I have an application in which I need to determine if a transformer is
suitable.

Perhaps a little more info about the project is in order.  Following
the transformer there'll be a bridge rectifier, smoothing cap... then
a linear regulator suppling constant current to charge batteries.  The
circuit is a bit more involved than only this (protection diodes,
charge controller, etc) but this is the subsection in question and the
rectified output of the transformer will need to stay above roughly
12.3VDC so I'm trying to figure out what constant current this
transformer (or others) can supply.  

emailaddr... at (no spam) insightbb.com

I have an application in which I need to determine if a transformer is
suitable.

Transformer is rated for 12VAC, 1.67A but I need DC.  I could put a
bridge rectifier and capacitor after it easily enough but how does one
go about calculating the DC output this is capable of?

I've previously used a x 1.41 factor to convert which would give
16.92V, minus the forward drop of a couple silicon diodes in the
bridge rectifier estimated at roughly ( 2 * 0.7V, ignoring changes in
diode forward drop at different current) which would leave (16.92 -
1.4) = 15.5V, but am I correct in thinking this is peak DC output and
there is a different calculation needed to arrive at the output
voltage if the load were drawing 1.67A?

** This is your error.

The 1.67 amp ( ie 20VA) rating of the transformer is only for AC output -
not some derived DC voltage.

It is also a " free air " rating -  meaning
there must be plenty of ventilation and no heat sources next to the
transformer.

Perhaps a little more info about the project is in order.  Following
the transformer there'll be a bridge rectifier, smoothing cap... then
a linear regulator suppling constant current to charge batteries.  The
circuit is a bit more involved than only this (protection diodes,
charge controller, etc) but this is the subsection in question and the
rectified output of the transformer will need to stay above roughly
12.3VDC so I'm trying to figure out what constant current this
transformer (or others) can supply.  I've alread accounted for other
looses like that through the regulator when coming up with the 12.3V
figure.

How much rectified DC voltage can that transformer maintain at 1.67A?

**   It would be overloaded delivering that amount of DC current and
overheat -  unless perhaps you blew cold air over it with a fan.

Normally, one applies a de-rating figure to the AC current rating of a
transforemer when it is feeding a bridge rectifier and capacitor filter.
The figure depends somewhat on the design of the particular transformer (
toroidal, E-core )  but is in the range of  0.5 to 0.7.

So, if you really do need 1.67 amps DC at 16 volts  -  then pick a 40 VA
transformer.

I have an application in which I need to determine if a transformer is
suitable.

Transformer is rated for 12VAC, 1.67A but I need DC. I could put a
bridge rectifier and capacitor after it easily enough but how does one
go about calculating the DC output this is capable of?

I've previously used a x 1.41 factor to convert which would give
16.92V, minus the forward drop of a couple silicon diodes in the
bridge rectifier estimated at roughly ( 2 * 0.7V, ignoring changes in
diode forward drop at different current) which would leave (16.92 -
1.4) = 15.5V, but am I correct in thinking this is peak DC output and
there is a different calculation needed to arrive at the output
voltage if the load were drawing 1.67A?

Perhaps a little more info about the project is in order. Following
the transformer there'll be a bridge rectifier, smoothing cap... then
a linear regulator suppling constant current to charge batteries. The
circuit is a bit more involved than only this (protection diodes,
charge controller, etc) but this is the subsection in question and the
rectified output of the transformer will need to stay above roughly
12.3VDC so I'm trying to figure out what constant current this
transformer (or others) can supply. I've alread accounted for other
looses like that through the regulator when coming up with the 12.3V
figure.

How much rectified DC voltage can that transformer maintain at 1.67A?

I have an application in which I need to determine if a transformer is
suitable.

Transformer is rated for 12VAC, 1.67A but I need DC. I could put a
bridge rectifier and capacitor after it easily enough but how does one
go about calculating the DC output this is capable of?

