Doubts on this circuit

Hello Everyone

I'm very new to this forum, so apologies if posted in wrong section.

I wanted a backup for my wifi router (12V * 1A = 12W) during power cuts , which is for two hours in my place Hyderabad, India.

So I searched the net and i found THIS .

I understand the circuit but there are some things that i dont get.

1. Does this circuit recharge the lead acid battery ?

2. What is a LM338T Linear voltage regulator with a Heatsink ?

3. The color code for 2.6Kohm is Red-Blue-Red & 2.5Kohm is Red-Green-Brown , is it right ???

4. Does it switch from main to battery and vice versa automatically ?

I'm a mechanical engineering student please help me out.

Thanks in Advance

2.5K resistor color will be Red-Green-Red. LM338 is an electronic voltage regulator and you have to put it, means screw it with a piece of Aluminium, bit big one for cooling the regulator. It will getting charge (according to circuit) and will feed also same time to the Router when the AC mains is present and will keep supplying power to the Router when the AC mains power is off.
Yes it will do switching automatically.

answer to questions: 1>> yes this circuit can recharge lead-acid bat..
2>> lm338T is a adjustable voltage regulator..the out put voltage can be set by changing value of R1
(in your circuit 2.6 K resistor)..generally voltage regulators are fixed with the heatsink which is a aluminium plate to dissipate heat .
3>> colour code for 2.6 k is ok but colour code red-green-brown is for 250 ohm resistor..
4>> the output of the circuit is directly connected with the terminal of the bat. so this circuit never switches the state ...when there is
mains supply available it charges the battery as well as supplies power to the load(router) when mains is unavailable then battery does
it alone...

Thanks for the useful replies. Clears many of my doubts.

How many cycles(Discharge & Recharge) does the 12V 7AH lead acid battery give ?

How do i know when to replace the battery ??

as you have said earlier that your wifi-router draws 1A current ..that means if 12v 7AH battery if fully charged ..it can supply 1A current for 7 hours...
it is generally specified the discharge and recharge cycles in the battery specification itself....if not ..when the backup of battery is drastically reduced ..then you can change the battery..

One more question.

The circuit in the first post shows to use laptop charger for input voltage, but i'm planing to use a step down transformer (220v to 18v 500mA) with a full wave rectifier for dc input voltage .

So how do i purchase a rectifier that suits this circuit ? Can i just use any rectifier ?

as you have said earlier that router requires 1A current so ...18V 500mA transformer will not do the job ... satisfactorily ..and also battery will charge slowly..so to overcome this problem you have to use at least 1A transformer..
>for rectifier you can make your own diode bridge rectifier which will be simple and cost effective..solution..one more thing while purchasing diode keep in mind that
rating of diode must be >=1A.

In general use a higher current rating or equal current rating when ever you are charging otherwise the charging time will be increased rapidly

when I ask for a capacitor in a radio shop they ask for voltage. So for the circuit above in the first post what voltage of electrolytic and ceramic capacitor should I ask for?

use electrolyte capacitor of 1uf (63 V)...and 0.1 uf ceramic capacitor..

one question from my side, can it auto cut the charging when the 7AH Battery is full?

Hello again. Finally I implemented the circuit in the first post with some modifications due to lack of availability of some components.

The first time i tested the circuit i removed the battery in the UPS of my desktop and used it. This worked perfectly.

Anyway i needed the battery for UPS, so i bought a new battery of the same brand (exide 12V 7AH) and tested for the second time my router got damaged, it is dead now.

While testing first time i used step down t/f for input but the second time i kept the step down t/f aside to finish the connections neatly.

While testing second time the last connection i put was router terminals to 1N5408 and instantly i saw sparks in the breadboard.

I also posted my breadboard connection screenshot , please find my mistakes on it.

Anyway I spent a lot of time on it and i wanted to know what did i do wrong.

Below are the orginal and modified circuits . Please tell me what to change and what to keep.

Original Circuit



Modified circuit



I made the circuit on a bread board even the bread had burns on it.







Please help me this time , May be i'll be careful next time.