Instructables

Portable USB Charger

Step 2: Schematic

Picture of Schematic
Here is a Schematic on the circuit.
 
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aekara3 years ago
hi threre i have a problem with the diagram

I cant understand the connections on the swich

Can you please send me a better schematic just to understand how to cannect the swich?

THANKS
slipknot rules (author)  aekara3 years ago
What part cant you understand?
I´m also unsure of how the switch is supposed to be connected. As it has 6 connection points, what goes where?

Rookie question i know but hey, I am one...
Hi,

Forget that it is double ganged (effectively two switches in parallel) for the moment. Lets look at any one of the 'two' switches.

On a switch one connector will be able to be switched to one or the other of the other two. Like this:

0----------- Sw pos 1
Common-----------------0
0---------- Sw pos 2

The switch contact is connected to the point I've referred to as common and moves between Position 1 and 2. OK so far?

In the diagram above we have a double ganged switch, so one physical switch to throw, but two sets of contacts associated with it.

Hopefully if you look at my crude diagram and think about the explanation then you should be OK. If you are still struggling then I would, respectfully and with serious concern for your safety, suggest that you seek advice from someone more knowledgeable who can assist you with this project. I mean no offence. As I commented before, lithium cells can be dangerous. If you are not 100% sure what you are doing - seek assistance - don't just 'have a go', this is a great little circuit but very simple and subsequently has little in the way of inherent safety. Again, not a criticism but an observation.
Hi!
Thanks for the comments and you concern :-). I would have figured it out from your post but I actually already built this a while ago and everything is working perfectly. I´ll post pictures of my "Läkerol charger" when i get around to it.

Some hands on testing when the mail delivered my parts and this video helped me figure it out http://www.youtube.com/watch?v=WGskIUxzcz8.

Next project will be a "Solar charger with status LEDs" for the "Läkerol Charger"

P.S. You are totally right about being carefull with DIY charging batteries. I have great respect for them. I'm a diver and a dive buddy of mine had his battery pack catch fire while using a proper CE marked name brand charger. It was a 10,4 Ah 14,8V litium-ion pack so no toy. Luckily the only casualty was a nearby piece of clothing...
The switch diagram didn't work too well, so lets try this:
Switch.JPG
The goblin3 years ago
The switch:

It is a two ganged switch. Both sets of contacts move with a single throw of the switch (shown by the dotted line)

In the current position the power to the 7805 IC is from the two batteries. To visualise this look at the negative terminal (smaller plate) of the battery nearest the 7805. This can be followed to the -ve input of the 7805. Follow the +ve plate and this is connected, via the switch, to the negative plate of the second battery, effectively placing it in series. The positive plate of the second battery then passes through the switch to the +ve input of the 7805. Assuming both batteries are charged to 3.7V this places 7.4V across the 7805 which can then produce a 5v output.

With the switch in the other position the 7805 is disconnected from the batteries, but they are then connected to the +v input, but are in parallel and can therefore be charged simultaneously from the one charger.

OUTPUT components:

The LED and resistor are used to demonstrate that there is an output. the resistor limits the current to about 10mA-ish.

Suggested potential improvement: (If I may be so bold)

Capacitors across the input and output would stop start up and shut down spikes and increase the life of the 7805.


NOTE: THERE IS A SMALL SAFETY CONCERN HERE! Imbalanced cells can result in imbalanced charging and a potential fire hazard exists. Lithium batteries are not nice when they go wrong (Google it!), so be careful when charging cells in parallel without a balancing system in place.

Not a criticism, just an observation. Clever use if the switch though!
slipknot rules (author)  The goblin3 years ago
Thankyou for such a great comment! :) yes I know capacitors on the input and output would be an inprovement but i didnt really have the space for them! And yes lithium cells can be very dangerous if used incorrectly, could you reccomend a suitable balancing system? :)
Cheers
Hi,

I'm no expert unfortunately, but I'd guess if you were looking for a balancing system it would increase the complexity and cost considerably. Personally I'd play it safe, ensure I used identical cells and charge them on the charger that was intended for them. My guess is you are using old laptop or even mobile batteries here (I appreciate that you can buy commercial off the shelf Lithium cells for projects). You'd have to be quite careful. Even the commercial chargers get called into question when it comes to balanced charging. If you want more info I'd look at a discussion on one of the many RC Chat Forums ref one of the many Chinese chargers available from HK via EBay. The IMax B5 always seems to generate some informative banter and differing opinions!
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