Protect Motorway Signs from Unwanted Power Surges

When Scrimsign Micro Electronics Limited (www.scrimsign.co.uk), one of the

UK's leading manufacturers of dot matrix displays for the public and private

sectors, found that its award winning signs were experiencing unforeseen

failures, the company's research department concluded that this was caused

by transients in the power supply.

Because many of Scrimsign's displays are located out of doors, in exposed

areas such as motorways and harbours, the quality of the power supplied to

these units cannot always be guaranteed. As such, they are often subject to

power surges caused by lightning induced transients.

New research tells us that 88.5% of all power disturbances are caused by

transient voltages. And, even though you might think that data and

production processes are safe, the truth is that they will have very

little - if any - protection against this potentially disastrous phenomenon.

Scrimsign had already addressed this issue by installing a transient voltage

suppressor device manufactured by a third party. These units were positioned

at site locations deemed to be at a high risk from mains borne transients,

but the problem still remained.

After consulting Advance Electronics, two issues were identified that could

lead to the problem being unresolved. Firstly, because many of Scrimsign's

electronic signs were sited, out of necessity, on rock this, inevitably,

cannot provide a good earth contact. Secondly, Scrimsign had specified a

competitor's transient suppression module that relied on metal oxide

varistor (MOV) technology. And it was this latter issue that was more of a

problem than first appears.

Most transient protection equipment relies on MOV based technology. Their

limitation is that MOVs are traditionally slow to react to the rapidly

increasing current of a typical transient waveform, giving insufficient

protection if used on their own. Moreover, just a single voltage spike can

easily destroy these devices - leaving sensitive microprocessor-based

components open to devastation. Only a properly designed TVSS module or

power conditioner can offer completely reliable protection against the kind

of environment that Scrimsign's designs are regularly sited at.

With the need to provide the utmost dependability firmly in mind, Advance

Electronics recommended to Scrimsign the installation of one of its range of

AGT constant voltage transformers (CVT) for each affected site.

Fundamentally, the constant voltage transformer is a very straightforward

device, with no electronic components. Its indestructible characteristic

makes it an excellent choice for situations where variations in the quality

of the power supply are all too regular, but where maintenance facilities

are at a premium - ideal for the remote motorway and harbour sites that are

typical of Scrimsign's installations.

A constant voltage transformer is also ideal for protecting sensitive loads

against direct lightning strikes on power lines, something that can occur

all too frequently in the self same locations where outdoor signs are often

mounted. Because the constant voltage transformer is self-protecting to its

supply, and to the critical load connected to it, the unit is effectively

immune to damage from the rising waveform of voltage caused by a typical

lightning strike.

The constant voltage transformer is also useful for riding out momentary

breaks in the mains. The microprocessor components contained in dot matrix

signs are highly sensitive to even the shortest of interruptions in the

power supply. Moreover, because the resonating circuit within a constant

voltage transformer stores a limited amount of energy, this renders them

ideal for filling in potential small gaps in the power waveform. In fact, an

average constant voltage transformer will be able to cover a gap of about

half a cycle in a typical situation. This will satisfactorily drive modern

microprocessor based equipment over a short-term break in the mains power.

And Scrimsign's view? They are delighted that their signs can now be located

anywhere that has a power supply, regardless of the quality of the mains.

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