I've previously used a x 1.41 factor to convert which would give
16.92V, minus the forward drop of a couple silicon diodes in the
bridge rectifier estimated at roughly ( 2 * 0.7V, ignoring changes in
diode forward drop at different current) which would leave (16.92 -
1.4) = 15.5V, but am I correct in thinking this is peak DC output and
there is a different calculation needed to arrive at the output
voltage if the load were drawing 1.67A?

** This is your error.

The 1.67 amp ( ie 20VA) rating of the transformer is only for AC output -
not some derived DC voltage.

It is also a " free air " rating - meaning
there must be plenty of ventilation and no heat sources next to the
transformer.

Perhaps a little more info about the project is in order. Following
the transformer there'll be a bridge rectifier, smoothing cap... then
a linear regulator suppling constant current to charge batteries. The
circuit is a bit more involved than only this (protection diodes,
charge controller, etc) but this is the subsection in question and the
rectified output of the transformer will need to stay above roughly
12.3VDC so I'm trying to figure out what constant current this
transformer (or others) can supply. I've alread accounted for other
looses like that through the regulator when coming up with the 12.3V
figure.

How much rectified DC voltage can that transformer maintain at 1.67A?

** It would be overloaded delivering that amount of DC current and
overheat - unless perhaps you blew cold air over it with a fan.

Normally, one applies a de-rating figure to the AC current rating of a
transforemer when it is feeding a bridge rectifier and capacitor filter.
The figure depends somewhat on the design of the particular transformer (
toroidal, E-core ) but is in the range of 0.5 to 0.7.

So, if you really do need 1.67 amps DC at 16 volts - then pick a 40 VA
transformer.

I have an application in which I need to determine if a transformer is
suitable.

Transformer is rated for 12VAC, 1.67A but I need DC. I could put a
bridge rectifier and capacitor after it easily enough but how does one
go about calculating the DC output this is capable of?

On Jul 9, 10:57 pm, "Phil Allison" <philalli... at (no spam) tpg.com.au> wrote:

emailaddr... at (no spam) insightbb.com
"Phil Allison"
** This is your error.
The 1.67 amp ( ie 20VA) rating of the transformer is only for AC
output -
not some derived DC voltage.

Ok, but I'm not asking what it's not, I'm asking what it is.

None of the information given is useful in making a determination on
this issue. Phil is telling you that the AC
rating is for 1.67A RMS, which amounts to 2.3A peak, but
that your rectifier will only conduct for a fraction of the duty
cycle, so
the 1.67A average output of the supply is all delivered in very
short time intervals (when the transformer output exceeds
the capacitor charge plus the diode forward drop). The
rectifier current could be 18A repeating peaks (even though
the average is lower), because the rectifier is almost always
turned off.

18A can melt the copper windings.

Go to http://www.hammondmfg.com/5cpwr.htm and download their Power
Transformer Selection Guide - it is a one page .pdf that shows various
circuit configurations and resulting voltage/current relations.

On Jul 9, 9:26 pm, emailaddr... at (no spam) insightbb.com wrote:

I have an application in which I need to determine if a transformer is
suitable.
Perhaps a little more info about the project is in order. Following
the transformer there'll be a bridge rectifier, smoothing cap... then
a linear regulator suppling constant current to charge batteries. The
circuit is a bit more involved than only this (protection diodes,
charge controller, etc) but this is the subsection in question and the
rectified output of the transformer will need to stay above roughly
12.3VDC so I'm trying to figure out what constant current this
transformer (or others) can supply.

Another thing I am wondering is if a basic, typical small brick AC-DC
switching supply is stable powering this kind of load. Stripping down
the circuit to the basic topology, what if it had for example:

AC-DC SMPS -> LM317 -> 1.25 Ohm resistor (LM317 in current regulating
config) -> 1A into Battery Pack + LM317 feedback

Since the SMPS is trying to regulate to it's spec'd voltage, let's say
that is 13V even if an uncommon value, will it run stable doing so and
if so, is that at the same constant output wattage from the SMPS the
whole time, with the linear regulator simply dropping more voltage,
creating more heat at the beginning of a battery recharge cycle since
it's suppling constant current to batteries that are at a lower
initial, low state-of-charge voltage?

