Voltage Measurement

The sensor node utilizes the linear optocoupler method mentioned in Section 2.4.1.2 to make voltage measurements. The linear optocoupler used is the IL300 from Vishay Semiconduc- tors. The IL300 has 0.01% servo linearity (The ratio between the photo-diode current and the LED current) and a high gain stability of ±0.005%/◦_{C [17]. The IL300 has a power}

consumption of < 15 mW with an isolation voltage of 5300 VRMS tested for 1 second. The LED in the IL300 is an infra-red emitter that emits at 890 nm and operates eciently with a drive current from 500 µA to 40 mA. The IL300 data-sheet recommends a drive current between 5 mA and 20 mA for the best linearity.

4.4.4.1 Circuit Design

The voltage being measured by means of the optocoupler circuit range from 0 - 46 V according to Table 4.1. The rst part of the voltage measurement circuitry is a voltage divider that

scales the input voltage (Vmod). With a safety margin added the circuit was designed to have

an output voltage (Va) of 4.5 V for a input voltage (Vmod) of 50 V. The voltage measurement

circuit diagram can be seen in Figure 4.11.

Figure 4.11: Voltage Measurement Circuit Diagram

Resistor R1 was chosen as 1 Mega-Ohm for low energy dissipation. The equation for resistor R2 can be written as:

R2 = R1 · Va Vmod− Va

= 1M egΩ · 4.5V

50V − 4.5V ≈ 100kΩ (4.2) Resistor R4 creates voltage Vb through the current IP 1. R4 is calculated at a certain LED

current (IF). R4 was designed at the following conditions: The input voltage (Va) is at a

maximum (4.5 V) and the LED current (IF) is 20 mA. The LED current (IF) was chosen

as 20 mA for the maximum input voltage, since 20 mA is the upper boundary for the best linearity - as mentioned in the device data-sheet [17]. The relationship between the LED current IF and the photocurrent IP 1 is shown in Figure 4.12 [17].

From Figure 4.12 it can be seen that the photocurrent IP 1 will be ≈ 150 µA for a LED

current IF of 20 mA at room temperature. The equation for R4 can be written as:

R4 = Vb IP1 = Va IP1 = 4.5V 150µA = 30kΩ (4.3)

Resistor R5 is chosen to be the same value as R4 since the IL300 has a transfer gain (K2/K1) of ≈ 1 [17]. The output voltage (Vout) will thus theoretically be equal to Vb which is equal

to Va.

Resistor R3 is the LED current (IF) limiting resistor. The output voltage of the Op-amp

(VU 1) is targeted to be 50% of the supply voltage (Vcc), or 2.5 V when the LED current (IF)

is 20 mA and the input voltage (Va) is 4.5 V. The Op-amp output voltage (VU 1) was chosen

as 50% of the supply voltage because this is the voltage at which a single supply Op-amp delivers it's highest output current. The typical LED forward voltage (VF) of the IL300 is

1.3 V as stated in the device data-sheet [17]. The equation for R3 can be written as: R3 = VU 1− VF

IF

= 2.5V − 1.3V

20mA = 60Ω (4.4)

Components The operational ampliers that are used in the voltage measurement cir- cuitry are OPA347 rail-to-rail single supply operational ampliers from Texas Instruments. The OPA347 has rail-to-rail input and output and operates from a single supply of 2.3 - 5.5 V. The input range of the OPA347 extends 200mV beyond the rails and the output range is within 5mV of the rails [38]. The OPA347 can source 25mA at an output voltage of 2.5V with a supply voltage of 5V.

An isolated power supply is required to power the 'high side' Op-amp. This is done with a CRE1S0505SC 1 Watt isolated DC-DC converter from Murata Power Solutions. The CRE1S0505SC has an input voltage of 4.5 - 5.5 V and an output voltage of 5 V. The device has a minimum isolation voltage of 1000 VDC and a maximum output current of 200 mA [39]. The circuit design of the isolated DC-DC converter can be seen in Figure 4.13. The DC-DC converter is powered by the main 5 V power supply. A capacitor is placed across the output for a smooth output voltage, while a Zener diode in the same position prevents the output voltage being higher than 5.1 V when the load on the DC-DC converter is less than 10% as stated in the device data-sheet.

Figure 4.13: High Side Power Supply Circuit Design

The circuit design of the voltage measurement circuit can be seen in Figure 4.14 and 4.15. A capacitor C7 placed in parallel with resistor R3 to form a low-pass RC lter that lters out

high frequency and noise components from the measured input voltage. The -3 dB cut-o frequency of the RC lter is:

fc=

1 2πRC =

1

2π · 100kΩ||1M egΩ · 100nF = 17.5Hz (4.5) The rest of the circuitry is similar to the circuitry from Figure 4.11. The circuitry is split up into a 'High Voltage Side' (Figure 4.14) and a 'Low Voltage Side' (Figure 4.15) with the isolation barrier between the two sides being the IL300 linear optocoupler. On the 'High Voltage Side' Red(+) and Black(-) refer to the PV module positive and negative connections. The output of the voltage measurement circuitry (A0_Voltage) is connected to one of the analog input pins (A0) on the microcontroller as seen in Figure 4.4.

Figure 4.14: Isolated Voltage Measurement High Voltage Side Circuit Design

Figure 4.15: Isolated Voltage Measurement Low Voltage Side Circuit Design