On Sat, 14 Mar 1998 18:42:09 +1300 Gavin Jackson writes: >Hi there once again > >Correct me if I'm wrong, but I though that the 78XX voltage >regulators can only take 3X the output voltage on their input, >so the maximum supply that a 7805 can have as its input >is 15V. They are rated for up to 35V, except the 7824 is rated up to 40V. But, there is a problem with higher input voltages: heat. Since they are linear regulators, the amount of power dissipated depends almost entirely on the load current and the difference between input and output voltage. For example, if your circuit uses 5V at 50 mA, with an input of 15V, the 7805 will dissipate about .05(15-5) W, or 500 mW of heat. This is about the limit for a bare 78XX device. Above that, a heat sink would need to be added to keep it cool. If the input voltage were 25V, the dissipation would increase to a full Watt, even though the load circuit is using only 1/4 Watt of power. So the 3x input voltage rule is a good rule of thumb for design, though not an absolute limitation. In the case of the boat project being discussed, heat isn't a big problem since the load current is only 5 mA. In this case, the 7805 would dissipate about 380 mW. This is too much for a 78L05 as someone suggested (max. dissipation on these is 100 mW), but a standard TO-220 regulator with little or no heatsink would do fine. One of the newer ones intended for automotive use should be used along with additional protection in order to withstand the likely high voltage spikes on the 24V input from other large electrical machines connected to the 24V battery. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]