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lead acid battery charger sch/pcb available

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Full Member level 5
Mar 12, 2002
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battery charger pcb

hi guys!

i have designed a 24v,4A low cost battery charger works in simple constant current mode as stand alone.also it is possible to control its output current/voltage by a microP or even a computer port,if an a/d card is available.

its docs (sch & pcb)in protel format is available.
if u want it respond to this letter!

24v lead acid battery charger schematic

i have designed a 24v,4A low cost battery charger myself.

I am interested in the schematics (preferrably as PDF) if it is a switching design without heat sink.


lead acid charger schematic

Dear Friends
Excuse me for lating.I was busy two past weeks.

About Heatsink : In This Range of Power(100W) or even lower than this heatsink is required , but its size ,
shape and orientation is not very critical .for example a small piece of Al Sheet may be sufficient ; it is
possible to find the Size by some calculations (see switching power supply design by A. Pressman) , but in
practice its size depends on several factors e.g. shape,orientation,color,fan(may be required or not).Thus
calculations offers a nearly size,therefore it maybe necessary to size it empirically.
Most of SMPS manufacturers are using Formed Thick Al sheets as heatsink,in this way not only good heat
sinking in small footprint is possible,but also it can be used as EMI filter.(see Attachment)

A little about The Charger:
it is an off line flyback charger designed for charging 24V lead-acid batteries @ 4A.Switching freq. of
75Khz reduces the size of charger and increases quality at the cost of EMI which must be filtered.
Unitrode UC3844 controller is selected for several benefits:
-it works in current mode thus is more stable than voltage mode SMPS
-it has short circuit protection
-due to internal under/over voltage look out there is no need to special supply for the IC
-also it is a low cost controller IC
for switch a high voltage NMOS (2SK794) is selected , thus MAX input voltage can be higher than 250V.
also the ckt works @ MIN input voltge of 170V.
input and output are isolated by a tight CTR optocoupler.
The core is TDK :pQ3230 ,it has a small size ,good heat sinking and good shielding.
Primary: 42 turns,0.7mm,in 2 layers
Scondary:7turns,2*0.9mm,in 1 layer
Primary aux :6 turns,4*0.5mm,1 layer
Secondary aux :6 turns ,4*0.5mm,1 layer
Output voltage @ 25°C=27.6V
Output voltage TC=-3.9mV/°C
Short circuit current=4A ;Duration=infinite
there are 4 controlling signal in secondary side:
CSEN=current sense,it is linearly proportional to output current (Io=0A:CSEN=0)(Io=4A:CSEN=2.5V)
VSEN=output voltage,it is linearly proportional to output voltage(Vo=0:VSEN=0)(Vo=27.6V:VSEN=
CCON=current control,by this signal u can control output current according to :
Io=0.4+1.44*CCON ; 0V<CCON<2.5V => 0.4A<Io<4A
VCON=voltage contrlo,by this signal u can control output voltage according to :
Vo=13.8+5.52*VCON ; 0V<VCON<3.5V => 13.8V<Vo<33.12V
if the board works stand alone or its control pins are open,it works in simple constant current/constant
voltage mode.Exciting VCON & CCON by a micro controller or other means can change output
characteristics ;also there is a DIPSWITCH in secondary side which can be used for controlling output
voltage level .RESERVED resistors must be calcuated for this reason.


Unitrode/TI App. Notes (
SLUA055.PDF : "Simple Switchmode Lead-Acid Battery Charger " contain useful info. about lead acid
batteries and charging algorithms.
offline charger designing and closing the feedback loop.
SLVA061.PDF : "Understanding Boost Power Stages in Switchmode Power Supplies" for flyback

Power Integrations INC. App. Notes (
All of App. Notes Are Recommended,specially for flyback transformer design and construction,EMI and

24v battery charger schematic

I'm interested in but, from where, I can download your design?

two switch flyback battery charger

i am searching 48volt/200 watt SMPS based charger design .Also i am looking for 40volt/1500 watt SMPS based charger design.Could anybody help me.

swiching charger pcb

I am also searching for project files

battery lead-acid charger schematic board


this is very good of you.
And i admire such pioneers as yorself...bringing in efficient switching designs.

But you will know that you cannot use uc3844 for your power (100W) like that.

You will know that you need a PFC front end.

So i am afraid you will need a boost PFC front end.........and then i would suggest a design like you've done but with a 2 switch flyback as in uk you will have 400V to switch.

you could alternativley use a flyback pfc using say L6561 or NCP1651 and then just use the uc3909 battery charge controller in buck configuration...this would be the cheapest solution.

anyway, i am sure you will know which questions i will ask you i have toiled over the uc3909 myself.

