SC_Intelligent_Solar_Charger.pdf

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SILICON CHIP

www.siliconchip.com.au

here are many people across

Australia, nay, around the

world, who rely on "free"

power from the sun, courtesy of the

solar cells mounted on their roofs.

For many of those, solar power is

their only source of power: typically,

these are people who live too far away

from the electricity grid to make con

nection economic. For them, the

somewhat questionable economic re

turns of solar power don't come into

the equation: if you want power, you

have to make it yourself.

As we mentioned in that feature,

the basic choices are hydro, wind,

bio-mass or solar. And while there

are plenty of micro-hydro systems,

wind generators and even some small

scale bio-mass systems, by far the larg

est percentage of people opt for solar

power.

However, there are many others,

city and country who, for many rea

sons -environmental, experimental,

(or perhaps just plain mental!) have

decided that they too would like some

of this "free" power.

The main difference between solar

power in the suburbs or towns and

remote solar power is the way the

power is used when it is generated.

Where increasing numbers of city

dwellers with solar power these days

probably have "grid-linked" systems,

invariably, remote solar power gen

erators must use some pretty muscly

batteries to store the power when the

sun is out, ready for use when it is (a)

needed, or (b) dark/cloudy/rainy/etc.

Typically, banks of storage batter

ies are used. In the past, a lot of peo

ple have used (expensive!) traction

type batteries (eg, fork-lift, etc) be

cause these are designed to be deep

cycled.

Such treatment destines your typi

cal car or truck battery to a very short

life.

In recent years, batteries have come

onto the market which are specifically

intended for power storage (eg, solar

power) applications.

Most systems use series and parallel

connected batteries to give both high

current and high voltage (well, higher

than six or twelve volts!) systems.

The reason for this is mainly in the

higher efficiency of

DCI

AC inversion

from a higher voltage and lower I2R

losses in the system.

24V

is common,

as is

48Y.

Above this, though, you

could start to get into difficulties with

running from

12V

solar cells.

That's not to say a

12V

battery sys

tem is not perfectly practical; in fact,

you can use a commercially-available

12V/240V

(or more usually

230V)

verter and save a lot of hassles. Some

of these are very efficient, these days.

And we aren't saying that anyone

in the middle of suburbia shouldn't

put in a solar power system, if that is

your want. Whether you want to save

the planet or not (or perhaps you've

come across some cheap solar pan

els!) you have every right to put in

your own system.

Where the situation does become a

bit muddied is when you want to

connect your solar system to your

home wiring, using existing power

outlets and so on.

The power authorities have some

pretty strict rules about how this is

done, especially in the way your sys

tem is isolated from theirs.

We suggest if you do want to put in

a solar power system, keep it com

pletely separate from the domestic

mains supply.

Besides, unless you're a licenced

electrician, you're not allowed to do

lN4004

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12VONlY

120k

INPUT

FROM

SOlAR

PANELS

(CON1}

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MBR1645

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INTELLIGENT SOLAR CHARGER

The intelligent charger is built around a specialised IC, an

L4949

made by

On Semiconductor.

can suit

12V

and

24V

systems.

A=rB

D~D

MARCH 2002

www.siliconchip.com.au

anything with your home wiring. But

that's another story in itself!

Charging the batteries

Having invested sometimes thou

sands of dollars in batteries, it is im

portant to "treat them right" to max

imise not only their life but also the

power you can store in them and get

from them.

"Treating them right" means not

only the way they are stored (eg, bat

teries don't normally like being placed

directly on concrete floors), main

tained (eg, distilled water level where

appropriate) but also in the way they

are charged and discharged.

has been fairly common practice

to simply connect the solar cells in

series with the battery, usually via a

series diode to prevent the battery

discharging through the cells when

they're not producing power.

As the solar cells are essentially a

constant current device, this is not a

real problem when the batteries are

either fully or partially discharged.

However, it is not good for the bat

teries when they are charged. The

solar cells don't know this and they

keep on pumping out power while

ever the sun shines. Result: over

charged batteries.

This will certainly lower the bat

tery life - and that's why you need a

regulator.

senses the state of charge:

while the batteries are less than fully

charged,

allows the solar cells to

pump in as much power as good

01'

Sol will allow. But when they are

nearly charged, it starts throttling back

the electrons so the battery won't over

charge.

Larger-than-life view shows the input and output connectors at the front of the

PC board along with the (optional) fan. This fan should not be needed for solar

panel systems (a small heatsink will suffice).

Circuit operation

This circuit is designed for either

12V and 24V systems with the chang

ing of just one link.

At the heart of the circuit is IC2, an

L4949 monolithic integrated 5.0V

voltage regulator with a very low drop

out voltage and additional functions

such as power-on reset and input volt

age sense. In this circuit we use the

5V regulator because of its extremely

low quiescent current. When there is

no power source (ie, solar cells) con

nected, the total current drawn from

the battery is around 300uA. We also

employ the voltage sensing compara

tor section of this IC as the main

switching device with hysteresis. The

power-on reset circuit is not used.

