• Bringing Our Products On The Road
• Ultra High Bright Display

• Industrial Computer Workstation
• Control Dimming on High Bright Displays

VarTech Systems has been busy with getting our products to you! In efforts to meet our customers and give you a chance to view our products first hand we have been actively involved in several tradeshows.

VarTech's diverse line of industrial monitors was recently displayed at the 2005 ATCA (Air Traffic Control Association) tradeshow in Dallas at the October/November tradeshow. It was the 50th Anniversary of the the show and as always provided important information to respective industry professionals. We also displayed our products at the annual ISA (Instrumentation, Systems and Automation Society) Expo in Chicago during October. The ISA Trade Show is held yearly and directed towards dispersing new information to both automation and control professionals. Thanks to all who stopped by to visit. The shows were a great success and gave us a chance to show you some of our new and exciting products including the new 32" widescreen display. If you will be attending the I/ITSEC Expo this year in November, be sure to stop by our booth #1349.

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Ultra High Bright Display Model VT170HBU-Series

The latest in hight bright displays is now available from VarTech. This new series brightly sports a display luminance of 1200 nits (cd/m2) while maintaining complete image clarity. The monitor preserves a crisp image under either direct sunlight or in any high ambient light conditions. The screen is easily readable under any lighting condition and can be equipped to include manual or automatic dimming.

Industrial NEMA 4x Computer Workstations

VarTech strives to offer a complete line of display systems which will fit into any industrial application. In our latest endeavors we have introduced a system that has been engineered to survive in the harshest environments, such as coal mines, rock quarries, oil rigs, chemical plants and the production floor of food processing and other facilities where complete wash down of equipment is required.

This 15" wall mount workstation is completed with an enclosed keyboard and pointing device.

True "Industrial" Grade -
Not a "Re-packaged" Commercial Version

Workstation Features:

• Completely sealed, airtight enclosures
• Water proof and dust proof
• Corrosion, shock and vibration resistant
• Optional touch screen available

The workstation can be configured to your exact specifications, including:

INTERNAL (inside the interface enclosure)
• Industrial MicroBox PC
• Small form factor desktop

LOCAL (separate enclosure with 25 foot cable)
• Larger form factor industrial computer
• Desktop PCs

REMOTE (up to 3000' away with KVM extender)
• Desktop PCs up to 500' away with CAT5 KVM
• Desktop PC up to 3000' away with fiber optic KVM


Click for More Information

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Control Dimming on High-Brite Displays    

The two most common techniques used to control the brightness of the cold-cathode fluorescent tubes (CCFLs) that backlight an LCD panel are analog dimming and digital (or PWM) dimming. In analog dimming, a dc voltage directly controls the output current of the inverter powering the CCFL, which in turn, controls the hi-brite display's brightness.

The pulse width modulation (PWM) dimming technique fixes the output current of the inverter and modulates the time that the inverter is on. Each technique has its own set of advantages and limitations-important design considerations that must be taken into account during system design and integration.

Analog Dimming
Analog dimming is the simplest technique to implement. An applied dc voltage directly controls the output current of the CCFL inverter. Generally, this voltage is electrically summed into the on-board voltage control loop or is used to supply drive current to the inverter's primary switches. In either case, the inverter operates continuously. In this mode, power supply requirements are reduced since there is little input ripple current and additional filtering of the power supply is not required.

However, the dimming range available when using analog dimming is quite poor, particularly for applications such as daylight- and nighttime-readable displays that require a wide dimming range. The display's minimum brightness would occur when the specified CCFL is operating at its specified minimumoperating current, often 30% to 50% of the rated typical current.

Since light output is relatively linear, a dimming ratio of roughly 2:1 or 3:1 can be accomplished. This is further complicated by large displays, typically larger than 17 in., with high electrical losses in the CCFL assembly that could effectively reduce the dimming ratio to as low as 1.5:1.

The dimming ratio is the ratio between the highest achievable brightness of a display and the lowest
attainable brightness level. Lower dimming ratios are perfectly acceptable for many applications.

Typically, office environments-where the ambient light levels are fixed-require a relatively limited dimming range. However, if the ambient light levels change significantly, such as in the automotive or aeronautical industry, very bright backlights are needed to make the display bright and readable in direct sunlight and the display must also not blind the user at night.

PWM dimming
Digital or PWM dimming is rapidly becoming the dimming technique of choice since it is less display-sensitive and offers more flexibility in choosing brightness levels. To make PWM dimming possible, an inverter needs to be specifically designed for it.

On many generic closed-loop inverters, the control loop is often too slow for an effective dimming range. When PWM dimming is used, the time that the inverter is on is modulated and the applied duty cycle roughly equates to display brightness, with 100% being the maximum.

Since the inverter is actually being turned on and off at the PWM frequency, care should be taken in the design of the inverter's power supply to take into account the low-frequency pulses of current produced by the inverter. Furthermore, magnetic structures may sing as a result of the windings compressing and expanding (magnetostriction) as the current through them changes rapidly at the PWM frequency, so acoustics are also a consideration.

One of the greatest advantages of using PWM dimming is a wide dimming range. Since the minimum tube current requirement is always met (the inverter is either fully on or fully off), the minimum brightness is a function of controllable duty cycle, which can be below 1%. With a minimum duty cycle of 1%, a dimming ratio of 100:1 is easily accomplished.

Some inverters offer on-board analog- to-PWM converters that allow for the increase in dimming range while interfacing to legacy circuits or potentiometers. However, when possible, the source of the dimming signal should be digital and at the desired PWM frequency.

If it is not, undergoing the conversion process from analog to digital or digital to analog to digital can be plagued by noise and tolerance problems. These are manifested in minimum brightness stability (flicker or brightness drift) and lack of repeatability (consistent brightness levels from assembly to assembly).

Soft Starting And The Signal Interface
If an extreme dimming ratio is not required, the disadvantage of acoustics and power supply ripple requirements can be reduced by choosing an inverter with soft-start functionality. Soft start gradually increases and reduces the CCFL current at each PWM cycle, which in turn softens the edge on startup and turnoff. The performance hit in dimming ratio isn't a killer either, as dimming ratios of greater than 50:1 can still be accomplished, which is sufficient for applications like portable tracking stations and POS terminals.

Interfacing signals to inverters is not always a straightforward task. It is important to note that many of the interface signals on the inverter side are not digital. Some interface to voltage levels outside of the digital range, are not compatible with TTLlevel signals or require current sourcing or sinking.

In some cases, it can be as simple as a mismatch in the PWM analog control range voltage or swapped polarity on enable or control signals. Instead of developing circuitry to accommodate the inverter, the knowledgeable inverter manufacturer will configure the inverter to accommodate the interface signals or offer interface solutions.

Articled Prepared by:
Jim Moronski
Endicott Research Group
Endicott, NY

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VarTech Vision November 2005
VarTech Systems Inc.
11529 Sun Belt Ct. | Baton Rouge, Louisiana 70809 U.S.A.
International: 001.225.298.0300 | Fax: 225.297.2440 | Toll-Free (U.S.A & Canada) 800.223.8050

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