I have some miss-understanding with some issues which i post in the shape of a diagram. kindly guide me in this title. TRANSFORMER for a Battery charger.

I attached the diagram . for high resolution see the PDF format .
THANKS

 

in fugure 3, the secondaries are paralled, giving 12 volts at twice the current. paralelling the diodes isnt usually done, use no filter caps for battery chargers such as car chargers, only for chargers used on electronic equipment chargers. diodes should be 35 amp 100 or 200 volt diodes.

 

You cannot parallel rectifiers as you have drawn. You need to buy a rectifier with the correct current rating.

Only Fig 1 would be useful as a battery charger because the current is full-wave rectified. The other two are only half-wave.

Depending on the battery, you are likely to overheat the transformer. There is no current-limiting, so if you connect this to a mostly-discharged automotive battery, the initial current will likely burn up the transformer or destroy the rectifiers.

There is much more to a battery charger than a transformer and diodes.

 

battery chargers used to use selenium rectifiers to limit current.

 

battery chargers used to use selenium rectifiers to limit current.

Can you go to a Radio Shack and buy a Selenium rectifier?

 

Your should derate the full-wave rectifier DC current output to about 60% or less of the transformer AC rating due to the high RMS (peak) current that a rectifier generates, which can overheat the transformer.
Never use a half-wave rectifier for high current from a transformer as it tends to saturate the transformer core and also cause overheating.

 

Depending on the battery, you are likely to overheat the transformer. There is no current-limiting, so if you connect this to a mostly-discharged automotive battery, the initial current will likely burn up the transformer or destroy the rectifiers.

I have had good results using this setup for car batteries. I would put a 20 amp DC circuit breaker in series with the output, but this one seems good with a 1 amp fuse on the input side of the transformer. Yes, a really bad battery will pop the fuse, but average, used, car batteries haven't popped my 12 amp breaker in 20 years.
I'm always watching the initial surge with a DVM and an amp-clamp so I can estimate the health of the battery, but I've never seen over 11 amps in the start period.

 

I have had good results using this setup for car batteries. ...

The desired behavior is very dependent on the exact turns ratio of the tranny. Think about the Sears buzz-box unregulated type of battery charger. They purposely build a lot of leakage inductance into the transformer to help with the current-limiting. They also set up the turns ratio so that as the battery voltage climbs toward the end of charge, the current drops off as the battery tops up.

If you substitute a higher quality transformer, then there is less current limiting because of the better flux coupling. If you substitute a transformer with a slightly higher turns ratio, then there will be higher initial current, and less tapering as the battery tops up.

 

Not to argue, just continuing a report on my experimental data.
A local surplus shop bought hundreds of 120 VAC to 12V toroids. 150 VA rated.
Classy looking stuff. Potted centers with a mounting bolt.
I had some FWB rectifiers, 200V @ 50 amps.
That and a 12A DC circuit breaker did the job for me.
I bought some Radio Shack empty boxes and made Christmas presents out of them.

A good car battery that merely got discharged by leaving the headlights on will take about 8 to 10 amps as a start surge and taper down to 3 amps, at which point it starts bubbling and I remove the charger.
A standard fool that does not watch can ruin a battery with this rig. I am not a standard fool. As GopherT says, I only make high quality mistakes.

 

#12, I would be interested in knowing the exact turns ratio of the trannys you obtained. Sounds like you lucked out, or the trannys were actually designed for this application...

 

The obvious answer is 10:1
Beyond that, I know almost nothing, except that they work.
Sometimes luck works better than brains.

 

I have had good results using this setup for car batteries. I would put a 20 amp DC circuit breaker in series with the output, but this one seems good with a 1 amp fuse on the input side of the transformer. Yes, a really bad battery will pop the fuse, but average, used, car batteries haven't popped my 12 amp breaker in 20 years.
I'm always watching the initial surge with a DVM and an amp-clamp so I can estimate the health of the battery, but I've never seen over 11 amps in the start period.

Yep similar experiences here as well except I skip the fuses and breakers all together.

My best example is a pair of high amp chargers I built years ago out of some pretty stout 2 - 2.5 KVA industrial transformers of which both can easily hold up at 12 volt outputs high enough to trip 20 amp circuit breakers on 120 volt feeds if given enough time and neither one breaks a sweat doing it.
All they have on their secondaries are a pair of 300 amp stud diodes from an old welder configured as shown in figure one. No breakers fuses or current limiting of any sort. In fact the primaries including the power cords are 12 ga and the secondaries are all 4 ga so there is no major degree of internal voltage drop happening inside the units.

Now as for the OP I set my battery chargers up as shown in figure 1 and thats what I would recommend.

As for paralleling diodes that seems to be a large myth being almost every off the shelf battery chargers either light duty or commercial uses a group of small button type diodes or stud diodes in parallel and they rarely give problems. To be honest I have seen more linear power supplies/charging systems, ranging from small power multi amp packs to industrial power supplies and welding power supplies that push over 1000 amps, than I can count that also use the parallel diode set ups as well so obviously it works just fine.

Personally I prefer to use over sized single diodes when possible but when I don't have the space or diodes that would work that way I have no issues with paralleling multiple smaller diodes to do the same job.

I say go with figure one and don't worry about it too much. set it up and put a good load on it while monitoring the diodes and transformer for excessive heat build up and if things pass call it good.

 

I know this is another wrinkle, I would go with either #1 or #3, but would include a ballast (current control, usually a resistor) and an active voltage regulator (for voltage regulated batteries like lead acid or lithium ion). You can omit the regulator for nickel-based batteries as you can overvoltage them safely. They still require the ballast though.

If you have more than one junction rectifier in parallel, one will hog current, pull a thermal runaway, and die. ...and there goes the other... Try it with LEDs and you'll see what I mean.

 

I know this is another wrinkle, I would go with either #1 or #3,............

I won't recommend #3 (or any half-wave rectifier) because of possible saturation problems with the transformer due to the DC current, as I mentioned in my previous post.

 

I've see a lot like #3, but as I previously mentioned, they usually also have a ballast resistor. All wall warts are like this.