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Ignoramus21666
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 Posted:
Wed Dec 07, 2005 9:12 am Post subject:
Re: Question about diode ratings in a rectifier bridge |
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On 6 Dec 2005 19:07:47 -0800, Mark <makolber@yahoo.com> wrote:
| Quote: | Bill is correct...
this is a very simple circuit but very complex to analyze
If you are using a cap input filter after the bridge rectifier then the
diodes do NOT conduct 1/2 the time, they conduct much LESS then 1/2 the
time. The cap delivers the AVERAGE load current and the diodes have to
recharge the AVERAGE load current... but they only conduct for a few ms
each so the PEAK current is MUCH higer then the average and the RMS
current is also much higher than the average.
This is what power factor correction is all about.
Four diodes rated at 10 AMPs RMS used in a bridge with a cap input
filter and a small conduction angle can proably deliver considerably
less than 10 Amps DC.
This is NOT a simple question.
By the way, this low conduciton angle and high RMS current also impacts
the transformer rating...
Cap input filter power supplies are not simple to analyze.
Do some reaserch on power factor correction, conduction anlge and RMS
vs AVERAGE values.
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How would one correct for it, add an inductor between the caps and
diodes?
i
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Mark
Guest
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| Posted:
Wed Dec 07, 2005 5:35 pm Post subject:
Re: Question about diode ratings in a rectifier bridge |
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ther eis no simple answer..
theoretically, an inductor will help but such an inductor is usually
not practical for low vltage high current supplies
you can use a smaller output C which increases the ouput ripple so the
diodes conduct longer but then you need to filter or reulate the output
to get rid of the ripple..
or you can add some series R which reduces the peak current but hurts
efficency and regulation...
probably the best idea in practice is to make sure the diodes are well
over-rated
....build a breadboard and get a current probe and a scope and look at
some voltage AND CURRENT waveforms...that is the best way to get a feel
for this...maybe make a spreadsheet and take some points off the
current waveform and convert it to RMS ....try that for some simple
rectangular waveform...say a 100 Amp pulse that is 1% duty factor...
the peak current is 100 Amps,, the average current is 1 Amp....you
calculate the RMS current... the RMS current is what determines how
hot the diode will get...
Mark |
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neon
Joined: 25 Feb 2006
Posts: 579
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Mon Jun 26, 2006 11:58 am Post subject:
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A bridge rectifier means that 2 diode must conduct the full load amps for each positive and negative cycle and each one must be rated for reverse voltage of AC peak. the current of whatever requirements will be the rating of the diodes stady state. Even a big cap seing peak voltage when turn on will not blow diodes because the time is too short to heat up. People is not the current that blows diodes it is the heat. A half wave bridge will supply half the voltage from the same transformer.Same rules apply.
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neon
Joined: 25 Feb 2006
Posts: 579
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| Posted:
Tue Aug 29, 2006 3:51 am Post subject:
Re: Question about diode ratings in a rectifier bridge |
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| John Popelish wrote: | All other things being equal (like ambient air temperature around the
diode or amount of heat sink per diode, etc.) then, yes. Each diode
in a full bridge carries half of the average output current. |
Half the current ? each diode carries full current for half a cycle and two are in series with the load when conducting and two are blocking. |
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neon
Joined: 25 Feb 2006
Posts: 579
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| Posted:
Sat Sep 16, 2006 11:54 am Post subject:
Re: Question about diode ratings in a rectifier bridge |
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| John Popelish wrote: | All other things being equal (like ambient air temperature around the
diode or amount of heat sink per diode, etc.) then, yes. Each diode
in a full bridge carries half of the average output current. |
Uin a full bridge all two diodes carry all the current but not at the same time/ |
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neon
Joined: 25 Feb 2006
Posts: 579
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| Posted:
Sat Sep 16, 2006 12:00 pm Post subject:
Re: Question about diode ratings in a rectifier bridge |
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| Tim Williams wrote: | "Ignoramus361" <ignoramus361@NOSPAM.361.invalid> wrote in message
news:fX1lf.9531$MM.1698@fe16.usenetserver.com...
| Quote: | That's great. So, to paraphrase your answer, I could build a 480 amp
rectifier bridge for balanced 60 Hz AC, out of four Ifav=240V
diodes, provided that I satisfy your conditions on equal heating etc.
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Sure. Don't forget you'll be burning about a kilowatt in that bridge.
Wait: "balanced 60Hz AC"? As in a center-tapped winding? Cut your losses
by half and use a single pair of diodes!
Tim
--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms |
you cut the power by half rectification so where is the saving of what? |
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neon
Joined: 25 Feb 2006
Posts: 579
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| Posted:
Sat Sep 16, 2006 12:03 pm Post subject:
Re: Question about diode ratings in a rectifier bridge |
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| Ignoramus361 wrote: | Suppose that I build a rectifier bridge to rectify 60 Hz AC.
Each diode has a rating of Ifav = X amps.
Ifav is described as the maximum mean on-state current.
Since in a full rectifier bridge, every diode conducts only half the
time, would it be proper to say that I can make the bridge to rectify
2X current, out of four such diodes?
Thanks
i |
two diodes carry the full load at one time then the other two carry the load. |
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neon
Joined: 25 Feb 2006
Posts: 579
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| Posted:
Sat Sep 16, 2006 12:05 pm Post subject:
Re: Question about diode ratings in a rectifier bridge |
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| John Popelish wrote: | All other things being equal (like ambient air temperature around the
diode or amount of heat sink per diode, etc.) then, yes. Each diode
in a full bridge carries half of the average output current. |
two diodes carry the load for half a cycle then the other two. |
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neon
Joined: 25 Feb 2006
Posts: 579
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| Posted:
Sat Sep 16, 2006 12:21 pm Post subject:
Re: Question about diode ratings in a rectifier bridge |
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| Ignoramus361 wrote: | On 5 Dec 2005 13:34:36 -0800, mjt@vex.net <mjt@vex.net> wrote:
| Quote: |
I'm not expert on electronics, but I think what he is getting at is
that theoretically yes - but that doesn't mean you should do it.
