Intrinsically safe LCD designs

Intrinsically safe LCD designs
March 16, 2017 Eric Hawkins
In Journals

An event that happened last month sparked an interest in writing about intrinsically safe LCD designs. Last month, less than a mile from where I live, a house exploded from a natural gas leak. Several days later I walked by the house and found the crumbled garage door several feet away from where it once hung. It was amazing to see just how powerful the explosion was. The link below shows the results of a cell phone when used at a gas station. (You can skip the first minute without missing any details)… http://www.intrinsically-safe-instruments.com/

I realized that many of our customers build products that operate in explosive environments. They require us to modify the LCD design to meet their unique needs.  Their end product needs to be an intrinsically safe design.

When arcing/sparks is a bad thing:

The goal for intrinsically safe LCD designs is the elimination of spark potential within components by limiting the stored energy in any given circuit. Even a low voltage battery powered product, such as a cell phone, can generate enough potential to ignite an explosive gas.

Although our displays are not “tested” to be an intrinsically safe LCD design; we are able to build the LCD to the customer’s specifications. Much thought is invested into what particular environment the LCD will be exposed to and what limitations the LCD needs. Some of these limitations can include:

  • Removal of a charge pump, bump circuit or booster circuit
  • Elimination of inverters (mainly used for EL backlights)
  • Avoiding capacitive touch screens
  • Minimum spacing between components placed on the PCB

Intrinsically safe LCD limitation: charge pump

I have heard the term ‘charge pump’ also referred to as a bump circuit or booster circuit. Regardless of the term, its function is to convert the incoming voltage supplied by the customer to the voltage/current necessary to drive the LCD module .

The charge pump allows the customer to supply one input voltage when the display may require two or more separate reference voltages. With the charge pump removed, the designer will need to supply the various reference voltages (12V, 3V, -5V, etc.) to drive both the logic and backlight. This complicates their design since they now require multiple inputs to different locations on the PCB.

There is no cost for us to remove the charge pumps from the PCB, but there is a one-time NRE fee to modify the circuit board in order to add the additional traces and pin-outs necessary to connect the multiple inputs. Once again, we do not certify that the display module is an intrinsically safe LCD; we build to the customer’s requirements.

Intrinsically safe LCD limitation: inverters

Inverters are necessary to generate AC from the DC power supplied by the customer. EL backlights (and now discontinued CCFL backlights) required an AC signal, but thanks to advances in LED technology and improved diffusers, EL backlights are being phased out of new designs and replaced with LED backlights. One main advantage of LED backlights is that they operate on a low DC voltage.

Character LCD displays do not require a charge pump since they can operate on a single DC input. If the character module also requires a LED backlight, the same power/voltage is able to supply both logic and backlight.

Note: We do not recommend connecting the LED backlight via pins 1 and 2 since these pins are reserved for logic. It is recommended to power the backlight via pins 15 and 16 or through a separate A and K on the side of the LCD glass.

Intrinsically safe LCD limitation: capacitive touch screens

The popularity of cell phones and tablets have increased the demand for capacitive touch panels that allow multi-touch operation, unfortunately these are not friendly in an intrinsic environment.

Need help with your next LCD design? Contact our US based LCD support center at 480-503-4295.