> I am using ST 7812 regulator the problem I face is the voltage out I > get normally is constant 11.9 or 12.03 V. Some of the parts after > use for fews day or 1 month, the voltage out drop to 8.9 V and it is > unstable waving between 8.9 to 11.9 V. What is the reason of this and > how to avoid this ? > > James James, just to awake a possible problem; In 2001 we bought hundreds of 7805 produced by TI in Mexico, all of them, without exception presented several different voltages, from 4.01 to 5.12V, not a single one did show a voltage between 4.95 and 5.05V, not one. Unfortunately we only found it out, after 5 months of purchase, so the supplier "could do nothing" about it. By other technical point of view, I can suggest you to check the current and temperature over such 7812. What is the Voltage at its input? what is the load to it? Are you using any electrolytic capacitor at the 7812's output? if yes, remove it, keep only a small 100nF to filter high freq noises. Any electrolytic capacitor should be used at the input of the 78XX family, not output. The reason is that it can cause regulation delays and problems and the 78XX "can" do some messy regulation. By the same reason, the 100nF capacitor is REQUIRED to avoid high freq to mess with the regulation. Always remember, 78XX family "should not" be used when required output current is in excess of 1A, and you should always calculate the dissipated power over the 78XX. Multiply the Current by the Voltage Drop over the 78XX. It should be lower than 1W for uses without heatsink, and even using heatsink keep it lower than 4W to keep it running smooth. 15V .------. -----o---| 7812 |---o----> 12V | '--.---' | 470 _|_ | _|_ uF --- | --- 100nF | | | _|_ _|_ _|_ Vdrop over 7812 = 15-12 = 3V Suppose Current = 700mA Power = 0.7 x 3 = 2.1W It would require some small heatsink. Wagner. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.