Application Note 107

Our line of Over/under Voltage Protection boards, including the "Smart" models, are designed to disconnect the load if the power line voltage goes outside the set limits. Most models let you set the protection voltage limits with an on-card DIP switch. This Note answers the most "frequently asked questions" and provides application information for the Over/under boards and our power line Surge protection boards.

Why do I need over/under voltage protection?

Unattended electrical appliances that operate continuously (such as FAX machines and personal computers) and those controlled by thermostats or other sensors (such as regrigerators, air conditioners and pumps) are subject to damage by high or low line voltage. Positive protection is provided by a circuit that senses the out-of-limits condition and then disconnects the load.

The TDL "Smart" Protectors monitor the power line voltage and open (or close) a relay when the voltage goes lower or higher than the set limit points. Even with the load disconnected, the board continues to monitor the line and reconnects the load 90 seconds after the voltage returns within limits. The 90 second delay helps avoid reconnecting the load until the line voltage is stable. Sometimes when normal service is restored after an outage, "brownout" or high line voltage, there are transients on the line. These are often caused by many loads all drawing starting current at the same time.

In some countries a common fault is loss of the neutral connection in the power company's wiring. When this happens, the building being served has one line-to-neutral voltage go high while the other line-to-neutral goes low according to the current being drawn on each side. For this type system (also common in the US) it is better to monitor the 115 VAC rather than the 230 line-to-line voltage.

Why so many "Smart" Protector models?

Each model was designed to do a particular job.

The SPPC-4A is for 208 to 240 volts, 50 or 60 Hz. It was especially designed for those countries using the higher line voltage with no neutral line. It controls either an off-card solid-state relay (SSR) or an on-card mechanical relay. The nominal voltage as well as four different limit "widths" are set with on-card DIP switches. A NiCd battery is not needed.

The other models are all for nominal 115 VAC, 50 or 60 Hz operation.

The SPPC-2A has a "pull-off" jumper to select either an off-card SSR or the on-card mechanical relay. DIP switches set the nominal voltage (110 to 125 volts) and four different limit "widths". In addition, a DIP switch selects one of four under-voltage drop out delay times. This helps prevent disconnecting the load during brief, non-destructive outages. This model needs a rechargeable NiCd battery to supply power during the delay time.

The SPPC-6 broadcasts the monitored line voltage over a 2-wire RS-232 line for remote monitoring or data logging. A NiCd battery is needed here too so monitoring can continue during an outage. Sample MS-DOS software for monitoring and logging is supplied on diskette.

The SPPC-7 is optimized for a very short dropout time, less than 30 msec. A battery is not needed.

The SPPC-12 has the same functions as the -2A except for the selectable dropout time, which is about 200 msec. No battery is needed.

All models include a self-test by connecting an off-card momentary contact push button switch.

How long does the battery last?

The models that need a battery trickle charge it all the time line power is applied. The battery manufacturers say you can expect "shelf life" at room temperature use, perhaps as long as five years. We've had units in operation here for over two years on the original battery.

We recommend (and use) a small 8.4 volt, 100 mAH NiCd which is the size of a 9 volt transistor radio battery. Suitable models include:

Panasonic P-9SPA (Digi-Key Corporation number P-148)
GP (Mouser Electronics 573-15F8K)
Power Sonics PS-TR7
Sanyo 9V-8.4
Plainview Batteries, Inc. PB-1605

What documentation comes with the Protector boards?

We furnish a schematic diagram, parts list, and parts placement drawing. The schematic diagram includes a suggested circuit for load protection.

Are custom models available?

YES. Please call, write, FAX, or email your needs.

What if I need to protect a high-current load?

The SSR or on-card mechanical relay can be used to control a power relay. Relays with contact rating from 50 to 1200 amperes are available from Automatic Switch Company, 1 City Blvd. West, Orange, CA 92688. Phone 714-937-0811 or FAX 714-937-1390. Power relays are also available from other sources such as C&H Sales company, 2176 E. Colorado Blvd., Pasadena, CA 91107. Phone 213-681-4925.

You can also get solid-state relays with a current rating to at least 100 amperes. Any SSR can be controlled by a Protector card because the DC operating current is in the 4 to 6 ma range independent of the relay's current rating.

If the power relay coil current is too high for the on-card mechanical relay contacts, you can use this circuit:

Figure 1

Do Protector cards provide surge protection too?

No. Although a surge is definitely an over-voltage condition, surge rise times are typically in the sub microsecond range; too fast for the Protector card to respond. We recommend surge protection for the over/under voltage protector card and for the load. As a minimum, metal oxide varistors (MOVs) should be connected line-to-neutral, line-to-ground, and neutral to ground (for a typical US system) as shown below.

Figure 2

MOVs are sacrificial components and are frequently vaporized by shunting a high-current lightning induced surge. The MOV internal structure can also be damaged by a less-than-fatal surge without causing any visible symptoms.

A passive lowpass filter using large inductors provides better surge protection as well as two-way line filtering. The high frequency (fast rise time) energy is dissipated as heat in the inductors. Our SURGE suppressors include MOVs at the output connectors for auxiliary protection. And we've added a fuse which blows and turns off a LED indicator if the MOVs conduct a large current.

These cards also suppress both incoming and outgoing power line noise. And in accordance with industry standards, measurements are made with 50 ohm source and load resistances. There is at least 20 dB rejection at 50 kHz and 35 dB at 100 kHz. This is important because time/frequency standard station WWVB uses 60 kHz and LORAN-C navigation is at 100 kHz.

Transients from starting or stopping large motors are primarily "differential" mode. That is, the transient voltage appears between line and neutral (in a 115 volt system). Lightning induced surges are often "common" mode. The voltage gradient from the strike cuts both the line and neutral conductors producing a surge voltage that is "common" to both wires. Our SURGE boards include inductors for both differential and common mode suppression. (In fact, a lightning strike can also induce a voltage into the ground conductor which lessens the efficiency of the MOVs to shunt the current to ground.) And the safety ground wiring in many buildings leaves much to be desired ... but that's another story.

The best place to locate your surge suppressors is at the building's electrical entrance panel because a good low-impedance ground connection should be present. This is also an obvious place for one or more Over/under Protector cards as the whole building could be guarded with an adequate relay or relays.

For a typical 115 VAC system, you would connect a TDL SURGE protector as shown:

Figure 3

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