On Jul 10, 1:50 am, ehsjr
e.h.s.j.r.removethespampunctuat... at (no spam) bellatlantic.net> wrote:

There is no "magic formula". You need to learn a whole
lot more about power supplies to calculate with precision
and to understand what needs to be considered and why.


Ok, but this should be resolvable to a reasonable level given the
example context of it being a typical E-core transformer and basic
silicon bridge rectified and capacitive filtered circuit. The output
at that point is the question. Even if some formula has an unknown
variable, or two or one hundred, the start would be to resolve those
which requires a working equation in which to place those variables.



I'll mention some general things.

First, with a bridge rectifier and capacitive filter,
figure the current you can supply to the load at
a little over 1/2 the transformer rating, in your
case, a bit over .9 amps.


This seems to counter the majority of small transformer examples out
there, does it not? Consider products powering just about anything
that uses a wall wart.
Consider a sub-20VA transformer rated in a wall wart for 12VDC, 1A
output. Granted, we could call that a little over 1/2 the rating but
just how much or little is the crucial issue, what variables determine
how much or little and how to express those mathematically.



Next, figure the ripple voltage, Vr. To figure Vr, seehttp://ocw.mit.edu/NR/rdonlyres/Electrical-Engineering-and-Computer-S...



Thank you for the link, though since I am not mass producing equipment
and so not overly concerned about small differences in component cost
or size, I'm essentially going to consider ripple current a constant
resolved later as would apply to any one design, while I am asking
about a general formula for conversion without regard to variables
that would change in different designs beyond the basic assumption of
a capacitor large enough to achieve acceptibly low ripple for example.


Next, find out how much headroom your regulator needs, usually
available from the datasheet. Then, the maximum voltage at
the output of the regulator will be
(Vsec*1.41) - 2Vf - Vr - Vheadroom = Vmax where Vsec is the voltage
at the secondary. Vf is the diode drop, which you can find
on the curve of Vf vs I on the datasheet. You should figure Vsec
based on worst case line voltage and worst case Vsec sag under load.

By the way, the 12.3 volt figure is odd - how did you arrive
at it?


This is what the example circuit would need at a bare minimum by
calculating similarly to what you have above plus other drops in the
circuit at (no spam) expected current levels, to still retain the necessary
minimum input voltage, plus or minus a margin of error as median
datasheet values where used.

However, I am not concerned about the entire circuit, not about drop
over a regulator, I am only concerned about resolving how the AC
transformer rating relates to DC output before anything further in a
circuit beyond a typical bridge rectifier and capacitor sufficient to
smooth to a hypothetical 0V ripple, and I am also accounting for the
(Often negligable) different in forward voltage over the rectifier(s)
at different current levels, but this too can be expressed
mathematically.

Essentially, I don't want information more applicable to one project
than to another. Only what remains true mathetically for all projects
which employ AC spec'd transformers of typical design and through
bridge rectification by the most common silicon diodes. In other
words, as you'd see in the vast majority of electronics already if
they're not using switching PSU.

I have an application in which I need to determine if a transformer is
suitable.

Transformer is rated for 12VAC, 1.67A but I need DC. I could put a
bridge rectifier and capacitor after it easily enough but how does one
go about calculating the DC output this is capable of?

I've previously used a x 1.41 factor to convert which would give
16.92V, minus the forward drop of a couple silicon diodes in the
bridge rectifier estimated at roughly ( 2 * 0.7V, ignoring changes in
diode forward drop at different current) which would leave (16.92 -
1.4) = 15.5V, but am I correct in thinking this is peak DC output and
there is a different calculation needed to arrive at the output
voltage if the load were drawing 1.67A?

