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arctic 06-07-2017 02:39 AM

Battery Charger in My Car
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First of all, thanks very much for reading my post.I want to make a car battery charge after I saw an article introducing something about it. Well, I would like to ask some help.

As the essay showing, This car battery charger circuit can be used to charge 12V and 6V batteries. If it is used a transformer that can deliver 4A to 5A at a voltage between 12.6V and 16V then we can get rid of the switch for 6V or 12V batteries. The schamatic is given below.

It aslo gave some maintenance tip about car battery.
The car battery charging current is automatically limited to 4.2A. If there is a 600mV voltage on R1 (4A thru it), then the T1 transistor starts to conduct. Excessive charging current is avoided because the current value on T3ís base is limited. The difference between applied load current (at T4ís collector) and real voltage of the battery is balanced thru T4ís collector-emitter junction.
The datasheet of 2N3055
The power input of 2N3055 is the product of load current and voltage difference already mentioned. When charging 6V car battery this power reaches a maximum of 40W. The rectifier diodes must be able to deliver 4A at 40V. T4 2N3055 must be mounted on a good heatsink in order to dissipate the heat.
However, I canít understand what byx21-40 is?
1) Is it like a diode rectifier ??
2).and what is 21-40 means like between 21-40 or just itís datasheet code (because i hardly found it on google)
3).and if itís being sold just as it is and what if I didnít find it in the market ?

Sorry for all the questions but Iím a mechanical engineer and electronics is a hobby for me so Iím not professional.
Thank you again,
Best regards,

Stubby79 06-07-2017 10:43 AM

That's the symbol for a full wave rectifier.
the -40 part looks to be just the voltage rating of said rectifier.
The only BYX21 data sheet i found was a BYX21-100, for a single diode rectifier, rated at 100 volts.

I'm not sure why you would want to make your own charger, unless it's just for an interesting project. You can buy an "intelligent" charger for less than you could build a simple one.

freebeard 06-07-2017 06:31 PM


First of all, thanks very much for reading my post.
Thanks for posting. The schematic is 100x45 pixels.That makes it hard to comment on the specifics.

Do you need to 'roll your own' for a specific mechanical engineering project?

You have the power to go back and delete those posts you scattered around the forum. Please consider it.

freebeard 06-28-2017 12:44 PM

Well, thank you for your thankful thanking. :)


Do you need to 'roll your own' for a specific mechanical engineering project?
I'm still curious why you made the original post.

I once charged a battery by taking a 14v wall wart, cutting the barrel connector off, and holding my hand on it to short-circuit thermal runaway.

For the lurkers:;d=1496817383

You've had two replies but the thread has had, at this time and location, 398 views.

Hilton 10-20-2017 04:33 PM

The symbol you indicate is a rectifier: which converts ac to dc. Usually, rectifier bridges are used in circuits having a drop of 1.4. So far in my life, I dealt with all rectifiers rated at 1.4 drops. Hopefully, that one is same. The specific part byx21-40 in google outputs a pdf by Farnell which unfortunately contains no info on the part. You can ask someone who uploaded it on electroschematics for that part?

JockoT 10-20-2017 04:40 PM

I wouldn't bother with this thread. I think artic has had his crayons removed and can no longer contact the outside.

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