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Old 1st January 2013, 07:33 AM   #11
DUG is offline DUG  Canada
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There is another reason.

Here is an example.

You have 16Vp from either a half wave or full wave rectifier.

You need 2V "headroom" on a 12V regulator at 1A (14V min)

With a full wave rectifier the capacitor only needs to be 3500uF and gets recharged at twice the line frequency rate. During the time this value of capacitor needs to be recharged the peak diode (and transformer) current is about 20A.

With a half wave rectifier the capacitor needs to be 7700uF and gets recharged at the line frequency rate. During the time this value of capacitor needs to be recharged the peak diode (and transformer) current is about 32A.

The capacitor needs to be bigger to supply the charge to the regulator for a longer period of time. Slightly longer than twice as long so slightly more than twice as large.

The larger the capacitor, the more current it takes to recharge it in the same amount of time.

Hope that helps explain what is happening and why half wave rectification is rarely used.

It is used more with lower current applications. IMO
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Old 1st January 2013, 08:28 AM   #12
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Quote:
Originally Posted by DUG View Post
There is another reason.

Here is an example.

You have 16Vp from either a half wave or full wave rectifier.

You need 2V "headroom" on a 12V regulator at 1A (14V min)

With a full wave rectifier the capacitor only needs to be 3500uF and gets recharged at twice the line frequency rate. During the time this value of capacitor needs to be recharged the peak diode (and transformer) current is about 20A.

With a half wave rectifier the capacitor needs to be 7700uF and gets recharged at the line frequency rate. During the time this value of capacitor needs to be recharged the peak diode (and transformer) current is about 32A.

The capacitor needs to be bigger to supply the charge to the regulator for a longer period of time. Slightly longer than twice as long so slightly more than twice as large.

The larger the capacitor, the more current it takes to recharge it in the same amount of time.

Hope that helps explain what is happening and why half wave rectification is rarely used.

It is used more with lower current applications. IMO


Very nice...
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Old 1st January 2013, 02:49 PM   #13
paulb is offline paulb  Canada
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Yes, so more filtering required and bigger peak currents. Still, this is the simplest solution to the problem and likely to be suitable.
When the OP asked how to use an existing transformer for a particular application, an answer of "get a different one" isn't really being all that helpful...
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Old 2nd January 2013, 04:46 AM   #14
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kinda depends on what he is powering, doesn't it?

since low VA transformers are not expensive, i would go for it.
and high VA applications don't usually go well with half wave - and more caps cost money
too... so there is a balance to be found.

Maybe the OP might say what he is going to run with it?
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Old 4th January 2013, 05:41 PM   #15
GoatGuy is offline GoatGuy  United States
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NigelWright got it exactly correct: Use of a half-wave voltage-doubler is the cheap-n-dirty way to get the voltages being looked for, -21 : 0 : +21 volts (before correction for peak-over-RMS)

NOTE though - it is definitely half-wave rectification, so the capacitors should be rather large to hold enough juice between waves. And also beware - BOTH the positive and the negative supply rails need independent regulators. And beware - that regulators that are complimentary are often happy to short the whole damned thing out if attached to the same heat sink.

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