SPC3-b1 High Voltage PV panel modification

Modified LVD section of the SPC3-b1 schematic: (with VR3, C14 and C9 added)

SPC3b1 HV PV schematic mod

Modified SPC3-b1 circuit board, note the cut trace on IC2 pin 4:

SPC3b1 HV PV circuit board mod

SPC3-b1 High Voltage PV panel modification

(C) 2012, G. Forrest Cook


The SPC3-b1 solar power center is limited to use with a PV panel with a maximum output voltage of 20V (see the SPC3 specifications for details), this range is acceptable for most common 36 cell PV panels.

Some PV panels (typically of the amorphous variety) have an open-circuit voltage that can exceed 20V. If the battery is disconnected and the LVD section is turned on, the supply voltage for IC2 (TLC27L4CN) can exceed the device's 20V maximum and destroy the IC, leaving the LVD circuit inoperable. This modification allows the SPC3 to safely handle up to 26V across the PV input.

The modification involves cutting one circuit board trace and installing a low voltage dropout (LDO) regulator and two associated capacitors between the power source and IC2.

Parts required for the modification include a Texas Instruments TL750L12CLP LDO regulator IC (VR3), a 47uF 35V (or 50V) electrolytic capacitor (C14) and a 1uF monolythic capacitor (C9).

To perform the modification, first cut the circuit board trace next to IC2 pin 4 as shown in the photograph. Use an utility knife to cut two lines across the trace, then scrape off the green solder mask between the cuts to reveal the board copper. Use a soldering iron to apply solder to the exposed copper and the copper will peel off of the circuit board. Scrape the exposed area clean with the knife.

Bend the leads of the TL750L12CLP regulator to fit over the circuit board pins as shown in the photograph. A piece of wire insulation or teflon tubing should be installed over the input pin of the regulator to prevent shorting to the nearby wiring.

The 47uF capacitor should be installed across the input of the regulator and ground, the negative side of the capacitor should connect to the regulator's center pin. The 1uF (105M) capacitor should be installed between the regulator's output pin and ground, this can be paralleled across the existing 100nF (104M) C9.

Verify that the modification looks the same as the above photograph. Excess solder flux should be chipped off of the board or cleaned with alcohol and a tissue. If the SPC3 will be used in a high-vibration environment, you may want to hot-glue the 47uF capacitor and regulator IC to the back of the circuit board.

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