solar panel simulator

8/8/2019 Solar Panel Simulator

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Solar panel simulator

Giovanni Romeo and Giuseppe Urbini

Istituto Nazionale di Geofisica e Vulcanologia

Roma, Italia

This is a funny kind of power supply, not stable in voltage or in current: it simulates the

comportment of a solar panel and can be very useful if you are playing around a solar powered

device in a raining day. We designed it for internal use, but it may have a general value.

We may roughly schematise a solar panel with the equivalent circuit shown in figure 1.

I dc

VCC out

Dn

D1

Fig. 1: Solar panel equivalent scheme. The current produced depends of the illumination and of the nature of the cells

(surface, material, geometry) and the voltage is tied also to the number of cells.

There is a simple way to ‘boost’ a generator saving its I-V characteristic: it’s shown in fig 2.

Fig 2: a generator booster. The output is transferred to the out using an amplifier (A), and the current through the load is

measured and used to control the reference generator current through the scale factor B. This circuit saves the I-V

generator shape, supplying the power defined by the scale factors A and B.

You may tune the values of A and B to obtain the wanted output using the I-V shape of the

reference generator.

ammeterA

BV

Z

)( ggg I V V =

load I

load g BI I =

( )load gout BI AV V =

open circuit voltage:depends of the cellsquality and their number short circuit current:

depends the of thecells quality and theillumination



The schematic in fig. 3 is the designed to work simulator, obtained by a generator (like the one in

Fig. 1) boosted following the principle illustrated in fig. 2.

+5

D4

R3

1k

R7

220k

R13

1

+5

R10

20k

0

0

R11

100

D8

RED LED

R875

R18

1K

+24

R14 0.1

U4A

LM339

10

9

4

1 8

2

+

-

V +

V -

OUT

R21

1k

-5

-5

D7GREEN LED

R1

220k

R5

200k

+5

-5

0

+24

U1

LM741

3

2

7

4

6

1

5+

-

V +

V -

OUT

OS1

OS2

-5

+24

+24

0

R4220k

U3

LM741

3

2

7

4

6

1

5+

-

V +

V -

OUT

OS1

OS2

R12 45

U6

ADM660

1

2

3

4

8

7

6

5

FC

CAP+

GND

CAP-

V+

OSC

LV

OUT

D1

0

-5

0

0

0

D51N4148

negative output

-5

D2

Q1

MJE520

D3

0

-5

0

U5

LM7805

1

3

2VIN

G N D

VOUT

0

R2

800k

U4B

LM339

8

6

4

1 8

3

+

-

V +

V -

OUT

D6

+5

R6

4k

+ C2

100uF

+

C1

10u

R9

220k

+5

+5

+24

+

C4

10u

R20

1k

+24

R19

1k

0

U2LM741

3

2

7

4

6

1

5+

-

V +

V -

OUT

OS1

OS2

0

R16

1k

+5

+

C3

100uF

R15

800

R171K

positive output

Fig. 3. The simulator designed to work. The circuit is 24 volts powered (bubble +24 and analog ground), using a

floating power supply (we used a DC-DC converter). The output is between ‘positive output’ and ‘negative output’.

U1 is used as current generator, and the constant current, flowing through a series of diodes (D1,

D3, D5) is obtained by reading the shunt R13; the diodes and the constant current source form the

reference generator (as shown in fig.1). D2, D4, D6 and U2 operate a temperature correction. U3

and Q1 (fig. 3) are used as output amplifier. The information about the load current is obtained bythe shunt R14. Both of the shunts, R13 and R14 work in the low side; this is not elegant but, since a

solar panel is a floating device, you need to power the whole circuit with a floating voltage source,

and the low side shunts work perfectly. The number of cells (open circuit voltage) is obtained

changing the output gain modifying the R10 value (A gain in fig.2); the resistor R12 regulates the

short circuit current (Idc in fig. 1) by changing the B gain (fig. 2). Op amps are not critical, and

many modern quad op amps may replace the old beloved 741.

The circuit shows the power coupling conditions using a two-colours LED. It lights green when the

load is too weak, red when it is too heavy, and orange when it approaches the maximum power

transfer, indicating that your load is well coupled with the simulated panel. This is useful to quickly

check MPPT charge regulators.

This LED is operated by a couple of comparators (U4 A and B) looking at the voltage of the diodesseries. This is done after the thermal correction.

Fig.4 shows the simulator comportment, the blue curve is the typical V-I curve of a solar panel.



The orange curve represents the power output. The traces red and green represent the LED status.

0 2 4 6 8 10 12 14 16

0

1

2

c u r r e n t ( A )

, p o w e r ( W / 1 0 ) , L E D s t a t u s

voltage (V)

currentred LED

green LED

power

maximum power point

Fig. 4: Simulator comportment. The blue trace shows a typical I-V curve of a solar panel; red and green traces show theLEDs actions. The light is green in case of weak load (low current, high voltage), red in case of heavy load (high

current, low voltage) and orange (both LEDs on) around the MPP. The LEDs thresholds can be tuned using R17 and

R18 in Fig. 3. The orange trace represents the output power.

solar panel simulator

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