555 timer based constant current battery charger

[swordfish12]

hi everyone,
I am having a problem in understanding the circuit of "555 timer based constant current battery charger".....please help me to understand it... the circuit is designed by Kenneth, the link is >https://blog.thelifeofkenneth.com/2013/01/555-constant-current-lead-acid-battery.html <.

thanks in advance

 

[MikeMl]

It is horribly complicated for what it does, and it has a bad problem which would prevent me from using it...

If you loose the AC input, the battery will discharge backwards into the charger circuit, bad design!

The 317 and resistors create a poor excuse for a current limiter.

The 555 is used as window comparator (Schmitt trigger). The two pots are used to adjust two different trip points: Cutin and Cutout. Presumably, they want the charger to turn on when the battery voltage sags below ~13.2V (Cutin), and turn off at ~14.5V(Cutout). The way the pots are wired, it is possible to misadjust the pots such that the Cutin is higher than Cutout (bad design!).

The 7805 is used as a reference voltage to which the battery voltage is compared. Why not use a "real" reference?

The 555 output switches on a 2n3904 common base dc amplifier that shuts off the 317 current limiter when the battery voltage reaches Cutout.

 

[swordfish12]

thank you very much for clearing many doubts
1)what is the use of using the zener diode (1N5818) in the circuit?
2)explain how exactly the bjt is used here....

and I thought that power In is dc voltage,why not use dc???

3)what are the advantages of using this circuit over the conventional charger that has only an ac input with rectifier and filters connected to the battery????I mean the conventional circuit is better because the voltage increase in the battery decreases exponentially and almost comes to nil when it is near the max voltage of the battery(correct me if I am wrong).........

 

[MikeMl]

The 1n5818 is a Schottky rectifier.

When the 555 output is low, the transistor pulls its collector within a Vce sat (~100mV) of its emitter. That pulls down the Adj terminal of the regulator, lowering its output pin to about ~2v. The rectifier prevents the battery from back feeding into the regulator's output.

Power in could be full-wave rectified, poorly filtered DC; cannot be AC.

An unregulated charger (like you describe) will destroy an SLA battery in a short time. A proper SLA charger MUST be regulated and have some of the elements as the referenced design, (but done better)

1. Current limited while charging a discharged battery.
2. Current limited until the SLA battery reaches ~14.5V.
3. Upon reaching 14.5V, the charger switches to a lower constant float voltage, about 13.2V and holds the battery at 13.2V for days to weeks.
4. If a load is put on the battery, discharging it below ~12.5V, then the charger should switch on (current-limited) until 14.5V is reached again...

 

[swordfish12]

hi thanks again for answering my questions and doubts....
I was thinking, and how is 7805 used as a reference voltage to compare battery voltage??(and why do we need the capacitor when it is dc everywhere in the circuit.)...
and it might sound funny but what is the work of the two 10k potentiometers in the circuit??

 

[MikeMl]

I think all three questions will be answered if you read this forum thread

For the battery charger, the Cutin voltage is ~13.2V. With 5.00V at the 555 Vcc pin, the 555 will switch on when the voltage at pin 2 is 1/3 of 5V, or 1.67V. The voltage divider consisting of the two fixed resistors and the trim pot reduces 13.2V to 1.67V. Similarly, the other divider reduces the Cutout voltage of 14.5V to 2/3 of Vcc or 3.33V. The LM7805 is used to make the 5.00V at the Vcc pin of the 555.

 

[swordfish12]

one question, can i use a bc107 instead of 2n3904 in this circuit

 

[MikeMl]

Do what I would have to do for you!

Google the data sheet for both transistors. Open them in two windows, side by side. Now read both data sheets comparing parameter to parameter.