Hi.

I guess this question comes around from time to time. I've done a bit of searching, but I did not find anything I could use.

I'm working on a project that is using 12vdc. Now I want to use the USB port of a computer, to give power.

But the USB is only giving 5vdc. How can I step up to 12vdc.

It's for my 555-timer and LED project.

 

You will need a DC-DC converter.

A desktop or tower computer can output up to 500mA current at 5v from the USB port.

Laptops output less; your mileage will vary.

The conversion from 5v to 12v will not be 100% efficient. However, some conversions get pretty close; maybe 90% to 95%.

Let's say you find a 92% efficient DC-DC converter.
Let's also say you have a laptop with a maximum 300mA output from the USB port.

300mA x 5/12 = 125; times 92% = 115mA available.

Since you haven't posted a link to your other project, I have no idea what your current requirements are.

 

Hi.

Thanks for the quick reply!

It's for lighting some LEDs.

Source volt = 10
Diode forward volt = 3.3
Diode forward mA = 20
Number of LEDs = 100

According to this calculator it says I need 680mA. So it is out of the question to use my tower PC?

 

You won't be able to use the USB port to power it, but you could use a "Y" adapter to get 12v and 5v right from the power supply. You can get those adapters at any store that sells computer hard drives.

It depends upon how much power your PC's power supply is rated for vs what it's using. However, if you haven't added many extras, less than 1A additional is probably do-able.

They look like this:

Each of these "Y" power adapters has three different colors of wire; black (ground) red (+5v), and yellow (+12v).

There is a male plug on one end, and two female plugs at the other. Snip one black and one yellow wire from one of the female plugs. That's where you can get your +12v, without making any changes to your power supply harness.

It would be a good idea to use a 1A fuse in line with the +12v supply, just in case there is a short. The PC power supply is protected from overloads, but it will turn off your computer if that happens. Better to just blow a fuse.

 

Hi.

That is how I'm powering it now. I've "jumpstarted" a powersupply. Yes, I am aware of the hazzard.

I thought it would be nice and clean to connect it via the USB port....

 

Oops - did you use 10v for calculating the resistors? You should've used 12v, or the actual voltage that you measure between the +12v line and ground. If you used 10v in your calculation, the resistors will have too low of a value, and you will burn up your LEDs.

 

I've measured the output of pin 3 in my 555 timer circuit. When I put 12v from the PC powersupply in, it gives approximately 10v out. That's why I calculated with 10v.

Is this wrong?

 

Why not use the 555 at 5V? It is possible you're application can use the 5V just fine, or you can boost the voltage after the 555 circuit.

 

I suppose I could let the 555 run on 5v, but how do I then boost up so I can drive all the LEDs? I've read your LED and 555 articles.

I didn't plan to have two powersupplies.

 

How many LEDs, what colors, and how are you driving them?

Another idea, I'm thinking of a separate 555 driving a voltage tripler, ever hear of the concept?

Yet another idea, did you look closely at this circuit?

CMOS 555 Long Duration LED Flyback Flasher

Use the final 7555 as a driver for the multiple LEDs. Think of it as a primitive buck boost power supply.

 

100 Ultra Violet LEDs.


Pin 3 goes to a transistor, a 2N2222 or a BC5x7 (Don't quite remember... :-( I'm at work right now) You helped me in another thread.

 

Reread my last post, I think I edited since your reply. How important is it these LEDs are continous instead of pulsed?

 

Yes, you edited while I answered.

If I understand correcty, do I not think that matters at all. The 555 is in monostable configuration, so the time the LEDs are lit (is this correct? not lighted?) is pretty much the same. I will fine tune the time with a 2M Ohm pot.

I have never heard of a voltage trippler, but tell me more.

 

I think the flyback circuit would be better, fewer parts, less current. You use the monstable to turn on the LED driver separately.

 

Yes, I can try this. Is the TLC555 the same as my LM555CN? And what can I use instead of the 2N3906?

 

A conventional 555 will work with this circuit, and any switching type NPN transistor should do the trick. A 2N2222A will handle up to 600ma, but a lot less will work.

You can also think about using a 556 for the total package.

 

Our OP still won't be able to run it from the USB port. There just isn't enough power available.

However, they could use a voltage doubler on the 12v or tripler on the 5v.

Ronald Dekker has a great resource page for hobbyists that want to experiment with boost and flyback converters:
http://www.dos4ever.com/flyback/flyback.html

 

Actually the flyback circuit will take a lot less, remember it is a form of joule thief. The LEDs will be pulsed, which will reduce overall performance, even though they appear to be continuously lit.

By comparison the voltage tripler would be horrendous on its efficiencies.

He could go to the 7555 or 7556 for it though, with equivalent results, for even less current. For the OP, this is a CMOS 555, as is the TLC555. Being CMOS, it draws a lot less current than a 555.

I don't remember the application though, how many LEDs will be lit? I'm assuming the voltage drop of the UV units is comparable to blue.

*********************************
OK, I feel stupid, 100 LEDs? Off a USB port?

USB2 could push quite few, but you'll need to carefully check the specs. According to Wikipedia, USB 2.0 does 5V at 100ma. Figure this at 500mw. A typical LED draws 3.6V at 20ma, or 72mw. Being generous, it could drive around 6 units at full power. Because the 555 pulses the LED, you might get 10.

You need another source of power I think, which means I should have kept quiet.

It would be interesting to see how much current my circuit draws. I'll measure it and get back with you.

You could do what I've done in such a case, take a blank drive bay cover plate, drill out a hole for a power connector.

Going through the math for 99 LEDs, 3 LEDs per leg at 10.8V, 33 legs with 20ma each, or 660ma.

 

You could do what I've done in such a case, take a blank drive bay cover plate, drill out a hole for a power connector.

Yes, it's something like this I guess I end up with.

I'm still designing the LED lay-out, and the timer circuit, so it may be fewer, or more...

Thanks for Your time and effort.