Did you add the caps yet?

If not, don't post about it again until you do, and have tested it WITH the caps.

[eta]
You may also need a 10uF cap on the output in addition to the 0.1uF cap. Your charger's circuit undoubtedly uses a switching supply, and the regulator can't keep up with all of the transients by itself. It HAS to have the caps on both the input and output to be stable.

I really don't want to have to explain this a 3rd time. I seem to be having to do that a lot lately for some odd reason.

 

I really don't want to have to explain this a 3rd time. I seem to be having to do that a lot lately for some odd reason.

Ouch! Okay, I'll get the caps and try that. I guess my pea sized brain does not grasp electronics very well.

One circuit shows a 10uf on the 12v side and a 1uf on the 5v side. In another post said to use a .33uf on the in side and a .1uf on the out side. The circuit from Jameco did not have any caps. This is the last one I built.
http://www.jameco.com/Jameco/PressRoom/altoids-fig1.jpg
Now the suggestion to add a 10uf and a .1uf. Are these in series or parallel?

I hope you can see where my confusion comes from.

Thanks for all your help.

Todd

 

Todd,
Your best resource is to first look at the manufacturer's datasheet for the part in question.

If you look at the datasheet for just about any manufacturer's 7805 regulator (actually, the 78xx series of regulators) you will see that they recommend a 0.33uF cap on the input, and a 0.1uF cap on the output as the minimum requirements to ensure stability.

Some go on to say that adding 10uF on the output will aid in transient response.

Using caps a lot larger than that can be counter-productive, as a really large cap can cause a regulator to go into "fold back" mode, or current limiting mode. Sometimes, using a larger cap is really necessary - or using a lot of little caps all over a board, but that's beyond the scope of this discussion.

Just take my word for it right now that it's the right thing to do.
0.33uF from IN to GND.
0.1uF from OUT to GND
Adding 10uF in parallel with the 0.1uF from OUT to GND will help a lot with your particular load, which is a switching-type battery charger that generates a LOT of transients.

There, I've explained it a 3rd time.

 

Hey if possible could you post the schematic for the 12VDV to 5vdc USB charger pls? I could really use a usb charger in my car.

 

See the link in post #22. Be sure to add the caps that SgtWookie points out in post #23.
Good luck.

 

thanks man...pity my phones usb charger cord was stolen out my car last night.....well that will be the next project!

 

Hi I see the diagram you directed me to actually shows a 9VDC supply. Is that a error or should I change the values to combine with a 12VDC supply?thanks

 

The 7805 will handle from 8 to 30 volts, the resistors and caps are on the 5v side which is constant, so no changes are needed.

Todd

 

Oh thanks , I guess I should have looked and learnt a bit harder instead of being spoonfed! shot

 

hey i have all the parts i need to make this but as i dont want to destroy my phone or anything else testing it, i just have to ask. On the jameco circuit diagram the female usb connector is marked 1-4 from top to bottom of the diagram. The usb connector i have doesnt have any numbering, however the wire holders for soldering definetly have a top side. Can i assume that if i turn the connector in the same direction the right way up that it would be 1-4 as shown in the diagram? I wont try hold anyone else liable. Just wondering if it was right on any of yours...

 

oh yes, dont worry im gonna add all the capacitors

 

There really is no way to tell by looking at the wire which wire goes where. You will want to meter them and be sure the 5v goes to the correct pin and the GND to the correct pin. Since you are only using this for charging, you are only using 2 of the 4 wires. the other two wires are data.

 

my problem is just that on the actual female USB connector, there are no numbers. I can see in the schematic that gnd must go to pin four etc . Im just not sure which pin is pin four (actually socket as its a female). the white plastic in the female which only allows the male to be inserted a certain way around goes the top right? can i therefore assume the side to be soldered is numbered 1-4 from left to right if the plastic is on top? I cant find a clear answer anywhere... just dont want to do it the wrong way around as i WILL end up with gnd on the + side and + on the ground side....

 

pinouts.ru is an invaluble resource for finding which pins do what in MANY components:

http://pinouts.ru/Slots/USB_pinout.shtml

 

Look at a USB port on your computer, pin 1 is on the left 4 on the right. Take a volt meter and read pins 1 (red) and 4 (black) on the computer. Wire your circuit then check with each pin that it matches the voltage.

Todd

 

why didnt I think of that, thanks guys, will bookmark that site too

 

haha that worked couple of sparks on the computer but i dont reckon the boss will mind TOO much haha

 

glad i asked too coz i was going to number the pins opposite way around, thanks again, this forum kix

 

I am using both the L7805 and have noticed the output voltage is stable at 5v when no current is running through it.
But when I apply 800ma of current to the output, the 7805 gets really hot and the output voltage goes down to 4.2v.

Is this common for this voltage drop or am I using cheapo regulators here?

 

Do you have filter capacitors?