Rockwell Automation Safety Guidelines for the Application, Installation, and Maintenance of Solid-State Control - Manual (Page 7)

Publication SGI-1.1 - August 2009
Section 2: General Authorized Engineering Information 7
information for selecting an appropriate fuse or other current limiting
means.
Applications that require short-term overcurrent capability (e.g., motor
starting) must observe the manufacturers restrictions on the number of
times the device can be subjected to overcurrent in a specified time
interval. The user should be aware that this specification may vary
depending upon whether the conditions call for hot starts or cold starts.
Hot start means that the solid-state component is at or near the normal
operating temperature due to previous operation history when the
overcurrent condition occurs. Cold start means that the solid-state
component is at or below 40º C when the overcurrent condition occurs.
2.7 Transient Overvoltage Solid-state controls may be affected by transient overvoltages which are in
excess of those specified by the manufacturer. Voltage limiting means should
be considered and may be required.
Comments: 2.7 — Transient Overvoltage
Solid-state devices are especially sensitive to excessive voltage. When the
peak voltage rating is exceeded, even for a fraction of a second, permanent
damage can occur. The crystalline structure of the device may be
irretrievably altered and the device may no longer be able to turn OFF.
The external symptom of this situation is exactly the same as that of an
electromechanical device with welded contacts.
Minimum Holding Current
Another characteristic of concern is the minimum holding current
requirement for triacs and SCRs. When the load current falls below the
minimum value, typically 25...100 mA, the triac or SCR ceases conduction
and passes only off-state current until again triggered. Thus, it may not be
possible for the circuit to turn on or conduct full-load current for very
light loads. In these instances, a load resistor called a bleeder resistor may
be connected to the output to provide the minimum load. In some
equipment special circuitry is provided to overcome this problem.