Virgil,
As you can tell, there are quite a few extremely valuable replies from the "Tech Gurus" here at ECN, which not only cover your original posted Q, but added more scenarios.
Great job(s) as usuall!!!
Glad we have this vast pool of knowledge here at ECN to assist when needed!!
Just to add some more info to this topic:
* Simple methods of dropping DC Voltage:
- Dropping Resistor(s),
- Voltage Dividing Network (an array of resistors),
- "Tapped" Power Resistors,
- Diode or Diodes in series with the load.
With exception of the Diode(s), these methods drop DC Voltage by
Dissipating True Power and therefore produce large amounts of heat and are considered "wasting power".
The Diodes simply have a
Forward Voltage Drop, which makes the output voltage lower than the input voltage (please excuse the extremely basic description!)
* More advanced and precise methods are:
- Voltage Regulators - either descrete Power Transistors with a "Fixed" Base current control, to IC type "Flying" Regulators,
- Switch Mode Power Supplies - from simple to complex types,
- Series Subtractive connections of power supplies.
For really extreme and precise stuff, this brings in DC to DC conversion - Inverting the DC into AC, then either stepping up or lowering the Voltage, then Rectifying it back to DC again.
FYI, some methods of increasing DC Voltage:
- Voltage Doublers - Tank Circuits / RC Networks (RC = Resistor / Capacitor),
- Dc to DC Converters,
- Induction coils driven by pulsed DC (DC switched on and off rapidly, such as an Automotive Ignition System),
- Series Additive connections of power supplies (dry cells, Batteries, Generators, etc.).
As to driving a Transformer with pure DC, that won't do very much after the primary becomes stable. Only while the primary is "Building Up" a charged state when the power is applied, or while the primary is "Decaying" it's charge when the power is removed, will there be a usable current flow on the Secondary side.
When the Primary is at "Steady State", there may be a very small current that can be found on the Secondary side, which can be very transient at times. This current is not very usable for a power application, so it's generally neglected as being present.
If the DC is coupled with an AC signal, then you can do a variety of things now!
On the contrary, having large levels of DC coupled with the AC output of an Audio Power Amplifier is a concern for good crossover design and loud speakers.
OK, I feel that I have gone off-topic far enough now!
Scott S.E.T.
p.s. how did everything turn out with that flame war situation regarding lethal shock levels and Transformer KVA ratings???
By the time I saw the thread, everyone at ECN had posted info to it, so I did not think I should add more [except for support!]
S.E.T.