Re: [CAD_CAM_EDM_DRO] power supply: Transformer specs
Posted by
JanRwl@A...
on 2001-10-16 17:03:37 UTC
"Small" (less than 2 KVA) transformers are generally made with a "de-rating
factor" of 5% to 10%, depending on copper-size and quality, iron-thickness
and quality, and country of origin and the extent of experience with delights
of the grape known by the maker thereof. "Open Circuit", most small
transformers will put out a rather-surprising excess voltage, particularly
when the local utility is "happy" at the moment, and is supplying you with
125/250 VAC (as mine does). This business of "increasing" the voltage by
connecting a capacitor must NOT be misunderstood by those of us who are not
"electronic geeks" like I try to be, now and then! The cap merely "stores"
the voltage at the "peak" which is 1.414 x RMS. So, a 24 VAC transformer
rated for use on 120 VAC, with NO load, would probably be putting out 27
volts. Full-wave rectify that, and put a large-uF cap across it (10,000 uF
or more), and you would have a good 33 volts!
BUT, when you LOAD the transformer, especially near its "current rating", you
will find two things: The "measured" DC will drop to near the "AC rated
voltage" of the transformer, and the ripple will go up! You do NOT get
"something for nuttin"!!!
Too, a "bridge rectifier" will "waste" about 1½ volts as heat, particularly
when more than 5 amps is being rectumfried. Sorry! Rectified.
Nuff. Read-up on it, and "do what the book say". Do NOT try to be "clever",
as it has been done, already, and people have burned fingers and ruined
motors and all that, already.
Regards, Jan Rowland, Troll
[Non-text portions of this message have been removed]
factor" of 5% to 10%, depending on copper-size and quality, iron-thickness
and quality, and country of origin and the extent of experience with delights
of the grape known by the maker thereof. "Open Circuit", most small
transformers will put out a rather-surprising excess voltage, particularly
when the local utility is "happy" at the moment, and is supplying you with
125/250 VAC (as mine does). This business of "increasing" the voltage by
connecting a capacitor must NOT be misunderstood by those of us who are not
"electronic geeks" like I try to be, now and then! The cap merely "stores"
the voltage at the "peak" which is 1.414 x RMS. So, a 24 VAC transformer
rated for use on 120 VAC, with NO load, would probably be putting out 27
volts. Full-wave rectify that, and put a large-uF cap across it (10,000 uF
or more), and you would have a good 33 volts!
BUT, when you LOAD the transformer, especially near its "current rating", you
will find two things: The "measured" DC will drop to near the "AC rated
voltage" of the transformer, and the ripple will go up! You do NOT get
"something for nuttin"!!!
Too, a "bridge rectifier" will "waste" about 1½ volts as heat, particularly
when more than 5 amps is being rectumfried. Sorry! Rectified.
Nuff. Read-up on it, and "do what the book say". Do NOT try to be "clever",
as it has been done, already, and people have burned fingers and ruined
motors and all that, already.
Regards, Jan Rowland, Troll
[Non-text portions of this message have been removed]