BATTERY CHARGER

Maybe I'm missing something...? To me...

The circuit is badly drawn.

The transistor should be regulating the voltage or current, but I can see no obvious voltage reference or current sensing. So at a quick glance (so I could well be wrong) I wouldn't use the circuit. Much better to use a circuit you understand.

I would suggest looking at application notes for voltage regulators, National Semiconductors have excellent ones... these devices can be used for voltage regulation or current regulation.

That was me...not logged in....

Hmmmmm interesting, the transistor appears to be permanently 'on' but depending on the current gain of the transistor and the resistor value it is acting as a constant current limit into the battery...

Very crude and subject to temperature and transistor characteristics spread... But basically its just there to limit the initial charging current and when the current drops below this value the transistor is permanently on...

At least that is what it looks like to me... ?

No Hendrik, most of what you say is correct....

Its a full wave diode bridge with a reverse polarity diode protection a whopping big smoothing capacitor and a led to show its on...

BUT the transistor has a resistor (330ohms?) between its base and collector NPN, so it would appear to be continuously 'on' - That is why I reckon its used as a crude current limiting device due to its low HFE gain etc...

John.

I thought so. thanks.

I have a 2 way radio communicator withe a charging cradle with a led. The led lights up when the radio is inserted but flickers and go dead when the batteries are fully charged. How do they do that?

It's obvious you know your stuff Electroman,

So. have some patient with me to.

I may be wrong, but to me, it seems it won't be continuously on, there seems a flicker period between the lag of the resistor, and the frequency change.

my mistake, I didn't count D1 and D4 correctly. anyway, like I said, I'm new at this

Hi Assalamu

I took a few moments to really look at your diagram, and below is my consideration in answer to your question. "From the circuit I want to know how the transistor is helping here; may we work wtih out it derectly?"

quite simply,
the transistor has a greater value on the emitter side, and therefore the bias is pushed toward the collector side. while the value coming from R2 remains greater than the breakover voltage of the flat battery, there will be battery charge. as they said, it looks like the LED is always on and maybe the addition of a voltage differential switch will correct that issue. (please excuse the use of non EE terms, though I can read those diagrams, I haven't yet developed my terminology.)

I took another moment to consider the LED

In dealing with the always on LED.

replace that with a normal diode and shift it between R2 and the transistor. Of course, you have to impedance match the LED and resistor for the correct breakover voltage value if that is affected.

I did a test on resistors, and it may be my multimeter, but the voltage didn't get affected in a series of 9 resistors, and I tested across each connection point (it was a little baffling). So, I'm not sure if there will be a great value difference.

There problem solved

I have some free time at the moment, as the parts I was waiting for aren't ready yet.

So, seeing as you wanted to understand what's happening in your diagram, I drew a partial to demonstrate the implementation of a voltage differential switch, plus added some letters for discussion so that we have references to lines in the circuit and what happens in those parts as things develop throughout the circuit.

This will help demystify things for you.

at junction "f" when the frequency is negative diodes D1 and D4 block, and the capacitor back feeds the build it collects during the positive cycle of the frequency to points "d", "i", and "a".
at "d" the constant positive rate is connected at the emitter part of the cathode which is connected to N of the NPN resistor. see that with both "d" and "a" feeding the transistor, that side reaches a 0 bias, and current doesn't flow
"g" cathode is connected to the other side of N and while the voltage in "e" is high then there is a 0 bias on that side also. sort of like a tug of war, as one side get weaker the stronger side impacts its influence.
because the voltage in "a" is constant, there is no potential for voltage differential, and so the addition of b and h are pointless. even though current may not flow, the voltage is as constant as the source supply. even in that position the LED will still be as constant as the source.

So you see, the only real answer is to remove the voltage switch and position the LED to position "a". I'll leave it to you to draw a solution. just be conscious of flow direction and you wont have any problems

I mentioned below in message 11 the issue about impedance matching, this is to do with current flow. exception: when a battery is low it does affect the ampage of the battery. this is the thing that can cause some confusion. however, I mentioned a very important point above about the constant supply so voltage is always as constant as the supply. here, in the instance of R2, the impedance serves the purpose of limiting the charge rate.

