Design ProcessDesign Process

Analysis & EvaluationAnalysis & EvaluationPart II Part II

Example Design: Solar CandleExample Design: Solar Candlebyby

Prof. BitarProf. Bitar

Existing Window Candle Block Existing Window Candle Block Diagram Diagram

Solar CellCharge

Controller

RechargeableBattery

1.2V NiCd

DC-DCBoost

Converter

LED3.2V

20mA

FlickeringControl

ModeSelection

PhotoSensor

Diodes/Zetex - ZXLD383ET5CTDiodes/Zetex - ZXLD383ET5CT

Typical Application CircuitTypical Application CircuitNote Control Pin!Note Control Pin!

DC-DC ConverterDC-DC ConverterEfficiencyEfficiency

Modified System Block DiagramModified System Block Diagram

Solar PanelCharge

Controller

RechargeableBattery

1.2V NiCd700 mAhrs

ZetexLED

Driver85% Eff.

LED20mA

3.2V(min)

SwitchingControl

ModeSelection

PhotoSensor

Timer

Changing Focus to ChargingChanging Focus to Charging

How much energy is removed from the How much energy is removed from the battery during a typical evening? battery during a typical evening? • LED requires 20mA x 3.2V x 6hrs = 384 mW hrs LED requires 20mA x 3.2V x 6hrs = 384 mW hrs

(power x time = energy)(power x time = energy)• Converter is only 85% efficient, so energy taken Converter is only 85% efficient, so energy taken

from battery is 384 mW hrs / 0.85 ≈ from battery is 384 mW hrs / 0.85 ≈ 452 mW hrs452 mW hrs

How much charge?How much charge?• Dividing by the battery voltage gives the charge Dividing by the battery voltage gives the charge

removed: 452 mW hrs / 1.2V removed: 452 mW hrs / 1.2V ≈ ≈ 377 mA hrs 377 mA hrs

The Prior Art Dissected

On to the Solar Panel RequirementsOn to the Solar Panel RequirementsAfter taking the Home Depot Landscape Light After taking the Home Depot Landscape Light apart, I made the following measurements (in apart, I made the following measurements (in

direct sun): direct sun): IISCSC = 50mA , V = 50mA , VOCOC = 4.3V = 4.3V

Solar Panel V-I CharacteristicSolar Panel V-I Characteristic

T

Solar Cell Voltage (V)0.00 1.00 2.00 3.00 4.00 5.00

So

lar

Ce

ll C

urr

en

t (A

)

0

10m

20m

30m

40m

50m

Solar Panel ConsiderationsSolar Panel Considerations

How much charge is restored if the panel is How much charge is restored if the panel is connected directly to the battery? What connected directly to the battery? What assumptions should we make?assumptions should we make?

How aboutHow about• 10 Hours of Daylight 10 Hours of Daylight • 50% Incident Light 50% Incident Light

This gives 50mA x 10hrs x 50% = 250mA hrsThis gives 50mA x 10hrs x 50% = 250mA hrs

Is this enough? Is this enough? We need 377 mA hrs. No. We need 377 mA hrs. No.

Charge Options?Charge Options?

Use two solar panels in parallel to boost Use two solar panels in parallel to boost the current (but we’re throwing away the current (but we’re throwing away voltage?)voltage?)

Use the existing panel with some sort of Use the existing panel with some sort of Buck Converter (will need to look at Buck Converter (will need to look at efficiency).efficiency).

Find a solar cell better suited for this Find a solar cell better suited for this application…application…

Found something at Found something at Futurlec.com !Futurlec.com !

Open Circuit Voltage (Voc): 2.2VOpen Circuit Voltage (Voc): 2.2V

Short Circuit Current (Isc): 100mAShort Circuit Current (Isc): 100mA

Dimensions:Dimensions:

61mm x 61mm61mm x 61mm

Price:Price:

$1.50 (100+ Qty)$1.50 (100+ Qty)

Modified CharacteristicModified CharacteristicT

Solar Panel Voltage (V)0.00 1.00 2.00 3.00

So

lar

Pa

ne

l Cu

rre

nt (

A)

0

20m

40m

60m

80m

100m

IISCSC = 100 mA , V = 100 mA , VOCOC = 2.2 V = 2.2 V

(V(VOCOC greater than V greater than VBATBAT))

A Possible SolutionA Possible Solution

Now we have: 100mA x 10 hrs x 50% = Now we have: 100mA x 10 hrs x 50% = 500 mA hrs.500 mA hrs.

Is this enough? We need 377mA hrs. Yes! Is this enough? We need 377mA hrs. Yes!

Solar Panel Update to System Solar Panel Update to System Block DiagramBlock Diagram

Solar PanelISC = 100mAVOC = 2.2V

IAVE = 50mAΔt = 10hrs

Q = 500mAHrs

ChargeController

RechargeableBattery

1.2V NiCd

ZetexLED

Driver

LED20mA

3.2V(min)

SwitchingControl

ModeSelection

PhotoSensor

Timer

And now the Charge And now the Charge Controller…Controller…

Solar PanelISC = 100mAVOC = 2.15VIAVE = 50mAΔt = 10hrs

Q = 500mAHrs

ChargeController

RechargeableBattery

1.2V NiCd

ZetexLED

Driver

LED20mA

3.2V(min)

SwitchingControl

ModeSelection

PhotoSensor

Timer

NiCd Charge Control Methods NiCd Charge Control Methods (Panasonic)(Panasonic)

Which Charge Method to Choose?Which Charge Method to Choose?

Semi-Constant Current ChargeSemi-Constant Current Charge• Most Typical Charge SystemMost Typical Charge System• Simple and EconomicalSimple and Economical• Typical Charge Time = 15 HrsTypical Charge Time = 15 Hrs• Typical Charge Current = 0.1 It Typical Charge Current = 0.1 It

(0.1*700 mA Hrs = 70mA)(0.1*700 mA Hrs = 70mA)

Time Controlled ChargeTime Controlled Charge• More reliable than Semi-Constant CurrentMore reliable than Semi-Constant Current• Slightly more complicated. Requires timer.Slightly more complicated. Requires timer.• Typical Charge Time = 6-8 HrsTypical Charge Time = 6-8 Hrs• Typical Charge Current = 0.2 It (140mA)Typical Charge Current = 0.2 It (140mA)

Semi-Constant Current Charge Semi-Constant Current Charge Seems ViableSeems Viable

With our low average current of 50mA, With our low average current of 50mA, and charge time of 10 hrs, the Semi-and charge time of 10 hrs, the Semi-Constant Current Charge method seems Constant Current Charge method seems viable.viable.

Also, if we are concerned about over Also, if we are concerned about over charge, we can extend the on-time beyond charge, we can extend the on-time beyond 6 hrs.6 hrs.

This method is more economical and may This method is more economical and may not require a timer for this application. not require a timer for this application.

Charge Controller UpdateCharge Controller UpdateSolar PanelISC = 100mAVOC = 2.15VIAVE = 50mAΔt = 10hrs

Q = 500mAHrs

ChargeController

Semi-Const.CurrentMethod

RechargeableBattery

1.2V NiCd

ZetexLED

Driver

LED20mA

3.2V(min)

SwitchingControl

ModeSelection

PhotoSensor

Timer