Perhaps a little more info about the project is in order. Following
the transformer there'll be a bridge rectifier, smoothing cap... then
a linear regulator suppling constant current to charge batteries. The
circuit is a bit more involved than only this (protection diodes,
charge controller, etc) but this is the subsection in question and the
rectified output of the transformer will need to stay above roughly
12.3VDC so I'm trying to figure out what constant current this
transformer (or others) can supply. I've alread accounted for other
looses like that through the regulator when coming up with the 12.3V
figure.

How much rectified DC voltage can that transformer maintain at 1.67A?
How much current can it maintain while staying at or above 12.3V, or
if there is another voltage to consider because we don't know the
other properties of this transformer, what voltage would that be and
at that voltage what is the DC current capability?

Please I'm asking to learn how to calculate this myself instead of
only the numerical answer.

Thanks,
JC

On Wed, 9 Jul 2008 18:26:12 -0700 (PDT), emailaddress at (no spam) insightbb.com wrote:

I have an application in which I need to determine if a transformer is
suitable.

Transformer is rated for 12VAC, 1.67A but I need DC. I could put a
bridge rectifier and capacitor after it easily enough but how does one
go about calculating the DC output this is capable of?

I've previously used a x 1.41 factor to convert which would give
16.92V, minus the forward drop of a couple silicon diodes in the
bridge rectifier estimated at roughly ( 2 * 0.7V, ignoring changes in
diode forward drop at different current) which would leave (16.92 -
1.4) = 15.5V, but am I correct in thinking this is peak DC output and
there is a different calculation needed to arrive at the output
voltage if the load were drawing 1.67A?

Perhaps a little more info about the project is in order. Following
the transformer there'll be a bridge rectifier, smoothing cap... then
a linear regulator suppling constant current to charge batteries. The
circuit is a bit more involved than only this (protection diodes,
charge controller, etc) but this is the subsection in question and the
rectified output of the transformer will need to stay above roughly
12.3VDC so I'm trying to figure out what constant current this
transformer (or others) can supply. I've alread accounted for other
looses like that through the regulator when coming up with the 12.3V
figure.

How much rectified DC voltage can that transformer maintain at 1.67A?
How much current can it maintain while staying at or above 12.3V, or
if there is another voltage to consider because we don't know the
other properties of this transformer, what voltage would that be and
at that voltage what is the DC current capability?

Please I'm asking to learn how to calculate this myself instead of
only the numerical answer.

Thanks,
JC

You should go to the library and make a copy of the paper:

"Analysis of Rectifier Operation", O. H. Schade, Proceedings of the IRE,
July 1943

This will give you an idea of what's involved in a detailed analysis of the
sort you're asking for.

I have an application in which I need to determine if a transformer is
suitable.

Transformer is rated for 12VAC, 1.67A but I need DC. I could put a
bridge rectifier and capacitor after it easily enough but how does one
go about calculating the DC output this is capable of?

I've previously used a x 1.41 factor to convert which would give
16.92V, minus the forward drop of a couple silicon diodes in the
bridge rectifier estimated at roughly ( 2 * 0.7V, ignoring changes in
diode forward drop at different current) which would leave (16.92 -
1.4) = 15.5V, but am I correct in thinking this is peak DC output and
there is a different calculation needed to arrive at the output
voltage if the load were drawing 1.67A?

Perhaps a little more info about the project is in order. Following
the transformer there'll be a bridge rectifier, smoothing cap... then
a linear regulator suppling constant current to charge batteries.

You should go to the library and make a copy of the paper:

"Analysis of Rectifier Operation", O. H. Schade, Proceedings of the IRE,
July 1943

This will give you an idea of what's involved in a detailed analysis of the
sort you're asking for.

---
There's also Motorola's Silicon Rectifier Manual, an excellent
reference and cheap at:

http://www.amazon.com/gp/offer-listing/B000HVEWGK/ref=dp_olp_3/103-7647257-4204638

JF

However, it is fairly beside the point that I am looking for
a universal equation that ignores all of this. Many people seem to be
saying the rest matters and yes of course it does - but it is still
theoretically expressable in an equation with the factors that change
as variables.