The following transformerless power supply uses an MJE13005 transistor for voltage regulation. It acts like an amplifier controlled by a base zener diode to set the output voltage. A 3uF capacitor limits the output current to around 150mA, and two diodes convert the AC input to pulsating DC.
LED Decoration Light Application
This transformerless circuit can power small LED projects like bulbs or string lights. But be very careful! Since it's not isolated from mains power, all wiring carries dangerous AC voltage. Only build this if you can properly insulate it with plastic covers and sleeves. You take full responsibility for the risks involved.
Circuit Request from Mr. Don:
Mr. Don needs a circuit to power LED light ropes. Each rope has 6 strings, with each string containing 65-68 LEDs in series. The final rope has LEDs spaced 4 inches apart. Mr. Don wants options for powering:
- One string (65-68 LEDs)
- A complete rope of 6 strings (390-408 LEDs)
He also needs advice on:
- The best circuit with surge protection
- Any additional components for circuit safety
- Clear circuit diagrams with part values (since he's not very technical)
Circuit Design
This driver circuit works for powering LED bulb strings with less than 100 LEDs (for 220V input). The LEDs should be:
- 5mm size
- Rated for 20mA current and 3.3V forward voltage
The 0.33uF capacitor at the input controls the current supplied to the LED string. In this case it provides around 17mA, which is perfect for the chosen LEDs.
If you connect more LED strings (same type 60-70 LEDs) in parallel using a single driver, you can just increase the capacitor value proportionally to maintain optimal LED brightness. Heres a breakdown:
- 2 strings: 0.68uF capacitor
- 3 strings: 1uF capacitor
- 4 strings: 1.33uF capacitor (and so on)
Adding Safety with Resistors (Optional):
Although not shown in the diagram, including a 33 Ohm, 2-watt resistor in series with each LED string is recommended for extra safety. You can place this resistor anywhere within each individual string.
- Serious Warning:
- These circuits are not isolated from mains AC and can cause fatal shocks if touched while powered on.
- Building and using these circuits requires utmost care. All parts of the circuit carry dangerous AC voltage when connected to mains power.
- These circuits are NOT isolated from mains AC and can be deadly. Only handle them when disconnected from power.