Light Table using BenQ GL2430 Backlight

Summary

This blog illustrates how the salvaged LCD backlight from a BenQ model GL2430 monitor could be used in the design of a light table. 

GL2430-B BenQ Monitor

Just Another Light Table
Online suppliers such as Amazon have amazing light tables at prices which would force one to question why salvaged electronics should be used. In reply, this is a blog relates to salvaging, reusing electronic waste and my requirement was for dual purpose light table, home and workshop - this meant a robust and repairable solution.

LED Driver
In a previous blog relating to the salvaging of parts from a BenQ monitor, the LED driver and backlight were confirmed operational before starting the salvaging operation.

Further bench testing was needed to determine how the LED controller could be reused. At the core of the controller board sits a Monolithic Power System LED controller - MP3389. After perusing the datasheet a section on dimming on page 9 caught my interest.

MP3389 Typical Application (Courtesy Monolithic Power Systems)

This section of the MP3389 datasheet describes dimming control using a PWM signal or a DC signal. Dimming was not an immediate requirement for this project, although it was advantageous to know if such a feature could be added if required.

MP3389 DC Dimming Control
Further on page 13 of the MP3389 datasheet is a figure displaying the DBRT (Brightness Control) input. In order to use dimming with an external DC input voltage, a capacitor must be connected to the BOSC (Dimming Repetition Set) pin.

MP3389 Dimming (Courtesy Monolithic Power Systems)

Checking the LED controller PCB it was apparent there was a resistor, R801, connected to the BOSC pin. This resistor was replaced with a small ceramic capacitor 0.033uF to set the required frequency.

MP3389 LED Controller Change R801 to Capacitor

The MP3389 datasheets lists the minimum operating voltage for the device as 5VDC. After the power connections were determine from the old BenQ loom, a benchtop power supply was set to 5VDC with a 500mA current limit.

BenQ Controller and Backlight

With the power supply limited to 500mA the intensity of the backlight was comfortable to look at in a room with fluorescent lights. Full current was close to 1.2A at 5V DC. With a variable resistor attached to the BRBT pin, the dimming voltage to the DRBT pin of the MP3389 was adjusted until the power supply indicated around 480mA.

Control PCB Mounting
By some odd chance the controller fit neatly into a plastic enclosure. This was a UB5 plastic enclosure from a local supplier.

Boxed MP3389 Controller

The variable resistor was measured between wiper and external terminals. This converted the variable resistor into two fixed resistors, 68k and 22k. As seen in the image below, the larger value resistor connected between the 5V supply and the dimming pin. The smaller value resistor connected between the dimming pin and supply 0V.

Boxed and Modified MP3389 Controller

A chassis mount DC jack was added to one side of the enclosure away from the PCB. Also a small slot was added for the loom and connector which attaches to the backlight assembly.

Frame Construction
An aluminium lipped tube was used for the construction of the frame. The lip was used to retain the backlight.

Lipped Aluminium Tube

To connect the frame together, plastic corner pieces were utilised.

Tube Corners

The aluminium tube was cut into four pieces. Two at 555mm and the other at 325mm. A circular saw made light work of the tube.

Cut Aluminium Tube

Fitting the plastic tube corners was achieved with an engineer's square to check for square and a rubber mallet to massage the aluminium frame over the tube corners.

Aluminium Tube Corners Fitted

The remaining frame parts were assembled.

Aluminium Frame Assembled

A cut-out was required for the cable assembly which connected between the backlight controller and backlight. An area on the frame was marked out then removed with a Dremel.

Aluminium Frame Backlight Connector Markup

Aluminium Frame Backlight Connector Cutout

Control PCB
Two self-tapping screws were used to mount the control PCB enclosure to the aluminium frame.

Control PCB Enclosure Mounted to Aluminium Frame

For the connections to the control PCB, the original cable from the BenQ monitor was reused. The red, orange and green wires were connected to the 5V centre pin of the DC jack. These were the main supply and enable pins to the MP3389. The brown and black were connected to 0V of the DC jack. The white wire dimming connection was left floating and terminated in heatshrink.

To complete the control PCB assembly the cable to the backlight was plugged into the controller and the plastic lid attached.

Mounted Control PCB Enclosure

The exposed backlight cable was to be protected with flexible plastic trunking. 

USB Power Lead
The current limit set by the dimming resistors on the control PCB was less than 500mA. This value allowed off the shelf USB chargers or USB ports to power the light table.

A standard USB cable was modified so that only the USB power connected to the centre of the DC jack and the black to the outside of the jack.

USB Power Lead for Light Table

Acrylic Cover Sheet
A removable acrylic cover, 3mm thick, was cut out to cover the backlight. Having a removable cover sheet was necessary with the light table being used around home and in the workshop. 

Acrylic Cover for Light Table

Securing the Backlight
To secure the backlight into the frame, a bead of silicon was used between the four corners of the frame and the backlight body.

Fixing LED Backlight

Acrylic Mounting and Feet
Four self-tapping screws and Nylon washers were used to secure the acrylic sheet.

Securing the Acrylic Cover

On the rear side of the light table, four rubber feet were added to each corner.

Rubber Feet or Light Table

Backlight Diffuser
Using only the backlight diffuser which appears to be a type of translucent plastic film produces good uniformity. There were other optical sheets layered on the front of the original LCD although these were not used.

