Thursday, September 1, 2016

12V/280mA Transforerless Power Supply Circuit using LNK340 IC

Electronic Kit School. - This is one of the transformerless power supply circuit for you who want to build yourself this transformer power supply. This transformerless power supply here mean that we not using transformer step down to low the 220VAC to 12V or other. But in here we will use special component to do this work easily.

In this article we will give you one circuit of 12V/280mA Transforerless Power Supply Circuit using LNK340 IC like in Figure 1 below. As mentioned in this title that we will use LNK340 IC to build this power supply.


Figure 1. 12V/280mA Transforerless Power Supply Circuit using LNK340 IC

Component Parts

  1. Diodes
  2. Resistors
  3. Inductor
  4. LNK340 IC

Circuit like in Figure 1 above show you about the 12V/280mA Transforerless Power Supply Circuit using LNK340 IC that can produce 12V output voltage with 280mA output current without using transformer step down.

From Circuitstoday site that also publish this circuit describe that LNK304 is a low component count, efficient off-line switcher IC that can support buck, buck-boost and flyback topologies. The IC has a built in auto start circuit for short circuit and open loop fault protection. Other features of LNK304 includes low temperature variation, thermal shut down,high break down voltage, good line & load regulation, high band width , wide input voltage range (85 to 230V AC) etc. In general the LNK304 has a better performance when compared to the many other discrete buck regulators.

The circuit diagram of a practical 12V/120mA transformerless switch mode power supply is shown above. Resistor R1, capacitors C1 and C2, diodes D1 and D2 and inductor L1 forms the input stage. D1 and D2 forms the rectifier section while C1 and C2 are input filters. Resistor R1 which is a fusible resistor limits the inrush current, increases differential mode noise attenuation and also serves as an input safety fuse.

The next stage is the regulator stage which consists of IC LNK304, diodes D3 and D4, capacitors C3, C4 and C5, resistors R3, R4 and R5 and inductor L2. D3 is the freewheeling diode while L2 is the output choke. C5 is the output filter capacitor which limits the output ripple voltage to a value as low as possible. The IC LNK304 is so configured that the power supply operated in the most discontinuous mode and that’s why a fast recovery diode (UF4005) is used as the freewheeling diode (D3). UF4005 has a reverse recovery time of around 75nS and it is well enough for the given situation.

The voltage drop across diodes D3 and D4 are practically same and so the voltage across C4 tracks the output voltage and this voltage is picked by the network comprising of resistors R2, R3 and is fed to the feedback pin. R2 and R3 sets the output voltage and for 12V output the voltage at the feedback pin must be 1.65V DC. The circuit attains regulation by skipping the switching cycles. When the output voltage rise, the current at the feedback pin also rises and when the current rises above the threshold value, subsequent cycles are skipped until the current at the feedback pin goes below the threshold and thus a constant output voltage is maintained.

The IC will auto restart if no cycles are skipped during a 50mS time period and this limits the maximum output power to 6% of the maximum over load power. That’s how over load protection is attained. Resistor R4 serves as a small preload which nullifies the effects of tracking error.


  1. Assemble the circuit on a good quality PCB.
  2. LNK304 is a very high efficiency switching regulator IC that has a hand full of applications.
  3. LNK304 is commonly available in SMD package (DIP is also available) and care must be taken while soldering it.


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