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For
our purposes, tolerance is defined as the variation
allowed from a nominal value. How much variation can
your system endure? The tighter the component values
the greater the cost. Tolerance value decisions should
be based on worst case analysis. Many tolerances can
be single ended (either minimum or maximum). TAKE
ADVANTAGE OF THESE WHENEVER POSSIBLE. All tolerances
should be set with customer/vendor correlation in mind.
Tolerances should never be set tighter than the method
and precision of measurement. Be careful in defining
the method by which the parameters are measured. AVOID
DOUBLE SPECIFICATIONS such as the following: open-circuit
inductance, leakage inductance and capacitance determine
frequency response. If frequency response is the required
parameter, don't specify the other three. This will
only tighten the design envelope and increase cost.
Typical Production Tolerances:
1. Open-Circuit Inductance: Most applications
require a minimum inductance only; the higher the inductance,
the better most circuits work. When this is the specification,
it is designed at no cost premium. Flyback Transformers
and Inductors require a gapped core, which can be provided
with a ± 10% tolerance and at no cost premium. A tolerance
of ± 5% or less requires individual tuning and is expensive.
2.
Leakage Inductance: The coil's physical geometry
and the number of turns determine leakage inductance.
A maximum value will satisfy most circuit requirements.
This is assigned after a pilot run on automated winding
equipment. If a ± tolerance is required it can be assigned
after the pilot run and the measurement method is agreed
upon.
3. Capacitance: A coil's physical geometry and the
dielectric between windings (similar to leakage inductance)
determine capacity. A maximum value will satisfy most
circuit requirements. This is assigned after a pilot
run on automated winding equipment. If a ± tolerance
is required it can be assigned after the pilot run and
the measurement method is agreed upon.
4. Resistance: Resistance is a function of wire
diameter and length. Tolerances for resistance are determined
by the turn count and wire size. When the resistance
of any winding exceeds 10 ohms, ± 10% tolerance can
be held at no additional cost. Tighter tolerances require
specialty wire and increased costs. If the value of
resistance is 10 ohms or less, the tolerance should
be held to a maximum value.
5. Open-Circuit Voltage or Turns Ratio: Modern winding
machines can achieve ± 1 turn resolution. A ±3% tolerance
is standard for this parameter due to measurement errors
caused by meter and source impedance differences. Tighter
tolerances can be achieved with an agreed upon measurement
method.
6. Full-Load Voltage: After a design is completed,
variations in output voltages are caused by turns ratio,
winding resistance and leakage inductance. A 5% tolerance
for this parameter is easily attainable. Tighter tolerance
can be obtained with an agreed upon measurement method.
7. Mechanical Dimensions: Envelope dimensions are
intended to insure fit. Specifying minimum or maximum
dimensions whenever possible will accomplish this. The
use of reference dimensions that imply no tolerance
control, conveys information with no added cost. These
practices will eliminate unnecessary fixturing or custom
tolerances from raw material vendors. Row to row dimensions
for PC mountable pins can be held ± .02" (most cases
dependent upon part size) without special handling and
packaging. Insulated flying lead lengths can be held
± 1/8" (lead length is 6 " or less).
MAGNETICS:
Size vs. Economics
Transformers and inductors are large components in this
age of miniaturized PC boards. Inevitably there is pressure
to reduce size. Minimum size requirements cost more.
Avoid size requirements that force the design beyond
common manufacturing methods. Unique methods are usually
time consuming and expensive. Achievement of size reduction
can be achieved with a good design and carefully specified
requirements.
Tips
for Minimizing Size and Cost:
- Determine
your requirements with care.
- Ask
only for what you need. DON'T OVER SPECIFY.
- Transformers
and inductors are extremely reliable components when
operated at full load conditions. AVOID UNNECESSARY
SAFETY MARGINS.
-
Utilize high-temperature insulation systems. TAKE
ADVANTAGE OF ALLOWABLE TEMPERATURE RISE.
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