Incidentally, a specification sheet

for this IC can be found at the manu

facturer's (ON SEMICONDUCTOR)

web site:

www.onseml.com/pub/prod 0.1824.

productsm ProductSummary BasePart

Number%253DL4949.00.html

Instead of typing all that, it is prob

ably easier to search for L4949 at

google.com as

will be the first item

to come up, in less than a second!

For the following explanation, as

sume that there is a supply voltage

present at the source (Solar Panel etc],

therefore the voltage at pins 9 and 13

ofICl would be at logic 1.

Pin 2 is the input pin for the battery

sensor section of the IC. When the

voltage at this pin falls to 1.24V the

open collector output pin 8 is pulled

internally to ground. This pin would

normally be connected in series with

a resistor and a Battery Low indicator

LED to a positive supply.

In this circuit pin 8 pulls the input

of IClb to logic 0 level via a 120kn

resistor so the output from this in

verting gate would be at logic 1. Since

both the inputs of ICld are now logic

1 the output would be at logic 0 and

K&W HEATSINK EXTRUSION. SEE OUR WEBSITE

FOR THE COMPLETE OFF THE SHELF RANGE.

SILICON CHIP

www.siliconchip.com.au

LED2 (Red) would light to indicate

that the battery was charging:

Because of the inverting action of

ICla, the level at the output of IClc

would remain at logic 1 and LEDl

would not light. Q4 is turned on via

the 120k.o resistor and the gates of p

channel Mosfets Q2 and Q3 are pulled

low via the 22k.o resistor. Q2 and Q3

conduct, allowing the battery to

charge. A small amount of current is

fed by the forward biased diode (D5)

and the 2.2M.o resistor to the voltage

divider network, thus effectively

slightly increasing the voltage at the

sensing pin, (pin 2). The addition of

this resistor effectively reduces the

hysteresis voltage of this part of the

circl.\it.

When the voltage at pin 2 rises to

1.34 V, the internal transistor at the

output is turned off and the voltage at

the input of IClb is pulled high (to

+5 V), again via the 120k.Q resistor.

LED2 is turned off and LEDl (Green)

is turned on, indicating that the bat

tery is fully charged. Transistor Q4

and the Mosfets are turned off so the

charging ceases.

For a 12V battery (LINK2 in) and

with the values selected in the resis

tor divider network and a centred

potentiometer, the voltage of the bat

tery being charged will need to reach

approximately 14.2V before the charg

ing is stopped.

Charging will will resume when

the battery voltage drops to 13. 7V.

For a 24V battery (LINK2 out), the

voltage of the battery being charged

will need to reach approximately

28.4 V before the charging is stopped.

Charging will resume when the bat

tery voltage drops to 27.4V.

2N555

0.330

::::It

•

UNK

IN FOR SOLAR PANas

OUT FOR POWER SUPPUES

UNK 2: IN FOR 12V BATTERIES

OUT FOR 2AV BATIERIES

Same-size views of the component overlay and matching straight-on

photograph. The 3.3W resistor in the pic below is actually in the "Link

position - but it doesn't matter 'cos they're in parallel.

Charging from a

supply

While the circuit is designed for

use with 80lar panels, it can (with a

minor modification) be used with

other sources of power.

Solar panels have a limited cur

rent output so it does not matter if

they are connected directly across the

battery: the current will be similar in

value when the battery is "full" or

"flat". When this charger is used as a

regulator for solar panels, the 0.33.0,

5W resistor should be shorted with a

link for most efficient operation. In

this case the only loss is due to the

"on" resistance of the Mosfets and

the low forward drop of the Shottky

diode/so

www.siliconchip.com.au

However if the charger is used in

conjunction with power sources that

do not have current limiting (for ex

ample a bench power supply or an

automoti ve battery charger) the cir

cuit can be made to current limit by

removing the link across the 0.33.0

resistor. When the voltage across the

current limiting resistor exceeds 0,6V

transistor Ql is turned on, thus re

ducing the gate voltage applied to the

Mosfets. This serves as a simple con

stant current source, the value of

which equals 0.6/0,33A. To increase

the current, reduce the value of the

resistor.

To minimise battery drain when

the solar panel is not supplying power,

the voltage at pins 9 and 13 ofICl are

logic low and both the LED's are at

turned off no matter what the state of

the battery is,

Two series diodes, D3 and D4,

were added to reduce the supply volt

age to IC2 by approximately 1.2V. This

is necessary for a 24 V battery as al

though the IC has a transient supply

voltage of 40V, its maximum continu

ous supply voltage is 28V.

