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 Post subject: Silicon Diode
PostPosted: Oct Fri 23, 2020 3:47 pm 
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Joined: Apr Wed 10, 2019 3:48 am
Posts: 412
Placed line voltage (125v) to a 1N5408 rectifier diode and measured the results with dmm:
67.5vac and 57.5vdc

So, it appears that the rectifier is still passing AC. Is this the case, or will the AC vanish under load?
Is this AC considered ripple?
Is a capacitor required to pull the AC out to leave just DC?
Is a capacitor required to smooth the half wave DC?


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 Post subject: Re: Silicon Diode
PostPosted: Oct Fri 23, 2020 5:18 pm 
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Location: Montreal, Quebec
A rectifier only passes one side of zero, whether it's positive or negative depends on the direction of the diode.

If you need DC, you need a filter capacitor (and old tube power supplies often added chokes).


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 Post subject: Re: Silicon Diode
PostPosted: Oct Fri 23, 2020 5:22 pm 
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Location: Stone Mountain, GA
Yes you have DC with 100% ripple.
A capacitor will smooth out the DC and reduce the AC component.

Hard to say what your meter reads means without including how the meter measures voltage. On non sinusoidal signals, most meters give meaningless results.

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 Post subject: Re: Silicon Diode
PostPosted: Oct Fri 23, 2020 10:39 pm 
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So, along with this diode, could an across-the-line non polarized capacitor and proper resistor work as a dedicated supply to strictly tube heaters? If so, then I suspect DC would be powering the heaters?

Also, what would happen Without the capacitor in the same scenerio?
Would the heaters get hammered with a combination of AC and DC raising the voltage to the filaments to approximately double?


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 Post subject: Re: Silicon Diode
PostPosted: Oct Fri 23, 2020 11:20 pm 
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Joined: Apr Wed 10, 2019 3:48 am
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Or, maybe im Just testing improperly since my circuit is being completed by my meter.
In other words, AC Line hot, diode, meter, AC line neutral. So, meter leads between diode and line neutral.

Maybe the test requires a complete circuit be built Line hot, diode, Load, line neutral, and use meter to test voltages?


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 Post subject: Re: Silicon Diode
PostPosted: Oct Sat 24, 2020 1:12 am 
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Location: Dallas, TX
i think you need to learn more about electronics, the info is out there.
Half-wave rectifier with smoothing capacitor, near the bottom.
https://www.electronics-tutorials.ws/diode/diode_5.html
By the way, with a very few exceptions tube heaters work on much lower voltage than 120V.

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 Post subject: Re: Silicon Diode
PostPosted: Oct Sat 24, 2020 2:11 am 
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Joined: Apr Wed 10, 2019 3:48 am
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Was hoping for Experienced response to these specific questions!


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 Post subject: Re: Silicon Diode
PostPosted: Oct Sat 24, 2020 3:41 am 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island NY
It would help if we knew what you are trying to accomplish.

A rectifier, by definition, is a device that only conducts current in one direction. So when AC is passed through a rectifier, you get either the positive excursions or the negative excursions, as mentioned above. At this point it is no longer AC because the current is unidirectional. It varies from zero to the peak voltage in whichever direction it conducts, then back to zero. This is called pulsating DC.

With a little calculus it is possible to show that the RMS value of half wave AC is Vpeak/2. So if you apply 120 volts AC, the peak value would be 120 x 1.41 =169.2 and dividing by 2 we get 84.6 volts. Most meters respond to the average or peak values of waveforms, but are calibrated in RMS volts for sinusoidal waveforms. The output of a rectifier is no longer a symmetrical sine wave so the calibration no longer holds. A true RMS meter, on the other hand, would give the correct answer. You are getting readings of 57 and 67 volts because your meter doesn't know how to handle a rectified half wave voltage.

If a capacitor is connected across the output of a rectifier, it will attempt to charge to the full peak voltage when the diode conducts, then it will discharge into the load when the diode stops conducting. This tends to hold the voltage constant so now it looks more like pure DC than pulsating DC. The DC voltage reached depends on the load across it and the size of the capacitor. Generally speaking, the capacitor is calculated based on the load current to provide a particular level of ripple. (The ripple is residual voltage pulsations riding on the DC voltage). The subsequent filtering networks are then designed to remove the ripple. It is usually uneconomical, in terms of the capacitor required, to remove all the ripple with one capacitor. It's more common to use two capacitors with a choke or a resistor between them to make a filtering network of sorts. In modern electronics the solid state voltage regulator circuit is often used as an active filter to clean the ripple up.

