I understand how a diode works as a rectifier in an amp however in a bias circui

[catnine]

the diode is reversed so instead of rectifing a/c to d/c to a positive d/c voltage it rectifies the a/c to d/c to a neg voltage . Now since  diode only passess current one way like a one way valve so is current the key word here ? ie the diode will pass voltage at a low current and that's why the low current a/c from the PT and the higher voltage a/c will pass through the diode?

[sluckey]

Key word(s) is one way.

[catnine]

Key word(s) is one way.

  Meaning what , that no matter which way the diode is placed it will pass voltage one way depending on the direction of the current flow?

[sluckey]

It takes a small amount of voltage (about 0.6v) to forward bias a diode's pn junction. Once that happens, the diode passes current in one direction only. The resulting dc voltage is developed as a result of that current flowing thru some resistance.

There are several different ways to explain a diode's operation. Take a look with Google and pick the one that's easiest to understand or best fits your beliefs.

[stratele52]

On a bias circuit you need negative voltage so the diode should be in the right way ; cathode side on the AC so only the negative  will pass trough.

[J Rindt]

Maybe I am wrong, and I am sorry if it is just me,  but from reading the responses,  I am not sure the gist of your question is expressed clearly. What are you asking exactly.?

[catnine]

Maybe I am wrong, and I am sorry if it is just me,  but from reading the responses,  I am not sure the gist of your question is expressed clearly. What are you asking exactly.?

 My question is this. In a diode type rectifier the diode the anode side is connected to the PT VAC and once the anode is more positive than the cathode current will flow and pass positive DC voltage that is only positive voltage . One diode is a single wave rectifier. When you have a diode on both HT leads from the PT cathodes connected together each diode alternates at AC 60 cycles and both pass only positive voltage . Now since a diode will pass current in only one direction yet is placed with the cathode on the positive A/C of the PT bias tap and a diode is a one way valve then the cathode becomes more positive than the anode so in the bias circuit the diode then rectifies A/C positive to D/C negative . What I am trying to understand is how if a diode is a one way valve how this applies in theory in a bias circuit.

 I have this in an old radio shack book ." Both P-type and N-type silicon conduct electricity. The resistance of both types is determined by the proportion of holes or surplus electrons. Therefore both types can function as resistors and they will conduct electricity in any direction".

 If I take an ohm meter I get a reading in only one direction but that is D/C so is it the A/C that allows the diode to conduct in either direction? Does it depend solely on how much voltage/pressure whether A/C or D/C if the current is low ? Or is a diode being a one way valve really only depend on which way it is placed depending on the flow so in a rect it rectifies A/C POS TO D/C + and reversed it passes and rectifies A/C pos into D/C neg ? There is the PN junction and I understand PIV is say 1,000 volts and a diode is rated for amps how much current it can pass before it breaks down.

[sluckey]

so in the bias circuit the diode then rectifies A/C positive to D/C negative

That's the part that's messing you up. You gotta remember that a sine wave contains a positive half cycle and a negative half cycle. In the bias supply the diode is connected such that it BLOCKS the positive half cycle because the diode is reverse biased. When the sine wave alternates to the negative half cycle, the diode becomes forward biased and PASSES the negative half cycle. Then the cap smoothes the negative half cycles into an approximation of dc.

[PRR]

I have this in an old Radio Shack book. "Both P-type and N-type silicon conduct electricity. The resistance of both types is determined by the proportion of holes or surplus electrons. Therefore both types can function as resistors and they will conduct electricity in any direction".

Perfectly true and PERFECTLY USELESS, stupid.

We have good cheap resistors. Why would we muck with exotic metals for resistance?

The magic comes when P and N are "intimate" at the atomic level. The magic is not in the P or N but in the P-N junction. Here current flows only one way.

Just think "one way valve".

Picture ocean tide. Up and down.

Dig a pit. Connect to tidal water with a one-way valve.

Valve one way, the pit will (after a full tide cycle) drain to the lowest of low tide.

Valve the other way, the pit will (after a full tide cycle) fill to the highest of high tide.