In each kit are one lOA Shottky

diode and two power Mosfets, The

total dissipation in the two Mosfets

would be approx, 0,15W at lA, rising

to 2.4W at 4A, Doubling the number

MARCH 2002

Parts List - Intelligent

Solar Charger

1 PC board, 98 x 70mm, code

K009B (Oatley Electronics)

1 U-shaped heatsink (or

fanl

heatsink - see text)

Semiconductors

1 4011 quad Schmitt trigger

(IC1 )

L4949 voltage regulator (IC2)

BC557 PNP transistor

(01)

MTP2955 P-channel mosfets

(02)

(Can use two - see text)

2N5551 NPN transistor

(04)

MBR1645 Schottky diodes

(01) (Can use two - see text)

3 1N4148 small signal diodes

(03-05)

15V 0.5V zener diode

Green LED (LE01)

Red LED (LE02)

Capacitors

3 1OOJlF 35VW electrolytic

1 100flF 16VW electrolytic

2 .033JlF MKT polyester (code

33n or 333)

Resistors (0.25W, 1%)

2 2.2MO

4 120kO 4 22kO

1 6.8kO

1 12kO

41 kO

1 1000 1W (for optional fan)

1 0.330 5W (only required if

power supply is used instead

of solar panel)

Optional:

1 12V fan/heatsink

resistor is supplied in the kit. With

this the current is limited to 0.6A, so

the dissipation in the two mosfets

would be a total of 1.5W for a 2.5V

voltage difference (this figure applies

when the optional Kenwood plugpack

is used).

Construction

Wi th the exception of the (optional)

fan, all components mount on a sin

gle PC board, coded K009B. As usual,

inspect the board before assembly for

any defects - shorts between tracks or

broken tracks - and if necessary, re

pair them.

Most of the construction is pretty

much standard: start with the lowest

profile components first (resistors,

small capacitors) and move from their

to the larger capacitors (watch the

polarity on the electrolytics) and then

the semiconductors.

Naturally, all semiconductors are

polarised so ensure they go in the

right way. Leave the two Mosfets and

one or two Schottky diodes for a mo

ment.

Whether you use sockets or not for

the ICs is up to you but if you do, be

careful to align the sockets the same

way as shown on the PC board over

lay, and be even more careful to get

all the pins in straight when inserting

the ICs.

Now's the time to decide what for

mat you're going to build the regula

tor in - ie, for a 12V or 24V system,

and whether it is for solar panels or

for use with a power supply.

For 12V, a small link shorts out the

120kO and 22kO resistors near the

lower right corner of the board (leave

the link out for a 24 V system). Of

course these two resistors are redun

dant and could be left out but for the

sake of ten cents, they might as well

be included.

The second choice (solar cells or

power supply) determines whether

the 0.330 resistor is in circuit or not.

For solar cells, it can be shorted out

via a link (left side of PC board) but if

you are going to use it on any device

without current limiting (or want to

make it dual purpose), keep the resis

tor in circuit (ie, don't solder the link

in).

The Mosfet(s) and diode(s) are the

last components to solder in. They

may look quite similar so don't mix

them up! The one or two Mosfets

(depending on your requirements)

mount at the top of the board with

their metal siders) up - that is, oppo

site to the way you would normally

solder them into a circuit. This is to

allow contact with the heatsink.

The one (or two) Schottky diode(s)

mount at the bottom of the board (clos

est to the connectors) and solder in

the "normal" way - metal side down.

Finally, solder in the two PC board

mounting screw connectors, CONl

and CON2 and the board is finished.

Setting up

To set the charge, you will need to

have the 12V or 24V battery connected

and the solar panel(s) or power sup

ply connected. You can set it with a

power supply and use the same set

ting for a solar panel but make sure

the 0.330 resistor is in circuit if you

do!

Turn VRl fully clockwise. Monitor

the battery voltage (with a multim

eter) and when the battery reaches its

correct charge voltage (14.2V or 28.4V

for 12Vand 24V systems respectively),

slowly turn VRl anti-clockwise until

the green LED lights.

Optional fan

you decide you want to fit the fan

(as shown in the prototype) this sim

ply clips over the PC board along

with its integral heatsink.

However, as we mentioned, for use

with solar panels this fan should not

be necessary - a small heats ink will

suffice.

The 1000 resistor on the PC board

allows the nominally 12V fan to run

from the higher voltage produced from

the solar panels (up to 18-20V).

of mosfets would reduce this total

power dissipation by

1/2.

Increasing the number of Mosfets

results in better efficiency but is

hardly needed. Other types of Mosfet

with a lower "on" resistance could be

used (an MTP2955 has an on resist

ance of 0.30).

As an example a 60W solar panel

is rated to deliver approxiamtely 4.3A

into a floating lead acid battery (14V).

With this panel the mosfets would

dissipate a total of about 2.8W. A small

heatsink would be necessary but a

fan is not.

The fan shown in our photographs

is an option, for use when the link is

removed and the circuit is used as a

constant current source. Here the to

tal dissipation in the Mosfets becomes

the supply voltage minus the battery

voltage times the current. A

lOll

SILICON CHIP

Wheredyageddit?

This design is copyright Oatley

Electronics (PO Box 89, Oatley, NSW

2223). Phone 02 9584 3563; Fax 02

9584 3561.

website: www.oatleyelectronics.com;

email sales@oatlelectronics.com

Kit/Component Prices

BASIC KIT: PCB and all components

but with one Shottky diode: $21.00

Optional clip-on fan/heatsink: $4.50

Extra Mosfets:

$3.00

Extra Shottky diodes:

$3.00

16.5V

1650mA

Kenwood plugpack with

non-standard mains connector: $4.00

Postage for any qty/mixture: $7.00

www.siliconchip.com.au

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