> No wonder everybody's into switch-mode controllers. A good one can be 95% efficient, depending on load. Not everybody. Some of us are very concerned about the electrical noise generated by a SMPS :-) On 22 November 2013 14:22, IVP wrote: > > But if the amps are too high then sometimes it can make a resistor > > get quite hot, I noticed. (I wondered which would make it hotter, > > by increasing the amps or increasing the volts. ) > > Hi Lindy, > > consider a common practical example, a 5V regulator like the 7805 > which can deliver 5V at 1A, in the TO-220 package. As it's a linear > regulator you can almost call it a resistor > > Say you have 8V available as the 7805's input and are drawing 1A. > The difference between the input and output is 3V and the 7805 is > passing 1A, so the wattage dissipated by the 7805 is V * I =3D 3W. > Only just about acceptable without a heatsink. Another way to look > at it is 8V * 1A (8W) in, and 5V * 1A (5W) used by the circuit > > Now do the maths for a 25V input. The difference between input and > output is now 20V and the wattage now 20W. This is the limit if you > can find an infinite heatsink. > > There are two options - reduce the current or reduce the input-output > differential voltage. > > If the load actually uses 1A you can't reduce the current, so the other > option is to lower the 7805's input voltage by adding a series resistor > between the 25V and the 7805 input and limit the 7805 wattage to > 3W. > > Already known is that 8V input causes 3W of heat, so the resistor > you add needs to drop 25-8 =3D 17V and pass 1A. By V =3D I * R this > gives 17 ohms, and the wattage rating of that resistor needs to be > P =3D I * I * R (or I * V) =3D 17W. > > Plus the 3W of the 7805 =3D the 20W that had to be shared > > You can see how much energy is lost as heat with linear regulators. > Of the 25 watts going in, 80% is being wasted. For a 12V battery > it would be about 60%. No wonder everybody's into switch-mode > controllers. A good one can be 95% efficient, depending on load. > > Joe > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 __________________________________________ David C Brown 43 Bings Road Whaley Bridge High Peak Phone: 01663 733236 Derbyshire eMail: dcb.home@gmail.com SK23 7ND web: www.bings-knowle.co.uk/dcb --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .