Laser Beam Alarm System with Rechargeable Battery for Laser

Step 3 Choosing the Correct Resistor to Work with your LM317T

Now this is important if you are going to use a rechargeable battery and a solar panel, if not you can skip this step but if you are, read carefully.

Ok, a rechargeable battery hooked up to a solar panel will always recharge as long as the solar panel is producing more voltage than what the value of the battery is. For example my 3.6 Volt battery will recharge as long as the voltage is 4 volts and above. My solar panel produced a healthy 10 Volts so that is good; I dont need to worry about not having enough voltage. What I do need to be careful about is current.

Lots of current will charge the battery very quickly but will cause overheating and will kill your battery quickly. Too little current and your battery will charge extremely slow or not at all.

A general rule of thumb is that the optimum flow of current that you should try to maintain is 10% of the batteries current output. For example my battery was 850mA/H (850 milliamps per hour). So, 10% of 850 is...850/10=85. In this case the magic number is 85mA. We want our solar panel to produce an output of no more than 85mA per hour. To do this we need to choose a resistor which will work with the LM317T chip that will give us that control level. To do that we need this table:

Look at the fourth image for the table. You may need to view it at full size to see it clearly.

What you do is find your magic 10% current value and match it up with the closest current value on the table (bottom row) then look at the value above it and that will give you a resistor value. It is this resistor value that will give you the current flow that you need. In my case the closest value on the table that matched up with mine was 83.3mA. Above that is 15 Ohms. That is how I obtained the value for my resistor. You may get the same or you may get a different one, it all depends on the battery that you use. If you need any help with this just message me or leave a comment and I will reply asap.

Comments

[benzy4iva]

hi rev...wit the 10% of the current rating of the batt,,i had mine to be 250.... am wondering whether to use the 266....or the.....223 column..which one should i use...

[mobhacker]

Thanks for that I will start ordering the bits tonight I think.

[revhead]

oh and about you asking about not being able to get the 5v reed relay, in australia i brought my components from a store called "JayCar" google search their store to find the home page and you will find it there. i know they do post their stuff but im not sure if they post to the UK. do some digging around on their site to see what countries they ship to. let me know how you go. CHEERS!

[mobhacker]

This is a nice bit of kit I'm going to have a crack at making it but even though I want it to charge I want to be able to charge it up using to phone's plug. Is it as simple as pulling out the charging unit in the phone and using that instead?

[revhead]

if you had an old phone that you dont use any more and still have the charger for it, yeah it should work no problems. what i suggest u do is keep the phone circuit bord in one piece, and just solder two wire leads in parallel to the battery. doing this is actually very good because it iliminates the need for the charging ciruit that i used which im a bit iffy about how well it works. nice idea, let me know if u get it to work. if u have any problems and need to troubleshoot just message me, im happy to help out. CHEERS!

[flying texan]

Why is it that the chart shows that you need higher resistance for lower current? to me it seems backwards, it makes more sense to have more resistance to dampen down more current.

[revhead]

im not to sure, i pulled that info directly from a web site. i dont hav a very good understandin of the chip that i used, havnt used one since then. i think to properly understand how to choose resistors you would need to understand the inner workings of the chip.

[flying texan]

I bet the chip is just a high tolerance anolog switching transistor rather than the most common digital ones that are on/off-only when you apply voltage to it's gate. this one is probably a transistor that controls current flow depending on how much voltage is applied to the gate, and I say high tolerance because it's able to be bolted to a heat sink. To me it sounds like it's synonymous to a variable resistor, but instead of calibrating a knob you calibrate voltage.

[kk khrab]

Could you plz help me with finding the correct resistor value i have a solar cell which produces i.3 V.