There are gurus in here that may help... but then again, some just don't know how to come down to learner level.

actually,

I reread the part about impedance matching and it still sounds confusing. impedance matching is a non issue in this, and I should never have brought it up. And the stuff about the battery, but it does may for some interesting discussion if you want to talk about it?

just be aware that the LED may alter the voltage if you switch between R2 and the Transistor (I'm not sure, and it may only be a function of the zenner).

and aren't the gurus mean?, I mean, where's the help. the real help!!

This is just a basic trickle charger with the component values given the transistor will present something like about 6.8V to the battery which is about the recommended value for a 6V sealed lead acid battery.

If you remove the transistor you will probably destroy the battery.

Hi,

Your circuit looks incomplete to me. If the secondary voltage is 8V RMS, then you will have way too much DC on the battery, and will overcharge it or destroy it. There is no regulation or current limiting now. A 7V or 8V zener from the transistor base to emitter would regulate the output and protect the battery. The current would drop the battery gets charged. I would still suggest some current limiting besides. I agree that the circuit should be re-drawn.

S

Rectified voltage is about 10.4

This would give about 7.6V on base

Vd across transistor about 0.8V

V across battery about 6.8V a charged battery would rise to7.2 V approx, charge rate would be limited by this and not over charge.

Above figures are only rough but close enough.

That's why the transistor is there, as a crude but effective current limit...

John.

As nobody seems to have picked up on the transistor being a current limit perhaps I should explain what I mean.

For a power transistor with a dc current gain of 10 (say) and its base being tied to its collector by a resistor as in the circuit above, let's say by a 100 ohm resistor.

if the transistor were to pass a collector current of 10 amps then its base current must be 1 amp to keep the transistor turned on... Obviously the voltage drop across the 100 ohm base resistor will limit the base current to about 30 or so milliamps, so this means the transistor will only be able to pass a maximum of 0.3 amps for charging...

For a higher gain transistor and a lower base resistor the charging current would be higher etc...

This is what I mean about it acting as a current limit, crude but effective!

John.

What I said before

The transistor VD referred to as 0.8 V was collector emitter VD not total VD which is dictated by base voltage or bias if you like therefore regulating voltage and current to the battery

I forgot the VD across the protection diode also which will alter previous figures a bit, but as I said they were rough anyway

yes, that is where the impedance needs to be matched... otherwise burnout time...

I've never seen an overworked transistor, I suppose they'll look like a blown fuse.

n emitter + p = n collector

have a look at the balance ratio

A burnt out transformer looks just like an unburnt out one...except maybe slight discolouration of the label !

Some have said 'The circuit is ok.... '

Well ....only insofar as you could achieve much the same with the transformer one diode and a length of wire. Or buy one equally bad for next to nothing.

If you are taking the trouble to build a circuit, it may as well be a decent one rather than an 'ok' ...it won't cost any more.

The transformer point is valid...but to call it 'impedance matching' seems a misnomer to me. It is a matter rating.... selecting a voltage with sufficient overhead to charge at the rate required and sufficient current capability not to overheat.

It depends on the battery...but some batteries are not tolerant of abuse and a well designed charger can give vastly improved life.....I know, 'cos I ruined my first golf trolley gel battery

Del (as usual) manages to hit the button accurately many times in his post, I could not agree more in what he has said.

I have several electronic hobbies (!), one of which is I design (for my own use only!) battery chargers for all types of Lead-Acid Batteries. Mostly 12 volt batteries, sometimes 6.....

You can say that (as Del did) most of the Lead Acid battery chargers sold in stores are of the transformer + a single diode type or sometimes a Diode Bridge (really fancy!!). These damage more batteries than anything else if left longer than the time needed to recharge the battery connected. That time is variable with battery size and state of discharge.

A good charger must be matched to the battery in terms of the best charging current to supply and as soon as the battery is re-charged, either the charger should switch itself off, or go to a mere trickle charge (again, the size of the battery must decide the amount of "trickle!").

It also needs to watch both the ambient temp as well as the battery temp, if you want to charge fast & safely....

Overcharging only reduces some of the acid back into hydrogen & Oxygen at best (gassing), but can also damage the plates. Car batteries have thin plates.

Over discharging a normal car battery will severely reduce its life span, especially if left in such a state for more than a few minutes. Normal car use, with Styrofoam insulation for the winter time, with no deep discharges/overcharges, will keep a car battery good for many, many years.....

Leisure or deep discharge batteries cannot deliver the sudden bursts of energy a car needs to start as well as a normal car battery can, but they can accept a deep discharge or two with no problems.....but of course are far more expensive generally, for the same AH.

