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Current Limiting Scheme When Using Solid State Relays In Phase Fire AC Controls

IP.com Disclosure Number: IPCOM000132436D
Publication Date: 2005-Dec-15
Document File: 2 page(s) / 147K

Publishing Venue

The IP.com Prior Art Database

Abstract

The electrical EN61000-3-3("Flicker Spec") requirement limits instantaneous current that can be drawn from the AC line. When incadescent devices with low start-up electrical resistance are controlled by a solid state relay (SSR) additional steps are required to insure compliance. This invention proposes to solve the current-limit problem with SSR's by measuring the peak current drawn during a given line cycle and then using the measurement to set the phase angle of the subsequent line cycle such that current drawn in the subsequent cycle does not exceed mandated limits.

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Current Limiting Scheme When Using

Solid

State

Relays In Phase Fire AC Controls

Low cost and ready availability makes commercial Solid State Relays (SSR) attractive devices for controlling AC loads.  Part of the compliance requirement for EN61000-3-3(”Flicker Spec”) requires that the instantaneous current drawn from the AC line be limited to some value.  If this limit is exceeded when using a Solid State Relay (SSR), the control circuit cannot react as the device can not be turned off mid-cycle resulting, in a violation of EN6I000-3-3.  This problem is particularly acute when controlling tungsten (fuser) lamps since their positive temperature/resistance coefficient results in excessive current being drawn from the AC line when starting and also varies with time as the lamp warms up. Previous SSR-based solutions to this problem utilized an empirically determined phase-angle firing curve, based on time, to limit start up current.  Such solutions are open-loop and are estimating that the system is in compliance.

This invention solves the current-limit shortcomings of using SSR’s by measuring the peak current drawn during a given line cycle and then using the measurement to set the phase angle of the subsequent line cycle such that current drawn in the subsequent cycle does not exceed mandated limits.  Initially, the phase angle is set to an extremely low value such that the current limit will not be exceeded.  During this cycle, the peak current is measured.  A mic...