Transformer ratings

[bbcmicro]

Help! I need clarification for transformer ratings. I've already got myself thoroughly confused.

One schematic I read calls for a "15v CT x 1A winding" I'm guessing this means 15v and 1A from the secondary coil, but I'm not sure.

I see transformers elsewhere rated as TX 15v/6VA for example. Is this also 15v from the secondary coil, with 6A?

What does CT mean? TX? VA?

Also this particular schematic shows 2 inputs, and 2 outputs, with a 3rd illustrated as being taken from the middle of the secondary coil. Is this standard?

???

[Dwight Elvey]

Help! I need clarification for transformer ratings. I've already got myself thoroughly confused.

One schematic I read calls for a "15v CT x 1A winding" I'm guessing this means 15v and 1A from the secondary coil, but I'm not sure.

I see transformers elsewhere rated as TX 15v/6VA for example. Is this also 15v from the secondary coil, with 6A?

What does CT mean? TX? VA?

Also this particular schematic shows 2 inputs, and 2 outputs, with a 3rd illustrated as being taken from the middle of the secondary coil. Is this standard?

???

Hi
The first statement "15V CT X 1A winding" means that it is a 15VA ( 15V * 1A ) transformer with
a center tapped ( CT ) secondary.
The transformer you list as 15V/6VA is only able to put out 0.4A. This is not enough to meet
the 1A rating and it doesn't have a ceneter tap.
I don't know what TX refers to?
Without looking at the schematic, I'm not sure what your looking at.
Dwight

[nige the hippy]

TX usually denotes "transmit" or "transformer" dependent upon context (I have to add that disclaimer as I've been "put on the autistic spectrum" by a former teacher of autistics/aspergers kids;-))

It's as dwight says, I would add that usually to avoid confusion a 15V CT secondary, would be described as 7.5-0-7.5V

If it had 2 separate secondary windings (more common) it would be 0-7.5V, 0-7.5V and you'd connect one of the "0"s to one of the "7.5"s (as the centre tap) to achieve the same result (remember to get the direction of the current the right way round in order to add them together)
Draw it!

VA is Volt Amps, it's Volts X Amps, but it's not quite the same as Watts, as it takes the relative phase of the voltage and current into account. Use the maximum of each to give VA.

If you're driving a purely inductive or capacitive load the power (watts) drawn is zero because the voltage & current are 90 degrees out of phase.

But if you consider that current drawn through (the resistance of) the windings generates heat,
and the number of turns of secondary wire determines the voltage,

then the total amount of heat generated in the transformer is proportional to length of wire (i.e proportional to number of turns, i.e. proportional to volts) X current through the wire (amps)

Hence the rating (heat handling capacity) of the transformer is expressed in VA

[Dwight Elvey]

TX usually denotes "transmit" or "transformer" dependent upon context (I have to add that disclaimer as I've been "put on the autistic spectrum" by a former teacher of autistics/aspergers kids;-))

It's as dwight says, I would add that usually to avoid confusion a 15V CT secondary, would be described as 7.5-0-7.5V

If it had 2 separate secondary windings (more common) it would be 0-7.5V, 0-7.5V and you'd connect one of the "0"s to one of the "7.5"s (as the centre tap) to achieve the same result (remember to get the direction of the current the right way round in order to add them together)
Draw it!

VA is Volt Amps, it's Volts X Amps, but it's not quite the same as Watts, as it takes the relative phase of the voltage and current into account. Use the maximum of each to give VA.

If you're driving a purely inductive or capacitive load the power (watts) drawn is zero because the voltage & current are 90 degrees out of phase.

But if you consider that current drawn through (the resistance of) the windings generates heat,
and the number of turns of secondary wire determines the voltage,

then the total amount of heat generated in the transformer is proportional to length of wire (i.e proportional to number of turns, i.e. proportional to volts) X current through the wire (amps)

Hence the rating (heat handling capacity) of the transformer is expressed in VA

Hi
All is true.
One could also take two 7.5v 1 amp transformers and wire them such
that you'd have the equivalent of a single 15V 1A transformer with CT.
Dwight

[bbcmicro]

Thanks for the info,
I'll need some time to digest this. Also a pencil and paper.

[bbcmicro]

Another question which isn't worth a whole new thread:

can earth/ground as from the ground pin on a 3 pin mains plug be used as logic 0? is it unwise?

[Dwight Elvey]

Another question which isn't worth a whole new thread:

can earth/ground as from the ground pin on a 3 pin mains plug be used as logic 0? is it unwise?

Hi
Yes and no. It can be connected but it should only be connected at one location.
This means that if you have multiple equipment with ground connections, they should
all share the same ground path.
Most applications, like wall transformer units do not connect to earth ground. Most
PC do connect. It isn't required in some cases because the unit has an approved
power issolation.
Dwight