STP-2000-2115 Signal Transformer Inc., STP-2000-2115 Datasheet - Page 64

• Manufacturer Part Number: STP-2000-2115
• Description: Power Transformers & High Frequency Magnetics Your One Source For All Your Magnetics Needs!
• Manufacturer: Signal Transformer Inc.

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How to Specify Power Transformer & Filter Parameters
ADDITIONAL FACTORS TO BE CONSIDERED IN TRANSFORMER SELECTION
Load Regulation
RMS. Possible Signal parts would be ST-4-36, PC-34-300,
PC-40-250. A precautionary calculation remains to be made.
That is, the increase in voltage at the filter capacitor (into the
regulator) caused by a high line condition. If we assume our
highest line voltage to be 130 V AC then the transformer
output (compared to low line) would rise by the ratio 130/95.
In the 5V supply, for instance, the following would happen:
V AC = 130 x 9.27 = 12.7V
In the dual complementary +15 V supply:
V AC = 130 x 19 = 26V
ulator, which results in higher regulator power dissipation.
The illustrated values are safe for the typical IC regulator
but should be checked in any specific application.
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The transformer secondary rating is 38 V CT @ 180 mA RMS. Possible Signal parts for this application would be
ST-4-36, PC-34-300, PC-40-250. However, a cautionary calculation must be made. This is computing the increase
in voltage at the filter capacitor (into the regulator) caused by a high line condition. If we assume the highest line
voltage to be 130 V AC then the transformer output (compared to low line) would rise by the ratio 130/95.
For the 5V supply, for instance, the rise would be expressed as:
For the dual complementary +/-15 V supply:
This increase in output must be absorbed by the regulator, which mandates a higher level of regulator power
dissipation. The above values are safe for a typical IC regulator but this parameter should be checked for your
specific application.
It has been assumed up to now that changes in transformer secondary voltage due to line voltage fluctuation have had no
effect on load current. Therefore the transformer would appear to be “ideal” and the transformer secondary voltage (VAC)
would always remain the same. Actually, all the calculated voltages are assumed to be present under full load conditions.
Since transformers are not ideal and have an internal impedance or “regulation” characteristic, variations in load current may
cause a problem. If the load should be light at “high line,” then there will be an additional rise in secondary voltage, beyond
that due to the rising line voltage. This effect is caused by the decreasing voltage drop in the transformer’s windings.
Most smaller VA transformers (<10 VA) have a load regulation of 20% or higher. This means that the transformer’s no-load
voltage will be 20% or more above its rated full-load voltage. This must be taken into account when calculating maximum
VAC (and DC voltage into regulator) under low load current conditions.
Due to inherent characteristics of transformers, “regulation” will vary inversely with size (VA rating). In larger transformers, size
is dictated primarily by the heat resulting from internal losses. In smaller transformers, (low VA rating) size is determined by
the allowable level of no-load to full-load regulation. Curiously, even though this is an important design consideration, most
transformer manufacturers do not publish load regulation data. The chart below tabulates load regulation for Signal’s standard
transformer series’.
It is possible to estimate the output voltage at intermediate loads since load regulation varies in an almost linear fashion.
For example, the 241-8-16 has a full load rating of 16 V @ 6.25 A and a regulation of 10%. Its no-load output would be 10%
more than 16 or 17.6 V. At half load (3.12 A) its output would be 5% more than 16 or 16.8 V. Similar estimates can be made
for any percentage of full load. Another fact to bear in mind is that it is possible to safely exceed the VA rating of many small
power transformers. If the added regulation (drop in output voltage) is acceptable; an “overload” condition may be permissible
because the design is regulation-limited rather than heat rise-limited. If this approach is being considered, the decision
should be reviewed by Signal’s Engineering Dept.
V
V
AC =
AC =
The increase in output must be absorbed by the reg-
130
130
95
95
95
95
X 19 = 26V
X 9.27 = 12.7V
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