As for powering tube heaters, it generally doesn't matter if you use AC, DC, or pulsating half wave DC. The heater is insulated from the cathode pretty well in most tubes. Of course if the tubes are directly heated, eg. they have filaments rather than heaters and cathodes, then you have to filter the filament supply well. Since the filaments are also in the signal path, any hum in the filament supply voltage will modulate the signals you are trying to amplify. It is also necessary to isolate and filter the supplies in such equipment to prevent unwanted couplings that result in oscillation. What does matter is that the DC or RMS value of whatever voltage is applied to the heaters should be within +/- 10% of their rated voltages.

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 Post subject: Re: Silicon Diode
PostPosted: Oct Sat 24, 2020 3:47 am 
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Codepug wrote:
Was hoping for Experienced response to these specific questions!

Which questions?

The ones in your first post? SHenion answered those.

The ones about powering tube heaters? Your description of the circuit is too vague for anyone to answer sensibly. Draw up a schematic and post a photo of it.

Or the one about your measurement technique?
You already have a complete circuit. Your meter is the load. Your measurements are more or less what one would expect, although as SHenion says, the exact voltages expected depend on how your meter measures the voltages. Average? Peak? True RMS?

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 Post subject: Re: Silicon Diode
PostPosted: Oct Sat 24, 2020 1:28 pm 
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Joined: Apr Wed 10, 2019 3:48 am
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Chris, Thank you very much for the excellent detailed response. This is what I was hoping for, and helps clarify a lot of the confusion I had with this topic.



You already have a complete circuit. Your meter is the load. Your measurements are more or less what one would expect, although as SHenion says, the exact voltages expected depend on how your meter measures the voltages. Average? Peak? True RMS?

Steve, I will have to find my meter manual and determine exactly how the voltages are being read. This would certainly add to my inability to grasp the results. Maybe not true rms.


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 Post subject: Re: Silicon Diode
PostPosted: Oct Thu 29, 2020 2:22 am 
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Joined: Apr Wed 10, 2019 3:48 am
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http://www.vintage-radio.com/repair-res ... calcs.html

”If you measure the heater voltage and current with this circuit, the results will be confusing. This is because the waveform is neither a pure sinewave nor pure DC, so most meters will simply read an average value rather than the true RMS value. With an analogue meter switched to a DC range, multiplying the reading by 1.57 (1.11 x square-root of 2) should give the correct value. It is fairly easy to multiply the reading by 1.5 in your head, which gives an error of less than 5%. With digital meters, it is more difficult to predict what they will make of this sort of waveform.”


Making even more sense now!
Especially when using the 1.57 multiplier.

Thanks all....


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 Post subject: Re: Silicon Diode
PostPosted: Oct Thu 29, 2020 11:40 am 
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Joined: Nov Wed 30, 2016 7:35 pm
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Location: Sunbury, Ohio 43074
I'm going to second the "learn the basics first" .. and you can answer your own questions :). An 'expert' answer is only of use if you can apply it to whatever it is you are trying to do...

which, btw, is exactly what? Test a diode? Figure out zener value? Design a rectifier or regulator?

Kudos to Chris for the usual detailed treatise on general subjects. You can also READ and find out exactly that info. ;-). .... and at the same time, other useful tidbits that you can apply to the next problem. :)

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 Post subject: Re: Silicon Diode
PostPosted: Oct Fri 30, 2020 1:26 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3997
Location: Lexington, KY USA
It's hard to learn all this electronics stuff at first.

Don't worry if you ask a question or two about even very basic things. Someone will try to help.

We almost need a "beginner's corner" on the ARF for tutorial threads.

Another forum that might be worth looking at is

https://www.electronicspoint.com/forums/

Ted


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 Post subject: Re: Silicon Diode
PostPosted: Oct Fri 30, 2020 2:04 am 
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Posts: 5073
Location: Littleton, MA
Usually Lurking wrote:
We almost need a "beginner's corner" on the ARF for tutorial threads.

The ARRL Handbook is a good book to use to learn the basics. You don't have to want to be a ham radio operator to read it, just ignore the transmitter stuff unless you are interested in that.

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