1. How did you decide the value of the R and C that connect to pin 14 of the uc3909 as on page 7, figure 2 of the uc3909 datasheet


As you know, these are stability components, whose value is critical.

Also, when the charger that you have built is swithced ON, your uc3844 will commence switching and the voltage on the secondary (to which the battery is connected) will start rising. However, since the secondary voltage will initially be less than 10.5V, the UC3909 will firstly go into low current "trickle charge" mode.......and the uc3909 will command the uc3844 to keep the voltage at that level which keeps the secondary current at the trickle charge level. As the battery voltage builds up the uc3909 will command bulk charging then constant voltage charging.

but what if the user clips the battery connection + and - together before switching the charger on?.....your charger will be starting into a short circuit.............the secondary voltage will not build up and the uc3909 will NEVER ACTUALLY POWER UP. The UC3909 will thus not feed back an OFF signal for the UC3844 to brake and thus your flyback transformer will quickly go into saturation and your primary fuse (if you fitted one) will blow.

The problem for you is selected a flyback topology......flybacks have high peak you will have had to select a primary fuse bigger than you would your short circuit current will likely damage your charger beyond repair.

Another reason that your primary fuse will unfortunatley be over-sized is because you did not use a pfc front have used a diode bridge.....these only have a narrow conduction angle, and thus high peak currents.....another reason why your primary fuse will be even bigger than you would like.

Not only that, but because you did not use a pfc front end, you have electrolytics after the diode bridge...and therefore high inrush current........another reason why your primary fuse will unfortunateley have to be oversized.

So your charger will be destroyed by the first user who clips or contacts the battery charger connection crocodile clips together.

this is the great problem of battery chargers.......the dreaded customer is being relied upon to make the most important connection of the intended circuit...the connection to the battery that is to be charged.....................and your customer will leave the clips dangling together at some which point..............


this is why customer proof topologies are the only way forward with chargers.
Of course, the way you can get round it is to put a fuse on the secondary just before the battery charger. But you are then again relying on the customer to put in the right size fuse when he/she replaces it......................and i am afraid that the great public dont know the difference between a 1A fuse, a 4A fuse and a 10A fuse....they all look the same, pretty much.
so you are back to the first problem
ok i hear you say.............
--------------------------------------------------- will put in a soild state means of detecting secondary overcurrent and disconnect the battery by means of a series FET when an over current occurs due to the customer shorting the output.

...ok....the "fuse-FET" will be put after the secondary electrolytic the battery
now that really is problem solved.

But you still have the problem of wht happens if

customer attaches a knack*r*d battery to your charger, one that can never charge up?

Customer attachs a fully charged battery to your charger the wrong way round...........


That gives a problem because that series FET that you fitted to cut off any short circuit currents was a needed it to be a MOSFET because you needed it to break the short circuit current quickly..........but MOSFETS have antiparallel diodes in them....................and when the customer attaches a fully charged battery the wrong way round....(remember a customer cannot tell if a battery is charged or not by looking at it so they assume its discharged) your antiparallel diode unfortunately conducts and does not act like a fuse any's not able to break the short circuit current because of its inbuilt diode....that dreadful intrinsic diode thats ALWAYS there....lurking like a bad penny............every mosfet has one......and......................

BANG goes your charger!!!!!!!!!!!!!!!!!!!!!!!!!!..........damaged beyond repair because you used a flyback topology with a diode bridge and smoothing caps at the front end and had to use an over-sized primary fuse.

So i am afraid that your circuit relies on the customer to design the most important connection in your circuit...the connection of the two crocodile clips that go to the battery.

the only way round it is to use a PFC front end......................and then you can get the rating of that primary fuse down really low that no damage occurs to the charger
however, the customer will still put the wrong fuse in and destroys the charger.

ok, so now we know that the only way round it is to use a PFC front end and use a series FET in the primary circuit, and a current sense detector to switch off this fet if an over current occurs.
boost PFC's have overcurrent cut-off, but it is no good because with a boost, the input is connected to the output caps via a diode all the time
flyback pfc's offer respite from this problem.

hello sir..
ur project very amzing..
can i get your schematic design for this circuit..
i hope i can get it.. pleaz email me at tq

Re: battery charger pcb

hi guys!

i have designed a 24v,4A low cost battery charger works in simple constant current mode as stand alone.also it is possible to control its output current/voltage by a microP or even a computer port,if an a/d card is available.

its docs (sch & pcb)in protel format is available.
if u want it respond to this letter!

hello sir..
ur project very amzing..
can i get your schematic design for this circuit..
i hope i can get it.. pleaz email me at

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