I personally would not recommend deep discharging even these batteries unnecessarily either.......

A good simple indication of the charging qualities of the car, is how often you need to re-fill the battery with distilled water. If you need to say in a climate like central Europe, more than twice a year (after the first year) something is wrong!

Lead Acid Battery science goes a lot further than I have mentioned here.

"The transformer point is valid...but to call it 'impedance matching' seems a misnomer to me. It is a matter rating.... selecting a voltage with sufficient overhead to charge at the rate required and sufficient current capability not to overheat."

Actually Del, if you were refering to the transistor, then it is valid to say impedance matching when regarding the throughput of the transistor, and setting the right feed resistance to the control.

other than that, I know of 2 instances where impedance matching takes on a distinct meaning. 1, when you're matching a resistor, capacitor, and/or diode in series parallel to get a unique result. and 2, when you need to overcome or employ the resistance to values in Q.

Yes I miss-read transistor as transformer at some point.

I feel that the concept of impedance matching is probably not the best way to explaining electronics to a beginner.

The key to teaching is to find the way of explaining which happens to 'work' for the pupil...so impedance matching may work for some.

Personally I find the simple concepts of up/down on/off work best.

You are right about adding a second transistor..this would introduce the concept of feedback and make the circuit worth of discussion.

I have learned that it is a huge waste of time to try and tech someone who doesn't want to listen. I once had the very educational and humbling experience of sending a young apprentice to sleep...!

He asked 'How does a transistor work?'

when he really meant 'What simple tasks in a circuit can a transistor perform?'

I was fresh out of college and full off irrelevant (to him) stuff like energy bands and depletion layers.

So you can see why I tend to be wary of questions and people who don't really want the answer.

yeah,

no need for an extra transistor in the above circuit though.

that circuit above will self regulate.

still and interesting thought, resistor in series, and parallel.

Well explained, its seems obvious when you tell me, but it wasn't before, thanks.

As nobody seems to have picked up on the transistor being a current limit.

IHey John!

In my defence..I spotted the circuit was a crock of shite in 30uS! I just couldn't be arsed to explain the function of something so wooly.

Simple series regulator & current limit circuits are ideal to learn the basics of electronics with.....but I didn't feel like being a tutor today.... oh what the hell... Toot toot toot.

Ah that's better !

I wasn't having a go at anyone in particular Del.... I was trying to answer the first poster's question about what does the transistor do and can it be used without it...

Anyway, seeing as your here now have you seen the video of the cat in the ceiling fan thread?

No, Oh that's good then

John.

My conclusion is from your replies and my own ideas, that conclusion is - its not much of a charger.

There are plenty of good designs for a lead acid battery charger around on the web, most are for 12 volts, but can easily be changed slightly to allow for 6 volts.

Remember that your charger should charge quickly, but without gassing.

It should then use a very small trickle charge (depending on battery style, size and age) to just maintain the cells charge, no more no less.....and still no gassing, so the trickle charge should not exceed say 6.6 to 6.7 volts.....I personally tend to go for 6.6...

....and it should never overcharge, though capacity will be better if a small amount of overcharge is used, but gas will also be produced.

Temperature changes the charging characteristics of a battery.

The gas produced is highly explosive.....!

Also a lot depends on the use to which the battery is put and is it charged in situ or not.....

Do not forget goggles and other safety equipment when working with normal lead Acid Batteries....even if a SLA Battery is used....

.....as usual, we need extra infos to answer ALL the questions.

First off, the drawing reminded me of my classmate's penchant for redrawing diagrams from the "usual" way they are presented. I had to redraw it for it to make sense to me.

Electro-man's analysis is mostly spot on. I wouldn't use the term "current limiter" though. The word "limit" infers that if the current should exceed a certain value, the transistor would keep it from increasing any further. Just my opinion, Electro.

It does look like a trickle-charge unit though. The current going to the battery is less than 0.3A if we assume that the battery starts out at 4 volts. As the battery voltage rises, the current will keep decreasing. This thing is going to take a long time to charge!

There's no mention of the current rating for the transformer. I'm assuming it's properly sized. If it's one of those itty-bitty transformers at 200mA, this could get hot at the very least.

The design looks fine to me. once the corrections are made.

I think limit stresses a point more than above or below it.

I know that the truth of this thread can be put down to Aquarius's simple real time circuit translation, and now people including me can see exactly how it works.