Some things to remember, device ratings are rarely 100% accurate, there
is varience in everything. If you assume that a device is 100% precise
then you're running a risk when a real part is slightly out. Running
devices at their absolute maximum limit can reduce their life time
because you're placing more stress on them. As mentioned before -
devices typically take on different properties with temperature.... so
how it reacts are 5 celsius could be different 30 celsius. Also don't
forget that your circuit should be tollerent of outside changes beyond
your control. For example, what happens if your supply voltage surges
briefly or if the alternating frequency changes or stops alternating
(because someone connected it to an odd supply etc). You don't want
the diodes going bang as a result.
The main point is that while a lot of things can work, that doesn't
mean its advisable to cut things that close - especially when you're
dealing with power circuits.
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Sure. Real life and being careful demands some degree of derating. So
that, say, a 240 amp diode is only used for 200 amps. But, if I use
the same degree of derating, could I use diodes that could be used to
conduct 200 amps after derating, to form a full rectifier bridge to
conduct 400 amps with the same degree of safety?
i |
you do not have any idea of how bridge rectifiers works . If the load is 240 amps then each diode must be rated for 240 amps not 200 amps. |
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neon
Joined: 25 Feb 2006
Posts: 579
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| Posted:
Sat Sep 16, 2006 12:24 pm Post subject:
Re: Question about diode ratings in a rectifier bridge |
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| John Popelish wrote: | Ignoramus361 wrote:
| Quote: | That's great. So, to paraphrase your answer, I could build a 480 amp
rectifier bridge for balanced 60 Hz AC, out of four Ifav=240V
diodes, provided that I satisfy your conditions on equal heating etc.
Am I correct?
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Yes. If each diode is heat sinked to meet the requirements of
Ifav=240 amps, half wave, then the bridge would be capable of 480 amps. |
you realy do not know what a bridge rectifier does you cannot rate each diode one half the current for an impirical load each diode carry the full load not one half. |
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neon
Joined: 25 Feb 2006
Posts: 579
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| Posted:
Sat Sep 16, 2006 12:28 pm Post subject:
Re: Question about diode ratings in a rectifier bridge |
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| Ignoramus361 wrote: | On Mon, 5 Dec 2005 15:33:12 -0600, Tim Williams <tmoranwms@charter.net> wrote:
| Quote: | "Ignoramus361" <ignoramus361@NOSPAM.361.invalid> wrote in message
news:fX1lf.9531$MM.1698@fe16.usenetserver.com...
That's great. So, to paraphrase your answer, I could build a 480 amp
rectifier bridge for balanced 60 Hz AC, out of four Ifav=240V
diodes, provided that I satisfy your conditions on equal heating etc.
Sure. Don't forget you'll be burning about a kilowatt in that bridge.
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Thanks. Yes, it would be a lot!
| Quote: | Wait: "balanced 60Hz AC"? As in a center-tapped winding? Cut your losses
by half and use a single pair of diodes!
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Well, by balanced I meant that 50% of time it is positive and 50% it
is negative.
Good point on the center tapped winding.
i |
balanced means that the center tap is set with equal turn ratio of the same type of wire. it doesn't provide +- any thing it just provide equal load capability. |
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neon
Joined: 25 Feb 2006
Posts: 579
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| Posted:
Sat Sep 16, 2006 12:33 pm Post subject:
Re: Question about diode ratings in a rectifier bridge |
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| Ignoramus361 wrote: | On Mon, 05 Dec 2005 17:17:53 -0600, John Fields <jfields@austininstruments.com> wrote:
| Quote: | On Mon, 5 Dec 2005 15:33:12 -0600, "Tim Williams"
tmoranwms@charter.net> wrote:
"Ignoramus361" <ignoramus361@NOSPAM.361.invalid> wrote in message
news:fX1lf.9531$MM.1698@fe16.usenetserver.com...
That's great. So, to paraphrase your answer, I could build a 480 amp
rectifier bridge for balanced 60 Hz AC, out of four Ifav=240V
diodes, provided that I satisfy your conditions on equal heating etc.
Sure. Don't forget you'll be burning about a kilowatt in that bridge.
Wait: "balanced 60Hz AC"? As in a center-tapped winding? Cut your losses
by half and use a single pair of diodes!
But then you need twice as much wire in the secondary (but only half
the cross sectional area) as in the case of a full bridge, and the
amount of reverse voltage the rectifiers have to stand off increases
by a factor of two.
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It seems that higher power draw of the full bridge, vs. a bigger
transformer and higher reverse voltage, is a tradeoff depending of
investment cost vs. use cost, in other words depends on how much the
project item would be used in real life. Though the extra
i
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neon
Joined: 25 Feb 2006
Posts: 579
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| Posted:
Sat Sep 16, 2006 12:47 pm Post subject:
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| For people that are discussing this very simple circuit. In a bridge rectifier all current must flow thru the load and two diodes conduct for a half the cycle then the other two conduct for the other half of the cycle. therefore the load current rating for each diode must equal load current. NOT one half of the current. The voltage rating of the diode must exceed the peak voltage input. the difference between a half wave rectifier and a full bridge is conduction time to the load. a bridge will conduct current to the load continiously while a half wave will conduct half of the cycle. the voltage RMS to the load will be .639 * peak for a bridge. |
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