On 3/10/2013 9:25 PM, Jim Mueller wrote:
On Fri, 08 Mar 2013 16:51:57 -0500, rickman wrote:

On 3/8/2013 4:30 PM, Tim Williams wrote:
wrote in message
...
Is this a current transformer or a voltage transformer?
.--------. .--------.
| | | |
| C||C
VAC C||C Load
| C||C
| | | |
`--------' `--------'

Voltage. How about this?

.--------. .--------.
| | | |
| C||C
IAC C||C Load
| C||C
| | | |
`--------' `--------'

Tim


I have to say I don't follow the distinction. It is a transformer, no?

The second one is a current transformer. They both consist of coils
around a magnetic core driving some kind of load. The difference is the
source of power and that causes them to behave very differently as well
as being constructed differently.

I can't say I understand the distinction.


Let's assume ideal components (a good place to start when learning a new
concept). The voltage transformer is driven by a source that provides a
constant voltage, no matter what the load. The transformer takes this
voltage and converts it to some other voltage depending on the turns
ratio; Vout = Vin * Ts / Tp. For example, if the primary has 100 turns
and the secondary has 20 turns and the primary is supplied with 50 volts,
the secondary will provide 10 volts. As the secondary load changes, this
voltage remains the same but the current changes. If the secondary is
open-circuited, the voltage still stays the same. If the secondary is
short-circuited, the current becomes infinite; that's why real voltage
transformers are protected by fuses or similar devices.

This is ok so far.


Now for the current transformer, it is driven by a source that provides a
constant current no matter what the load. The transformer takes this
current and converts it to some other current depending on the turns
ratio; Iout = Iin * Tp / Ts (note the inversion of the turns ratio).
For example, if the primary has 1 turn (a common number for real
transformers) and the secondary has 5 turns and the primary is supplied
with 5 amps, the secondary will provide 1 amp. As the secondary load
changes, this current remains the same but the voltage changes. If the
secondary is short-circuited, the current still stays the same. If the
secondary is open-circuited, the voltage becomes infinite; that's why
real portable current transformers have a shorting switch on the
secondary that the operator must close before disconnecting the load.

I don't follow how any of this has to do with a difference in the
transformers. Bth transformers obey both equations you have presented.
Both transformers change the voltage as well as the current, no?


Also, note the difference in the number of turns, voltage transformers
have a lot of turns and current transformers have few turns.

I don't see how this follows from what you have written. What is there
about these two transformers that define the number of turns? The
current transformers I am interested in using use 100 or 300 turns. Is
that a lot or just a few?


For a loop antenna with an external resonating capacitor, a voltage
transformer would be connected in parallel with the loop and capacitor;
all three in a parallel circuit. A current transformer would be
connected in series with the antenna and capacitor so that the three form
a series circuit. If the loop itself is used as the primary of the
transformer and another winding is used as the secondary, the distinction
between the two types is blurred. Also, a real antenna is neither a
voltage source nor a current source but something in between.

What I have gotten from this is that Tim's original usage of the terms
implies how the transformer is connected to the antenna. As you say, a
voltage transformer will be connected across the coil in parallel with
the capacitor and a current transformer will be connected in series with
the antenna and capacitor.

I was planning to use the antenna wire itself in the middle of the
antenna loop as the primary of the transformer. So I guess that will be
a current transformer. I may try a simulation to see just what happens
with a parallel connection.

--

Rick