His employment of incorrect assumtions and the correction helped clarify misassumptions. credit to Aquarius!

You're the bomb teacher!! thanks

MEAN ? ME MEAN! Borrocks...!

If you really want to build a rubbish circuit fine!

I offered advice as to where to find a better one way back in the thread....

If you don't want to take my advice ...be it on your own head!

What is the point in wasting time and effort explaining how a bad circuit works?

BWFT

I resent the criticism of us Gurus...

I design electronics for a living and am extemely willing to help on a sensible series regulator design as they are perfect for learning. You will learn nothing but poor design from this circuit.

If you care to look at the NS application notes for voltage regulators you will learn a lot more than you will glean from this thread..

Funny...I said that back at trick one...

You don't have to take my advice...but don't abuse it.

If you really want I can parrot what what you evidently wish to hear...

'Oh yes it's a lovely circuit, the series transistor provides loads of stability and it will never ever damage any battery connected to it!' mwah mwah...

Now where is my valium...?

I agree.

You are absolutely right Del.

I must apologise to all the Gurus, without which there wouldn't be forums like this. "frustration" is a thin excuse at best.

It was a rather silly and short sighted comment really, so Electroman, Andy, Del, and all the other gurus, sorry 'bout being and "a".

As you can see, I did a bit of profile redecorating.

ciao

Welcome back!

The circuit is rubbish (I have said that before too), there are far better circuits around for free on the web. Easier to understand and would do a much better job as a charger than this does.....

Leaning how it works is fine as it tells us that it is not worth building, for that I thank all concerned.

Actually andy, I think the criticizm wasn't a bad thing. and it was a question. Del, she's not mean. Andy, you're not mean either.

I do think there's a limit, like actually doing design work that others could earn money from. Finishing their work for them. Thats a no! But, in cases where one can learn a thing or two, some of the Gurus are way up in the clouds.

come on del, as you see, there's a lot of benefit.

You know what, I have a battery charger that I had to fix cause one of my bros went a connected the thing backwards to the battery. blew out the rectifying circuit.

It's one of those really old designs with a bridge rectifying circuit built in plain view. the old way, not these premanufactured jobs. i.e... you can see the diodes.

anyway, I replaced one of the diodes with "shock" a resitor. A! a resistor? yip a resistor and it works! I'm still scratching my head about that one

actually, its not that puzzling, but it does show that todays design are inefficient

Any charger halfway worth it's salt, protects itself from a wrongly poled battery in of two simple ways so that no damage is done, except maybe to a fuse.

The charger you are using would appear not even to have had that (and its not alone!!).

This should warn you that maybe it needs trashing and something better made or bought......

A good charger is still cheaper than one car battery......and could last a lot longer!!

have you heard the saying, if it ain broke, don't fix it? why would I trash something that does what it was designed to do?

I could add a transistor, and thus make it self regulating. that will improve the design. Other than that, there's nothing wrong with it.

Any handyman worth his salt will correctly attach the charger.

The most basic problem with the circuit is that there is no voltage regulation, zener or feedback. It depends entirely on the source voltage being accurate and then the voltage drops of diodes and the transistor VBE. The forward drops decrease as the heat up, the input line voltage is quite variable in many areas and the transistor gain will change consideably with temperature. In a faily ideal situation it will probably work fairly well but it looks like an example circuit than can be a starting point. For higher reliability and longer battery life I would add some overvoltage protection by regulating the maximum output voltage with feedback or a zener on the transistor base. A diode for revers connection protection on the output would be a good idea as well. Several improvements are warrented - the best way to look at a something like this is ask what could go wrong and determine how to prevent it. Never assume it will work because you want it too.

Firstly, well said Guest.

Secondly, please join and become a name......

and excellent point! well done!

May I suggest that you allow me (and anyone else who has a good circuit that can be freely distributed) to look for a charger circuit for you on the web, I will post it here and then let everyone pick it to pieces first and see if it is worth using, or maybe with a slight redesign or what ever?

But only if you wish......its your Blog first of all.....

But I think that the original design you showed is really not worth the trouble to fix.....starting from scratch would be quicker.

Hi Andy this website will give you plenty of grist for your mill.

http://www.mpoweruk.com/testing.htm

Nice link, I have stored it for future usage, thanks.

But I was thinking more on the lines of a practical circuit for him to construct, rather than the circuit that he already has....I have seen many, there are some good ones around, but I want him to choose, not me.....