As one of the university projects, I designed and constructed a transformer and AC-DC power supply which I have presented a summary of my work here:
The first step in transformer construction was to prepare the bobbin. The primary coil was using #28 AWG wire, with an expected 470 turns (67 per layer, for 7 layers) and a single layer of insulating tape between each.
Attributes & Constraints of the product
- Input: 3 prong wall connection, 120V, 60 Hz AC, range: 100 V≤ Voltage ≤ 126 V
- Output: Regulated 15V DC (± 3%), 30W (Banana connectors)
- Safety: Transformer (isolation), Fuses (at In/Out), Surge protection
- Line regulation: 3%
- Load regulation: 3%
- User friendly: ON/OFF switch, LED indicator, 3 prong wall plug
- Reliable: Follows the Regulation
- Efficient: Minimize components
- Economical: low cost of components
Functional Block Diagram
- Banana plug terminals (positive/negative color coded)
- Input fuse, Fuse holder
- Output breaker
- ON/OFF switch (postponed to final product)
- Yellow LED with 1.5kΩ resistor
- Step down/Isolation (Transformer)
- MATLAB script
The coil was then covered with three layers of insulating tape to separate it from the secondary coil.
The secondary coil was wound using 20 AWG wire, which was originally calculated to have 88 turns (29 per layer, for 3 layers).
Bill of Materials
- Bridge Rectifier – Vishay/General Semiconductor, KBL01-E4, QTY: 1, price: $1.11
- Filter Capacitor – Cornell Dubilier Electronics (CDE), 338-1626-ND, 4700μF, 50V Alum, QTY: 1, price: $2.75
- Volt. Regulator – LM2576T-15-ND , LM2576-15, QTY: 1, price: $3.86
- Inductor – Pulse , 553-1574-5-ND, 220μH, QTY: 1, price: $4.12
- Capacitor – Panasonic – ECG, P5156-ND, 25V, 1000μF, QTY: 1, price: $0.76
- Diode – ON Semiconductor, MUR410RLGOSCT-ND, 100V, 4A, Ultra-fast, QTY: 1, price: $0.73
- Heat Sink – Ohmite, WA-T220-101E-ND, QTY: 1, price: $1.77
- Fuse Holder – Littelfuse Inc, F1497-ND, QTY: 1, price: $1.29
- Fuse – Littelfuse Inc, F2541-ND, QTY: 5, price: $ 1.08
- LED – Vishay/Semiconductors, 751-1116-ND, QTY: 2, price: $ 0.49
- Resistor – Panasonic – ECG, P1.5KBACT-ND, QTY: 1, price: $ 0.11
- Breaker – Tyco Electronics, PB1068-ND, QTY: 1, price: $ 2.85
- Filter Cap – Panasonic – ECG, PCE3898CT-ND, 100μF, 25V, QTY: 1, price: $ 0.73
- CAN/CSA-C22.2 no. 231 series-M89
- Fuse Holder
- C22.1-02 C22.1AB-02
- Class 1 & 2 Devices
Testing and Documentation
Prototype testing was carried out using two digital multimeters (DMM) and an AC power meter. The testing configuration is summarized in the figure below. An AC power meter was used to simultaneously measure input voltage and current from a Variac to the power supply. Two DMM are used to measure the corresponding output voltage and current. Variable loading was achieved by connecting one of many available resistor modules to the power supply output, including a rheostat (0-12 Ohm) and a resistor bank (100-900 Ohm). These modules were also combined to create a greater number of test cases. No load measurements were also recorded.
Voltage regulation may be determined using measurements at the rated, minimum and maximum conditions. Line regulation is given by
and the load regulation is given by
Vo(Vi,max) – Output voltage when the input is 126 VRMS.
Vo(Vi,min) – Output voltage when the input is 100 VRMS.
Vo(Vi,nl) – Output voltage with no load connected.
Vo,rated – Rated output corresponding to nominal input voltage of 115 VRMS.
The resulting line regulation and load regulation calculation give LR = 1.26% and LDR = 1.36% at the rated output voltage of 14.95 V.
The efficiency, η, is calculated as the output power over the input power,
Note that the input power requires the inclusion of cosφ, the power factor, in order to calculate the real power. The product of output voltage and current gives the real power, since both the current and voltage are in phase.