# LTspice simulation of behavioural voltage sources

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#### desan2012

##### New Member
Hi everyone,

I am using LTSpice IV version 4.231.

I have a polynomial fit to a current-voltage characteristic curve and would like to simulate in LTSpice using B sources.

The polynomial fit is:

#### alec_t

##### Well-Known Member
Welcome to ETO!
Set the bv source value to V=05407*i(n)**5+4.8027*i(n)**4 etc, where n is the identifier of the component in which current x flows.

#### desan2012

##### New Member
many thanks alec_t.

Do I place the identifier 'n' on the schematic?

The equation is a polynomial fit for the I-V characteristics of a photovoltaic cell.

I carried out a couple of experiments in the lab and plotted the I-V curve at a specific irradiance.

Would 'n' therefore be Rs or Rp on the PV model?

#### alec_t

##### Well-Known Member
Would 'n' therefore be Rs or Rp on the PV model?
I don't know what PV model you're using, but if you're interested in the Rs current then put Rs in place of n.
Here's an example with a LED and a bv source.

#### MikeMl

##### Well-Known Member
Are you saying that at a given irradiance,
the Voltage V from the panel = 0.5407*I**5 + 4.2807*I**4 + 14.649*I**3 + 16.603*I**2 + 8.4911*I + 261.88, where I is the Current output of the panel?

The attached .asc file is how to model that:

I1 is the independent variable of the simulation. The red trace shows the MPPT.

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#### desan2012

##### New Member
Many thanks to alec_t and MikeMI.

My apologies for not getting back yesterday.

The PV equivalent circuit is the single diode model with Rs and Rsh.

My apologies for the misunderstanding. The equation is actually:

I=0.5407v^5+4.8027v^4+14.649v^3+16.603v^2-8.9411v+261.88

where I is the output current supplied by the PV cell and V is the voltage across it.

#### MikeMl

##### Well-Known Member
...
My apologies for the misunderstanding. The equation is actually:

I=0.5407v^5+4.8027v^4+14.649v^3+16.603v^2-8.9411v+261.88

where I is the output current supplied by the PV cell and V is the voltage across it.
That would say that the short-circuit (V=0) panel current is 261.88A. I would like to see your panel

The way I interpreted the equation is that the open-circuit (I=0) output voltage of the panel is 261V and per my plot, the short-circuit (v=0) output current of the panel is ~4.2A, which is more like real panels I have worked with.

Which is it?

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#### desan2012

##### New Member
MikeMI let me cross-check with the graph and get back to you.

#### desan2012

##### New Member
Another misunderstanding. When V=0, the panel current is 261 uA (micro amperes).

For simplicity, the PV cell is the AM-1816CA. I am using it for indoor PV energy harvesting.

#### alec_t

##### Well-Known Member
According to the spec, at 200 Lux you can harvest ~250uW. Doesn't seem much. What do you plan to do with the energy harvested?

#### desan2012

##### New Member
The AM-1816CA can produce a short-circuit current of 261 uA for a light intensity of 500 lux incident on it.

The intended application is Internet-of-things. Looking into powering low-power sensors e.g. proximity, CO2, temperature, light level sensors.

#### MikeMl

##### Well-Known Member
Another misunderstanding. When V=0, the panel current is 261 uA (micro amperes)...
Units are important..., so are boundary conditions.

Here it is again:

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#### desan2012

##### New Member
Appreciate the assistance MikeMI, alec_t and ronsimpson

Will keep in mind to always specify units and boundary conditions in the future.

Kind regards.

#### desan2012

##### New Member
MikeMI, could you explain how you derived the spice equation. New to LTSpice.

Read in an article online that the polynomial fit curve needs to be converted to Laplace transform for use in LTSpice??

#### MikeMl

##### Well-Known Member
You are making it too hard. Read the Help file for
B. Arbitrary Behavioral Voltage or Current Sources.

and

.DC -- Perform a DC Source Sweep Analysis

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