Light Table Test

During cleaning of the acrylic with a dry cloth, the backlight diffuser became attracted to the acrylic and lifted off the backlight. An application of silicon helped reduce the lift of the backlight.

Salvaging electronic parts - Part 3 BenQ GL2430 Monitor

Summary

This blog continues a series of salvaging electronic parts, this time focusing on a BenQ model GL2430 monitor and what components could be salvaged. 

GL2430-B  BenQ Monitor

The monitor build date was from early 2011.

GL2430 Name Plate

Salvaging
The dismantling process for the monitor was skipped. The two major and two smaller printed circuit boards (PCB) removed from the monitor were: Flat panel driver board, Power supply board, Backlight driver board and Audio breakout board.

Flat panel driver board
Shown below is the flat panel driver board, single sided multilayer circuit board. This board contains the Realtek flat panel driver IC boxed in blue. Realtek part RTD2483RD. 

GL2430 Flat Panel Driver Board Top Side

Boxed in red of the image are a mixture of five serial memory devices ranging from ST Micro 24C02 EEPROM to a Winbond 2Mbit Flash W25X20. These devices are usually very easy to salvage.

Boxed in yellow are some of the numerous inductors on the board.

The semiconductor in the top right hand corner of the image is a Diodes Inc PAM8603 - 3W stereo class D amplifier. Possibly not worth salvaging as there are parts available with lower distortion ratings.

In the bottom left hand corner of the board are a pair of TVS arrays from the manufacturer InPaq 1045QU. Again probably not worth salvaging.

The crystal on the board is common video 14.318MHz type and could be repurposed if any crystal frequency is suitable.

Remaining on the board are a mixture of connectors, passives and unidentified semiconductors. If you were really scratching to find a MELF diode or 220uF Lelon Electrolytic then this board could be added to the spare parts box.

Power supply driver board
The power supply board is populated with components on both sides, double sided with a single layer PCB design. Heavy through hole components on the top side of the PCB and smattering of glued surface mount components on the solder side.

GL2430 Power Supply Board Top Side

Boxed in red is the mains common mode choke and towards the centre of the board, the switching transformer. Both these devices are manufactured by a Taiwanese transformer manufacturer. These parts can come in handy for research and design projects and a worth salvaging.

The green boxes highlight a few resistors which could be extracted for the spares bin. There are no signs of overheating or other physical damage which may have been caused by a fault on the PCB.

In the blue boxes are the diodes and bridge rectifiers. The bridge rectifier is listed as an obsolete part on supplier's websites such as Mouser so may be good for the spare parts box. The two larger axial diodes are Vishay part UG4B; a reputable brand worth salvaging once properly tested.

The two devices on heatsinks are the mains side switching MOSFET K4101 and secondary side dual rectifier diode FMX12S. At least one of these devices has discolouration in the PCB surrounding the heatsink. Heatsinks could be salvaged for other purposes.

Remaining on the top side of the board are various connectors, mains voltage rated varistors and mixture of capacitors. Usually components which have been operating at mains voltages for an unknown amount of time can left on the PCB if the operational state of these parts is unknown.

GL2430 Power Supply Board Bottom Side

Shown above is the solder side of the power supply board. To the left of the image is the switch mode controller. Across the board are a number of other parts, all glued down with epoxy. Using epoxy to glue surface mount components in order to simplify the assembly process is standard practice. The epoxy can make repair and salvaging parts difficult.

Backlight driver board
Below is an image of the backlight driver board. The main LED driver, boxed in red, is an MP3389 from Monolithic Power. Device is worth salvaging or even the entire board itself as it is a self-contained unit which could easily be reused.

GL2430 Backlight Board Top Side

Boxed in blue is an unbranded inductor which is always good to have in the spares box.

Shown in the yellow box is a SinoPower MOSFET APM1110 which was not located on the company website. Specifications are nothing to be excited over although part would be worth salvaging for prototyping.

The remaining passives such as the radial capacitors are from Lelon making the remainder of the board a contender for the spares box.

Audio Connector board
Lastly is the small 3.5mm audio connector board. The connectors are a standard pinout and could be salvaged or the entire board repurposed for a bespoke project.

GL2430 Audio Connector Board Top Side

Design Notes
A section of the power supply mains input section was chosen for some brief notes on PCB design.

GL2430 Power Supply Board Main Input Section

Boxed in white, top left hand corner of the above image, is the one of the mounting holes with exposed long pads coated in solder. This is a good feature for eliminating star or copper washers however the electrical resistivity can suffer due to the smaller contact area and surface oxidisation of the solder. It should be noted that on the component side of the board is a through hole nut allowing direct access to the mains earth connection.

In the orange boxes are several slots on the board to improve the creep distance between component pins. Slots in the board are an effective and cheaper solution to conformal coating.

Shown in the blue box is attention to detail by the PCB designer. A small pullback was applied to the copper surrounding the two mounting holes for the IEC mains connector.

Boxed in purple is a section of silk screen showing the isolation plane between mains AC and isolated DC voltages. The silk screen for the isolation plane and most components is present on both sides of the PCB making component identification and servicing easier.

Lastly the red box shows three cascaded surface mount resistors used in series to discharge the mains input capacitor connected between active and neutral. The PCB designer was mindful of creep distance and to some degree spacing between these components.