Dave Hills (dhills@onramp.net) wrote: : minilinx@netaccess.co.nz wrote: : > : > Hi, : > I am experiencing problems with over voltage pikes destroying the : > power supply to a micro processor controlled circuit. : > The current conditions and ciruit are as described below. : > : > Voltage = 24V (nominal) : > : > In series with the +Ve input to the device are a 330R/2W resistor, : > 1N4007 Diode and a 1W/12V Zener Diode. Following this is a 20V/1W : > Zener diode and 100uF capacitor in shunt. This then feeds a LM7805 : > regulator. : > : > Referring to an application note by SGS-Thompson regarding protection : > for 12V automotive systems, I expect that voltages as high as 300V are : > possible for up to 300mS. I assume that the 24V system from the : > diesel engine may produce spikes well in excess of this. : Get yourself a copy of international standard "ISO 7637-2". This covers : the 24 volt vehicular requirements for immunity from electrical : transients. It provides all the waveforms and voltage levels you will : have to contend with. A good L-C filter on the input is a must to : attenuate the high voltage fast transients normally encountered. The LC filter will not necessarily help. A fast-rising tranisient will couple right on through most simple inductors due to the interwinding capacitance. This is why zeners or transzorbs are mandatory. A zener is *extremely* fast, since there is no Trr or Tfr time to contend with (it is always reverse biased, so removal of minority carriers from the PN junction is irrelivant). Most microcontroller-based electronics that I have seen in volume production (used in an automotive environment) relies on this approach, not LC filters. Bob.
Date: 5 Dec 1996 21:02:21 GMT
Original Subject: Re: Hints for Automotive Electronics required