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possible 1N007 fault

MTJB

Member
But it isn't 120V mains, it's still 240V mains but only switched on half the time and the current in the (on) half cycle will be the same. Into a resistive load I can't see why this isn't half power.

MTJB, I stated "assuming perfect diode".

Mike.
Nigel is correct in his statements. To look at it another way. 1/2 of the sine wave means the circuit is seeing only 1/2 of the voltage, and the wattage halfs because the pulse rate frequency halfs also. So only 1/4 of the power is consumed by the load. Do the math! P=(E x E) ÷ R.
 

Nigel Goodwin

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The half cycle is identical to the full cycle 240V (only a half cycle) so the instantaneous current will be the same - so won't the instantaneous power be the same for the half cycle it's on and zero for the other half cycle? Sorry to harp on but I can't see how this works.

Mike.
Watts = Volts x Volts divided by Resistance.

Going back to my long ago college days, pre-computer (except for an old valve one that occupied three rooms in the building somewhere, no idea if it was ever used or not) one way of 'measuring' RMS voltages was to plot it out on graph paper, and then count the squares within the wave form (this is essentially what RMS is, the equivalent heating of a DC voltage, you can count the squares for DC as well). So if you full wave rectify the waveform, then the area under the curves is still exactly the same, same number of half sine waves - and obviously if you half wave rectify it the resultant wave is only half of the previous count. Hence, half the RMS voltage.
 

MTJB

Member
Nigel. So if a TUBE is a VALVE over there across the pond; how do you explain how a TUBE over here amplifies???Hi,Hi! :) A question submitted in one of my old classes!
 

Nigel Goodwin

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Most Helpful Member
Nigel. So if a TUBE is a VALVE over there across the pond; how do you explain how a TUBE over here amplifies???Hi,Hi! :) A question submitted in one of my old classes!
The first valve was the diode valve, created in the UK by Fleming, hence the name 'valve' because it's one way flow is essentially identical to a water or air valve. The first amplifying valve the triode, was an American and Austrian (independently) development of the diode a couple of years later.

So 'valve' makes sense because of the original diode, where as 'tube' doesn't make a lot of sense at all :D I suppose the encapsulation could be cut from a glass tube?, but they are really too short to be considered a tube.
 

Nigel Goodwin

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Most Helpful Member
And both are the same except it's only for 50% of the time! What am I missing?

Mike.
You're missing the voltage is only half - check my RMS explanation in post #22.

I can't see how you're getting the voltage as the same?, even volts p-p is obviously only half as only going from zero to +ve, and not -ve to +ve.
 

Pommie

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But it's not half, it's exactly the same. How or why is it half? It's going from 0 to V+ which is not 120V RMS. As I said, sorry to harp on about this but I just don't understand it. A diode can not change the mains profile.

Mike.
 

Nigel Goodwin

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Most Helpful Member
But it's not half, it's exactly the same. How or why is it half? It's going from 0 to V+ which is not 120V RMS. As I said, sorry to harp on about this but I just don't understand it. A diode can not change the mains profile.
How would 0 to V+ be the same as V- to 0 to V+?, obviously it's only half. And assuming 240V mains, the output of a half wave rectifier is 120V RMS.

And it's not changing the mains (unless you have a high current draw on it), it's simply changing what appears on the output of the diode.

Historically TV's used half wave rectifiers, but after a while the electricity companies insisted that they used full wave ones - back in the old two pin plug days it didn't matter, it was random choice if a TV used the positive half cycle, or the negative one, so it balanced out. But once 13A 3 pin plugs were in universal use the VAST majority of TV's were running off the positive half cycles, leaving the negative half cycles unused - and this created a negative bias on the electrical system which the electrical companies didn't like
 

Pommie

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How is 0V to V+ different from 0V to V-? Is one 40% and one 60%? Seriously, I don't undestand how it's not 50%. Early morning here so in the morning I'll try to educate myself using LTspice. However, I can't see how a half cycle into a resistive load can not be half the power.

Mike.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
How is 0V to V+ different from 0V to V-? Is one 40% and one 60%? Seriously, I don't undestand how it's not 50%. Early morning here so in the morning I'll try to educate myself using LTspice. However, I can't see how a half cycle into a resistive load can not be half the power.

Mike.
Because voltage and power are different - half the voltage means quarter the power.
 

alec_t

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Most Helpful Member
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tvtech

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All I know is Nigel is wise. But he doesn't know everything. My participation here on ETO is falling.

It's like I don't feel like popping in anymore. I've never been a smartass. And I've never tried being one. I'm what I am.
 

MTJB

Member
The first valve was the diode valve, created in the UK by Fleming, hence the name 'valve' because it's one way flow is essentially identical to a water or air valve. The first amplifying valve the triode, was an American and Austrian (independently) development of the diode a couple of years later.

So 'valve' makes sense because of the original diode, where as 'tube' doesn't make a lot of sense at all :D I suppose the encapsulation could be cut from a glass tube?, but they are really too short to be considered a tube.
I think the understanding here, is that the next development in glass or metal tubes over here after the triode, was the "traveling wave tube"; which actually was a tube of glass, and the terminology stuck from there on. Boy, I think someone needs to post some kind of a visual, of an AC sine wave converted to pulsating DC by a "half" wave rectifier diode to get the picture! o_O
 
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Nigel Goodwin

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Most Helpful Member
I think the understanding here, is that the next development in glass or metal tubes over here after the triode, was the "traveling wave tube"; which actually was a tube of glass, and the terminology stuck from there on.
Unlikely, as that was 25-30 years later, tetrodes, pentodes etc. followed from triodes.
 

alec_t

Well-Known Member
Most Helpful Member
So does W=V*V/R no longer apply?
It still applies. For the raw AC, W for one half cycle is exactly the same as W for the next half cycle, so if you remove alternate half cycles (which is what the half-wave rectification does) you end up with W/2 average.
 

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