| Regulation & Temperature Rise |
DUTY CYCLE |
| A smaller transformer can be used when the load is
intermittent and the duty cycle is much shorter than the
thermal time constant of the transformer (which is
several hours). The power rating of the transformer is
calculated as: |
 |
VOLTAGE REGULATION |
| The output voltage from the toroidal transformer
increases slightly at partial load. By using larger
diameter wire or a larger core, the voltage drop can be
reduced and the regulation improved. |
 |
| TEMPERATURE RISE |
| MIRACLE transformers are designed for a temperature rise of 60
degrees Celsius above ambient temperature at full load. At lower
output power, the temperature rise decreases significantly. |
 |
| If required, we can provide the toroidal transformer with a
thermal protector, which automatically turns off the transformer at
a set temperature. |
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Insulation & Shielding |
INSULATION SYSTEMS
MIRACLE Toroidal Transformers are constructed with
reinforced insulation and withstand 4,000 V RMS for 1
minute. Minimum creepage distance is 8 mm.
STATIC SHIELDING
When the toroidal transformer is used in an extremely
noisy environment, a static shield might be needed to
reduce the capacitive coupling between the primary and
secondary. The noise suppression decreases with larger
core sizes.
|
 |
The static shield consists of copperfoil laminated between
polyester tape.
Since the shield adds layers to the winding window in the
transformer, a larger core size might be required.
LOW MAGNETIC STRAY FIELD
MIRACLE toroidals have a very low magnetic strayfield, with the
primary and secondary windings uniformly wound around the entire
core. For extremely sensitive applications the radiated field can be
reduced further by winding a magnetic shield around the
circumferenceof the transformer.
|
 |
A - Without magnetic shield
B - With magnetic shield |
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Unregulated Linear Power Supplies |
Different rectifier circuits can be used when building
unregulated linear power supplies.
The most common circuits are shown below. |
1. Dual Complementary Rectifier |
A dual complementary rectifier is the best choice, when a positive
and negative DC output of the same voltage is required. The
secondary windings are bifilar wound for precisely matched
resistance, coupling and capacitance.
 |
2. Full Wave Bridge |
The full wave bridge rectifier is the most cost effective, since the
entire transformer secondary is used on each half cycle and no
center tap is required.
 |
3. Full Wave Center Tapped Circuits |
A full wave center tapped rectifier is commonly used in high
current, low voltage applications, since there is only one voltage
drop in the circuit. However, since only one secondary winding is
used at a time, the power rating of the transformer has to be about
30% greater than for a full wave bridge transformer.
 |
4. Full Wave Center Tap With Choke Input |
Choke input filters are commonly used in high current applications,
since they reduce ripple and allow better utilization of the power
capacity of the transformer.
 |
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Regulated Linear Power Supplies |
Regulated linear power supplies are used to provide a
constant output voltage for different loads and varying
input voltage.
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HOW TO SPECIFY THE TRANSFORMER |
A simplified formula to determine the AC voltage and current of the
transformer is as follows:
Vdc = Output DC voltage
Vreg = Voltage drop in the regulator = 3 Volt
Vrec = Voltage drop in the diodes = 0.7
VoltVrip = Ripple voltage = 10% of Vdc
Vnom = Nominal input voltage = 117 Volt
Vlow = Low line input voltage = 98 Volt
0.9 = Rectifier efficiency
The transformer AC voltage and current, when used in the
various rectifier circuits, is calculated as shown
below:
| Rectifier circuits |
RMS voltages (V) |
RMS curent (A) |
| Dual complimentary |
VAC = 1.03VDC + 3.47 |
IAC = 1.8 X IDC |
| Full wave bridge |
VAC = 1.03VDC + 4.13 |
IAC = 1.8 X IDC |
| Full wave center taped |
VAC = 1.03VDC + 3.47 |
IAC = 1.3 X IDC |
|
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