LM309K regulator

Hi Henk Usually the filter cap on the rectifier is enough to safely absorb the energy of the transformer. Still, it is something that you need to check and actually verify. If the range of line voltages that the unit is to run on put the input voltage near the maximum of the rating, it needs to be checked. Where I personally saw this effect was in a unit that had 5 of the modular power supplies ( linear w/ transformers ) all wired in parallel. When the switch was turn on and off, the stored energy in the cores needs to go sumplace, just like the coil in an ignition system. What makes it worse is something like a zero cross switch which would be worse as that is when the maximum energy is in the cores magnetic field. We were not able to get the manufacture of the modular power supplies to solve the problem fast enough so we shipped the machine but put those varistors on the inputs to clamp the voltage swing. We learned later that sufficient sized input caps would also work. For these, one had to make sure that it wasn't too small as this would actually enhance the spike, as the capacitor does in an ignition system does, buy reducing the arcing of the switch contacts. The diode, as you mentions, was the typical way to solve the back voltage problem. It is a good idea in most any case. Dwight _________________________________________________________________ Don?t miss your chance to WIN $10,000 and other great prizes from Microsoft Office Live http://clk.atdmt.com/MRT/go/aub0540003042mrt/direct/01/