Full Wave Rectifer !!

[DickDV]

magdyfayad, I know I'm jumping in rather late in this discussion but maybe I can offer some help.

First, it is generally considered undesireable and especially UNSAFE to take line voltage of any configuration with a neutral and rectify it. If you were to do that, the DC power rails have significant AC voltage on them with respect to ground. Rather, the good practice is to use a transformer with a floating secondary and, if your DC power is to have a ground reference, then ground it on the DC rail, either - or +. If the DC power supply is not required to have a ground reference, then let the whole system float.

I re-emphasize that using AC power with a ground reference on it to make DC is bad practice and seriously hazardous.

 

[magdyfayad]

thanks DickDV for this advice , as you want to say to make good rectification the best way is to take two hot wires with center tapped transformer and two diodes or at least to take two hot wires and four diodes , but do not take one hot wire and the another is a nuetral , is my understand correct or not DickDV ??

wait keithkyll for your answer for my last post

thanks all

 

[keithkyll]

thanks , but i do not understand well the first answer

assume as at USA we applied 120 VAC between the line and the neutral to the BRIDGE ( FOUR DIODES ) :

I assume you have the black lead of your meter connected to neutral.

1- at the maximum of the positive half cycle , the positive DC output is 170 vdc and the negative DC output is zero voltage , is not right or not ??

Correct

2- at the maximum of the negative half cycle , the positive DC output is zero vdc and the negative DC output is - 170 voltage , is not right or not ??

Correct

while if we applied the 240 VAC between the two hot wires to the BRIDGE ( FOUR DIODES )

USA AC, European AC, or 3 phase?

3- at the maximum of the positive half cycle , the positive DC output is 170 vdc and the negative DC output is -170 voltage , is not right or not ??

Only in the USA or 3 phase 208 or 3 phase 240. In Europe with the neutral at zero and the hot at 220, the voltages would be zero and +340.

4- at the maximum of the negative half cycle , the positive DC output is 170 vdc and the negative DC output is - 170 voltage , is not right or not ??

Only in the USA or 3 phase 208 or 3 phase 240. In Europe with the neutral at zero and the hot at 220, the voltages would be zero and -340.

 

[keithkyll]

magdyfayad, I know I'm jumping in rather late in this discussion but maybe I can offer some help.

First, it is generally considered undesirable and especially UNSAFE to take line voltage of any configuration with a neutral and rectify it. If you were to do that, the DC power rails have significant AC voltage on them with respect to ground. Rather, the good practice is to use a transformer with a floating secondary and, if your DC power is to have a ground reference, then ground it on the DC rail, either - or +. If the DC power supply is not required to have a ground reference, then let the whole system float.

I re-emphasize that using AC power with a ground reference on it to make DC is bad practice and seriously hazardous.

That's the way the did TV's in the late sixties. Transformerless and series filaments.
The DC supply is floating in a SMPS.

 

[magdyfayad]

thanks keithkyll


i assumed if we applied the 240 VAC between the two hot wires to the BRIDGE ( FOUR DIODES ) , the two hot wires are in phase as at USA homes - as i think all electerical books asumed when explaining full wave rectifer the two wires are both hot - then the resullt is :


1- at the maximum of the positive half cycle , the positive DC output is 170 vdc with respect to ground and the negative DC output is -170 vdc with respect to ground , is not right or not ??


2- at the maximum of the negative half cycle , the positive DC output is also 170 vdc with respect to ground and the negative DC output is - 170 vdc with respect to ground , is not right or not ??


3- as we noted - if the above two notes are correct - the neagative dc output if we use two hot wires in phase is fixed at - 170 VDC with respect to ground , i.e : if we connect the neagative dc output to the ground as it makes short circuit for both the positive half cycle and the negative half cycle , while if we use only one hot wire and the other is the nuetral it is also wrong to connect the neagative dc output to the ground as it makes short circuit also but only at the negative half cycle , is not right or not ??


4- all the discussion about full wave rectifer on electeral books and all links , it considered the two wires are hot and are out of phase - single phase but the two wire are hot - as used in USA homes , as i did not find any electeral book or any link say about if there is only one hot wire as at Egypt or at Europe , this note is right or not ??



thanks

 

[keithkyll]

1, 2, 3 = Correct.
4. Hots are not out of phase. Two wires (hot, neutral, I don't care) are single phase. 3 wires (all hots) are 3 phase. For this discussion, there is no such thing as 2 phase.

USA is 120 volt. Many other countries have 100 volts or more. Phase never changes, only the voltage.

 

[magdyfayad]

i meaned by out of phase it in the same phase but the amplitude of the voltage is shifted by 180 degree , by this consider is note four correct or not ??

i.e : i think all electerical books asumed when explaining full wave rectifer the two wires are both hot but in phase with 180 degree for the voltage , as i did not find any electerical book say about if there is only one hot wire as at Egypt or at Europe , this note is right or not ??


thanks

 

[keithkyll]

Ok. 4 is correct. I think the UK has Neutral=0 and Hot=220 (with respect to Earth).

 

[magdyfayad]

so , the main reason for my miss understand as i do not know all the electerical books asumed when explaining full wave rectifer the two wires are both hot but in phase with 180 degree for the voltage , as i thinked mistakely it assumed only one wire is hot and the another is the nuetral


then , do you agree with me the all electerical books when explaning the full wave rectifer , it considered only the USA connection , i.e : the both wire are hot with voltage shift 180 ??

thanks

 

[keithkyll]

No, I do not agree. It does not matter if the wires are hot or neutral. The only thing to consider is the voltage BETWEEN the 2 wires. Forget about Earth. Forget about Neutral. It does not have a thing to do with it. You are spinning in circles confusing yourself with Neutral, when I tell you it does not matter.
Take a battery with a pot. Turn it all the way up, then back down to zero. Reverse the leads. Turn the pot all the way up, then all the way down again. This is single phase AC.
Did I mention ground or earth anywhere? No. It doesn't matter. Only the voltage between the 2 wires.

 

[DickDV]

keithkyll, yes, and they also did AC/DC radio filaments that way too. Resulted in a lot of electrocutions when the AC plug was put in backward placing the hot side on the chassis.

You will notice that its not done that way any more, doubtless for safety reasons.

For the same reason, a DC supply with one side or the other or both referenced to AC neutral is plain foolishness.

 

[magdyfayad]

yes , you are right it doesnot matter


but there is a difference with respect to ground as for the two hot wires the value of the positive dc output is fixed with respect to ground at the same value at both the positive half cycle and the negative half cycle and the same for the negative dc output ,

while if only one hot wire the value of the positive dc output is not fixed at the same value with respect to ground at both the positive half cycle and the negative half cycle and the same for the negative dc output

is not right or not ??

thanks

 

[keithkyll]

Right. The point I'm trying to make is that we don't care. Only the final DC voltage from the bridge is important if we are talking about the basic function of the circuit.
We care about hots and neutral only when we are sizing filter caps for noise, or designing safety isolation. Neutral is also important when designing the jumper for 100/220 operation.

 

[magdyfayad]

thanks

 

[eebads2391]

Keithkyll ,

How is the batterys output ac ?