Car Charger

There was a Car charger circuit somewhere on here but I can't track it down now. It was very similar in design to an old unit I built about 1984. It works just fine but I would like to know if someone can find it here and perhaps help add current limiting to it.

This is the link I saved:
My DocumentsBattery ChargersCar Charger for 12V Batteries - EEWeb.mht

TIA

Here's the schematic. Is there some way to add current limit to protect the SCR?

No pix??? Guess bmp doesn't work so here is jpg

here's the link http://users.otenet.gr/~ athsam/car_12v_battery_charger.htm

as far as modifying the circuit why not try asking the author. at least you arer giving credence that you are trying to use his circuit design.

Correct Jerry, BMP doesn't load here, but diagrams are best in GIF or PNG, both for size and resolution.

Ok I searched the site and don't see an e-mail address.

OK so going from BMP to PDF is slam dunk!!

Copy the bmp, paste into word document, then print to PDF format, upload the PDF, slam dunk, do it from time to time.

I'm going to go out on a limb and offer the following (see below). This assumes a 17VAC transformer (that is truly 17VAC under a 5A load). Put a hefty heatsink on both transistors (but especially on the 2N3055).

If this works (totally untested), then it will limit the current to around 5A (might need to adjust R3, that's a *huge* guess based on a Vbe of .8V (conservative -- might be more like .7V). And if R3 _does_ need to be adjusted, be sure to compute the power requirement).

These components are going to get HOT so be careful!

Another possibility is a "crowbar" circuit and perhaps replace the fuse with a resettable circuit breaker, but I'm not good enough at analog design to generate that off the cuff (and I may not have been good enough to produce the below, either -- proof will be in you _pudding_ it together

OK, I ran a few simulations on my circuit and tightened it up a bit (see below). Notice that I changed Q2 to a 2N3053. Also, the base resistors became a 57Ω resistor (i.e. only one 5Watt resistor). Also, this is designed to limit at around 5A, but you many need to adjust R3 down a bit (like to .11 or .10) if the current limiting that it provides cuts in at too low a current (i.e. not letting the charger deliver the full 5 Amps). Probably no need for a heatsink on Q2 but monitor the heat, just the same and if it gets to hot to touch then throw a heatsink on it. [*when I say too hot to touch, use common sense! Or better yet, use a non-contact temperature probe -- if it goes above, say, 120°C put a heatsink on it, then test it agan*

Q1 will definitely need a hefty heatsink -- especially to handle the case where the output gets shorted to ground!

Thank you. Think I get how it works. Pretty slick. The circuit was designed for 5A but I'm actually working with a charger that's capable of 20 amps. Sorry I should have mentioned that in the original post. I really don't need that much cuurent though. I would be happy with 10 amps and can accept 5 A. I see your choice of transistor is good for 15 A. For such low values of R3, I should be able to devise a loop of heavy wire. I did that for a shunt on the ammeter. Works great. Have no problem finding good heat sinks and fans for cooling.

I'll be away for about 10 days on vacation so won't get to test for a bit.

Thanks again.

Consider that P = I²R In other words, the Power goes up by the current _squared_, So, increasing the current requirement has a serious impact. I would go with a different strategy for currents that high.

Also, that 15A rating is only part of the picture -- you also have to consider the power dissipation which is a factor of both current and voltage -- and because the 2N3055 will be in it's _active region_, it may not be able to handle that much current (though a good heat sink and a fan will go a long way in pushing that capability up).

The car charger for me is 300 watt power inverter from Bestek. Since I need to charge camera and laptop when travelling around the city. So the power inverter is necessary for me. This tool is easy to handle, just connect with car cigarette lighter charger outlet. We can charge in safe way. Or you can catch more information on Bestekmall. Have a nice day.