# DC applied to primary of transformer

Discussion in 'Electronic Basics' started by Thaqalain, Jun 10, 2005.

1. ### ThaqalainGuest

I want to apply DC safely to the primary of a transformer,how can i
do?by
*When limiting resistance is in series.
*When limiting resistance is in parallel.
*when neither of above

2. ### John PopelishGuest

The windings of a transformer act just as any other piece of wire
where DC is involved, so think of how you could safely connect wire
across a DC supply.

If the supply is a voltage source (voltage essentially constant, no
matter how much current is drawn from it), than adding other loads in
parallel with the wire would only provide additional paths for more
current, without limiting or reducing the current through the wire.

Resistances connected in series (the wire is just a low value
resistance) add. So a series resistor will reduce the current that
passes through the transformer winding.

If, however, the DC supply is a current source (which means that the
current is nearly constant, regardless of how much resistance is
connected across it because it varies its voltage to whatever is
necessary to drive that current), then paralleling the wire with other
resistances would detour some of that total current to other paths,
lowering the total voltage the supply needed to produce to maintain
the fixed, total current. This lowers the fraction of that total
passing through the transformer.

Since your mention of "applying DC" does not specify whether the
source is more like a voltage source or a current source, or something
third choice.

Tell your teacher that you learned this on the Internet, not in their
class.

3. ### ThaqalainGuest

This was test question for ems comapny,they don't describe
Volatge/Current Source?here is original question:
DC is safely applied to the primary of a transformer:
*When limiting resistance is in series with primary.
*When -------------------------Parallel------------.
*When neither of preceding applies

4. ### ThaqalainGuest

Thanks a lot Sir,I appreciate your detailed response due to that we get
some knowledge by these forums.I am civil engineer working for ems cos
in ON.

5. ### John PopelishGuest

Does my answer make sense to you?

Since voltage sources are so much more common than current sources, I
suspect they are expecting the first answer.

6. ### Tom MacIntyreGuest

Generalization...A.

Tom

7. ### mikeGuest

True or false?
Transformer cores are sized to be the smallest/lightest/cheapest
they can be and still not saturate at rated drive levels.

If true, then any significant amount of DC current is a bad thing.
Matters not whether it was placed there by Mr. Thevenin or Mr. Norton.
mike

--
Return address is VALID but some sites block emails
..
Wanted, PCMCIA SCSI Card for HP m820 CDRW.
FS 500MHz Tek DSOscilloscope TDS540 Make Offer
Wanted 12" LCD for Compaq Armada 7770MT.
Bunch of stuff For Sale and Wanted at the link below.
MAKE THE OBVIOUS CHANGES TO THE LINK
ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/

8. ### Rich GriseGuest

IOW, (D) None of the above. There is _no_ safe DC value that can
be applied to the primary of a transformer. Unless, of course, it's
a transformer that was designed for single-ended class A output. In
that case, it'd be series resistance, or more strictly speaking,
series transconductance.

If this is a test question, then Thaqalain needs to reread the
textbook before re-taking the test. Or ask the teacher what
answer they're trying to extract from the students.

One of the ways test questions are recognizable is by the date.
It's FINALS WEEK! ;-D

Cheers!
Rich