converter. You can find either a tiny circuit board with the whole system or build it custom with your own coil and SMPS chip... I would not go with a charge pump system like what's normally called a "doubler" or "tripler" because of th current levels. Charge pumps are better for low current levels.

5. ### tom66 Senior Member

May 9, 2009
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@sage.radachowsky: That part of the standard is rarely implemented. A USB port can actually provide as much as 1.5A, and sometimes even 2A without negotiation or setup. This is because external HDDs, FDDs, CD/DVD drives etc. draw lots of current.

Of course you cannot draw 1.5A from all the ports. You can probably draw 2A to 3A in total.

@OP: if you were talking about charge pumps you might be out of luck. You could build one with a few op-amps (like I did) _but_ supplying any more than about 200mA is difficult because the capacitors often can't take the pulses of upwards of 5-6 amps... they would just explode.

6. ### Wendy Moderator

Mar 24, 2008
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Actually voltage doubler and triplers are DC out, not AC. They use AC to pump up the capacitors, which means you need an oscillator such as (dare I say it?!?) a 555.

To the OP, the 555 reference was a joke, but it is also how I might do it. It couldn't do the amps though, so more would be needed. I tend to be the guy who designs with a 555 for everything around here. If you don't know a 555 is a very common chip, a kind of swiss army knife of electronics.

Personally I think adding a extra connector feeding out of the case for +12 is the best answer. It would be all off the shelf parts, no major mods needed.

7. ### tom66 Senior Member

May 9, 2009
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@Bill: Use a push-pull driver. I was able to simulate a 1A (!!) charge pump increasing 10V to about 18V, using a 555. At this current level, the capacitor was passing about 6-10A in pulses, which is difficult even for top of the line caps to do.

The disadvantage of a 555 timer is the lack of feedback. It is difficult to control the output so it follows a voltage.

My op-amp idea consists of using an op-amp as a comparator and an op-amp as an oscillator. They drive an AND gate which creates a primitive switching power supply PWM signal (not really PWM though... more like just plain 'ol pulse modulation.) It's possible to build a buck, boost, buck-boost, flyback and charge pump power supply, possibly even some others, with just a few modifications.