I read in !emc-pstc that IEEE-EMC User Group <[email protected]> wrote (in <2D1037012914D4118DB8204C4F4F5020275AB4@ITLLTD01>) about 'Corrective Actions for EN 61000-3-3 Plt; Pst Failures', on Sun, 20 Jan 2002: >A product (800W input power) having a heating element that is controlled by >ON/OFF action (1-2 sec. interval) of a thermostat failed the subject >requirements due to large input current variation resulting from the above >action.
It's unusual to have such a powerful heater in a product with such a low thermal inertia that the thermostat cycle is as short as 1 to 2 s. >What will be a practical corrective action in this case? To answer properly, more details about the product would be required. It might be possible to split the heating element into two parts, not necessarily of equal powers, and to have one permanently on (apart from an over-temperature cut-out) while the other one is switched in and out by the thermostat. The cycle time should then become much longer, and the 'dmax' value is reduced. Another solution is to use the electronic control method known as 'burst firing', in which the thermostat switches on a control circuit that sends trains of complete cycles of the supply current to the heater. With different designs, the switching of the load is either very fast (e.g. 5-cycle bursts of current, lasting 100 ms on 50 Hz mains) or very slow. In either case, the Pst value is low. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk After swimming across the Hellespont, I felt like a Hero. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
