6v-12v AC/DC to 5v regulated DC <100mA, the easy way?

BradtheRad said:

To reduce mains AC amplitude via capacitive drop is feasible though risky. The capacitor must be non-polarized. Either place two electrolytic type back-to-back, or solder a gang of ceramics (100nF) in parallel.

The load can be:
* tube heater
* transformer
* full diode bridge
* diode & 6V zener diode filtered to yield regulated DC

View attachment 174171

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Some considerations in connecting two polar caps in series to make "non-polar" -



Regards, Dana.

Using a rotary converter would require specially built motor and generator devices, converting AC to motion then motion back to AC would be very inefficient when it does exactly the same as a transformer. Using a DC generator would imply a commutation system was employed and that certainly would be a cause of interference!

Regarding the bridge blocking DC, consider these scenarios:
1. the AC connections of the bridge are across the AC and the DC connections are left open. The bridge does indeed completely block current.
2. same AC connection but link the + and - outputs together. Now you have a circuit with two diodes in series across two diodes in series wired the other way around. You get a voltage of 2 *Vf in both directions held across the AC connections and any voltage higher than that forces conduction.
3. same AC connection but wire a constant current load across the + and - outputs. Now you drop 2 *Vf but at a constant current so the overall voltage across the AC arms is higher while the current is the same.

Brian.

betwixt said:

2. same AC connection but link the + and - outputs together. Now you have a circuit with two diodes in series across two diodes in series wired the other way around. You get a voltage of 2 *Vf in both directions held across the AC connections and any voltage higher than that forces conduction.
3. same AC connection but wire a constant current load across the + and - outputs. Now you drop 2 *Vf but at a constant current so the overall voltage across the AC arms is higher while the current is the same.

Brian.

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I am not sure if you write these 2 points as being something bad or just considerations. Please let me know Brian.
In the first, you short circuit the bridge??

The point I'm making is that a bridge across the AC still passes the current through the two AC connections when you draw current from the DC connections.

For example, if you draw 300mA from the DC output of the bridge, you are also drawing 300mA from the AC connections. So by using a constant current DC load you can maintain a constant current between the AC connections and it can be used in a series heater chain.

Equally, by maintaining a constant voltage across the DC connections, you can draw as much current as can be provided from the AC connections. With care you can satisfy the requirements of the heater circuit while still generating the DC supply needed for the isolating DC-DC converter.

Brian.

betwixt said:

The point I'm making is that a bridge across the AC still passes the current through the two AC connections when you draw current from the DC connections.

For example, if you draw 300mA from the DC output of the bridge, you are also drawing 300mA from the AC connections. So by using a constant current DC load you can maintain a constant current between the AC connections and it can be used in a series heater chain.

Equally, by maintaining a constant voltage across the DC connections, you can draw as much current as can be provided from the AC connections. With care you can satisfy the requirements of the heater circuit while still generating the DC supply needed for the isolating DC-DC converter.

Brian.

Click to expand...

Oh that is new to me and very interesting!
The constant current load seems straight forward, but how about the constant voltage?
Does this mean that if for example I connect a 7805 at the bridge output, This alone will suffice to pass through the bridge (in series heater connection) whatever current is limited by the nearby tubes, to pass through it?
IS it that simple, or you mean something else by the "constant voltage"?