Diodes and Power Supplies

Do you guys have any pointers for remembering when to subtract the .7 volts needed for some diodes in the circuits.

if you put a diode you always have to substract 0.7v when that diode is a part of the circuit (when there is current through it).

Diodes always have a forward voltage drop, but the actual value of the drop is dependent different factors, including the kind of diode and the amount of current through it. The manufacturer's datasheet will help you out, and will specify Vf @ If. 0.7V is a "typical" voltage drop of garden-variety diodes.

I don't know how to answer your question as to when you need to "remember when" to subtract 0.7V for a diode drop, except to say you always need to remember it when a diode is in your circuit. Are you doing a Kirchoff's Voltage Law (KVL) calculation? Most of the time in power supply circuits (I'm not talking switchers, those are more complex, but in simple rectifier circuits) there is plenty of head-room and the voltage drop is not important, and sometimes can even work to your advantage.

If I've missed the point, give more details.

j.