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4hv.org :: Forums :: General Science and Electronics
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transformer question

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haxor5354
Sat Oct 08 2011, 02:11AM Print
haxor5354 Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
Transformer

will those 2 setups output the same current?
i've tested it and they both output the same voltage, but i dont have the tools to test current
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Mattski
Sat Oct 08 2011, 02:39AM
Mattski Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
With the first setup only one winding will conduct for each AC half-cycle. With the second they are in parallel and will both conduct for each half-cycle.

The second has lower winding resistance because the two windings are in parallel. It will not heat up as much due to resistive loss, so it can probably handle more current. But there is slight added voltage drop because the full-wave bridge has two diodes in series instead of one.

Rating the current for a transformer is tricky though, it also has to do with the magnetic flux in the core and that will be the same for the two setups.
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Dr. Slack
Sat Oct 08 2011, 06:44AM
Dr. Slack Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
As ever with these questions, the answer is "it depends".

First off, they don't give the same output voltage. The centre tapped (CT) on the left only has one diode drop, the full bridge (FB) on the right has two. So under light and modest load, CT gives more volts than FB. As the FB has half the copper resistance in the secondary, it might give more volts at high current, depending on exact values.

If the current limitation is diode peak current, then they both have the same output current capacity.

If the current limitation is diode heating, or transformer secondary heating, then the FB might win, as the I2R losses are lower in an element that conducts I for all of the time as in the FB, than 2I for 50% of the time as in the CT, but has fewer lossy diodes - note the diode drop power varies linearly, it's the diode residual resistance power that varies as I2R.

In practice, most people use only FB from habit, but CT has a definite advantage at low voltages where the diode drop is a significant fraction of the output voltage.

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Shrad
Sat Oct 08 2011, 10:06AM
Shrad Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780
low voltage schottkys nowadays help to take advantage of whatever mode you prefer I think
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haxor5354
Sat Oct 08 2011, 11:24AM
haxor5354 Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
Dr. Slack wrote ...

As ever with these questions, the answer is "it depends".

First off, they don't give the same output voltage. The centre tapped (CT) on the left only has one diode drop, the full bridge (FB) on the right has two. So under light and modest load, CT gives more volts than FB. As the FB has half the copper resistance in the secondary, it might give more volts at high current, depending on exact values.

If the current limitation is diode peak current, then they both have the same output current capacity.

If the current limitation is diode heating, or transformer secondary heating, then the FB might win, as the I2R losses are lower in an element that conducts I for all of the time as in the FB, than 2I for 50% of the time as in the CT, but has fewer lossy diodes - note the diode drop power varies linearly, it's the diode residual resistance power that varies as I2R.

In practice, most people use only FB from habit, but CT has a definite advantage at low voltages where the diode drop is a significant fraction of the output voltage.



20111008070804057

i suppose a 60A bridge rectifier is more than enough.
the transformer came out of an old Marantz stereo amp than can output dual 38 volts.

anyways, last night i already modified if from CT to FB
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Sulaiman
Sat Oct 08 2011, 11:30AM
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
With current (?) copper vs. semiconductor prices,
at 50/60 Hz the full bridge rectifier usually wins,
probably at all frequencies.

Although meant for valve stuff, I find this Link2 online simulator useful for linear power supplies.
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