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SYSTEM
SUPPORT SERVICES 11046 State Road 52, Hudson, FL 34669 ● (727) 243-3656 |
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SYSTEM
SUPPORT SERVICES 11046 State Road 52, Hudson, FL 34669 ● (727) 243-3656 |
| Product(s) Affected: | Comtrol 1 and Megaplex Supervised Contact Boards |
| Model or Part Number(s): | 30-0059, 30-0076, 30-0121 (All Versions) |
| Release Date: | December 4, 1987 (Originally released by COMPUTROL) |
| Subject: | Transient protection for supervised contact board inputs |
All of the Multi-level Supervised Contact FIMs, part numbers 30-0059, 30-0076, and 30-0121 contain CMOS analog multiplexer ICs in their input circuitry, which are sensitive to input voltage transients.
The CMOS multiplexers pass the DC voltage which results from applying the constant current source across the fixed line termination network to the detection circuit. The multiplexers can be damaged if the DC voltage across their inputs exceeds ½ volt above the supply voltage for these devices, or 15.5 volts.
To protect the multiplexer ICs against damage by transient voltages, some form of transient suppressor is required. This suppressor, or "transorb," must protect the multiplexer input circuits, without interfering with alarm and tamper voltage measurements on the line inputs.
A suitable choice for protecting the multiplexer ICs is the General Semiconductor transorb, part number 1.5KE13A. This is a 1500 watt device with the following characteristics:
| Standoff Voltage | 11.1 V |
| Min Breakdown Voltage | 12.4 V @ 1 mA |
| Max Breakdown Voltage | 13.7 V @ 1 mA |
| Reverse Leakage Current (Max) | 5 microamps |
| Maximum Clamp Voltage | 18.2 V |
| Maximum Peak Pulse Current | 82 Amps |
The 30-0059, 30-0076, and 30-0121, in their multi-level configurations, have an upper threshold of about 10 volts, above which, the board considers the line to be open (trouble or supervisory condition.) Any input voltage from 10.1 volts up to catastrophic failure will yield the same supervisory output. (The 30-0121-2 has an upper threshold of about 8.4 volts.) The protection device must allow the input to exceed the 10 volt threshold to allow the supervisory condition to be sensed (the standoff voltage of 11.1 volts allows this) but should then clamp the input to a safe level.
Ideally, this clamp voltage should be 15.5 volts or less. Unfortunately, protection devices with adequate speed and sufficient current carrying capability to protect the inputs are not available with such narrow ranges of operation. The actual voltage at which the transorb will clamp will depend on the current being shunted. It will begin operating between 12.4 volts and 13.7 volts, and should not allow the voltage to rise to the maximum clamp voltage unless the transorb is shunting the full 82 amp Peak Pulse Current.
For the transorb to provide full protection, it must have a proper ground. These devices represent less than 0.25 ohms when fully on. This desirable characteristic can be totally compromised by inadequate grounding. The result can be loss of input protection, or interaction (crosstalk) between channels.
It should be noted that transient voltage conditions that cause the input voltage to rise above 10.1 volts (8.4 volts for 30-0121-2) will cause transient supervisory conditions which may or may not be reported. Supervisory alarms are usually verified by repeated polling at intervals which will negate the effects of most voltage transients on the wiring.
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Revised: April 24, 2006 .