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weuhfs

Member Since 24 Aug 2016
Offline Last Active Mar 03 2017 03:07 AM
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Topics I've Started

Ways to charge my lithium batteries safely

22 February 2017 - 02:32 AM

Off topic here, but I wonder if anybody can help me.

 

I'm building a device that needs to run from Lithium batteries.

 

I am well aware they need to be charged/dis-charged properly to avoid damage and explosions.

 

To my knowledge a charging system has two components which are the managed charging and cut-off once charged and discharge protection when the batteries voltage drops.

 

I found a board that uses the common TP4056 controller to charge the lithium battery and cut-off once charged. It also has discharge protection at 2.5V.

I have two questions:

 

So this means I can hook up the above (or any) Lithium batteries to the charging board as well as my device and it would run well from the batteries until they got too low, at which the board would cut off the supply to the device leaving the device isolated from the batteries until charged again.

 

It says the discharge protection is at 2.5V isn't this well below the safe discharge voltage of a lithium cell. I thought you should never go below 3V?

 

   I apologize if this question is rudimentary however I'd like to make sure when dealing with constant use of lithium batteries.


Tips to put color LED Bulbs in Solar Garden Lights

06 December 2016 - 02:46 AM

I'd like to take the boring white LED bulbs out of my solar garden lights and replace them with colored LED lights. Here are some advises.

LEDs have something know as the forward voltage. This voltage is dependent on the particular LED you are using, but they tend to be related to the color. Generally, Red, Orange, Yellow and Green tend to have lower forward voltages (1.6V - 2.2V), while Blue, Violet and White tend to have a higher one (2.5V - 3.7V). Note that individual models can be different, and high brightness LEDs can be higher.

Any circuit designed to drive an LED has to be setup so that the right forward voltage drops over the LED, otherwise you are driving it with too little or too much current. There are different ways to drive LEDs, the simplest being a current limiting resistor. The solar garden lights I've looked at did it that way, but some models could be more sophisticated.

If you replace the LED with one with a similar forward voltage you should be fine. If it has a different forward voltage, you could be over-driving it and possibly burn it out.

Without knowing anything else about the circuit, it's hard to tell, but I would think you might have a better chance replacing the white with a blue LED than one of the other colors (Red, Orange, Yellow, Green). If the circuit simply uses a resistor, you could of course replace that with the appropriate valued resistor for the LED you are using. Or you could simply try it out and see how long they hold. It's possible that you are over-driving a red LED, but it will live long enough for the burn.

Here's the deal with solar garden lights: 

The circuit isn't like those simple battery + LED + resistor circuit that you have to set to a specific value to work. The solar panel and the battery don't provide much voltage, but the white LED needs a higher voltage to work. So the lights have an oscillating power circuit in there that converts the low-voltage from the battery to a sufficient voltage for the LED, along with some current. It's relatively efficient, which is definitely needed for those solar lights.

What does that mean for you? You'll just have to see if it works by experimenting. The circuits can usually "auto" adapt to the different voltage drop from the color-changing LED. So there's a good chance the auto-color-changing LED might just be a drop-in replacement for the white LED. Whether it uses more power or all the colors of the lights work... well, you'll just have to see.

How to do it? Rewire the white LED with the color changing one.

For myself, I'd probably end up just building something that would do the same thing because those solar garden lights are relatively expensive for what you get. It gives me an excuse to buy a bulk amount of parts too.

LED's are pretty robust devices and you've got to work hard to zap them! I have seen them massively over-driven and still working.

I've just taken one of my lamps apart and it has a very crude regulator between the solar cell and the LED. LED's are polarity sensitive so you have to solder them into the circuit board the right way round. If you look inside of the plastic LED envelope the lead that ends with a bigger metal tab is usually the Cathode which has to be connected to the negative supply. Often the cathode is also the shorter lead. Before you unsolder the original LED check which is the cathode using the same method.

Good luck!!


Let me tell you why can't a single resistor be used for many parallel LEDs

25 August 2016 - 01:42 AM

I have been asked for many times that why can't you use a single resistor for a number of LEDs in parallel instead of one each?

The main reason is because you can't safely connect diodes in parallel.

So when we use one resistor, we have a current limit for the whole diode section. After that it's up to each diode to control the current that goes through it.

The problem is that real world diodes don't have same characteristics and therefore there's a danger that one diode will start conducting while others won't.

By the way,you can find kinds of  resistors from http://www.kynix.com...ct/Cate/84.html

So you basically want this                                                                                             

 3FRWT.gif                     

                         


And you in reality get this                                                                           shCWb.gif

 

 

As you can see, in the first example, all diodes are conducting equal amounts of current and in the second example one diode is conducting most of the current while other diodes are barely conducting anything at all. The example itself is a bit exaggerated so that the differences will be a bit more obvious, but nicely demonstrate what happens in real world.

The above is written with assumption that you will chose the resistor in such way that is sets the current so that the current is n times the current you want in each diode where n is the number of diodes and that the current is actually larger than the current which a single diode can safely conduct. What then happens is that the diode with lowest forward voltage will conduct most of the current and it will wear out the fastest. After it dies (if it dies as open circuit) the diode with next lowest forward voltage will conduct most of the current and will die even faster than first diode and so on until you run out of diodes.

One case that I can think of where you can use a resistor powering several diodes would be if the maximum current going through the resistor is small enough that a single diode can work with full current. This way the diode won't die, but I myself haven't experimented with that so I can't comment on how good idea it is.