Capacitive dropper circuit

Circuit design

Dropper and rectification

With 240 volt mains power this reactance limits the RMS current to 165mA although in practice voltage drops across other components in series will reduce the voltage drop across the capacitor and hence the current flow, but it provides a starting-point for other calculations. The next component to consider is the smoothing capacitor C2 and in order to keep the ripples in the rectified DC output above 80 volts at all times a value of at least 6.35μF is required of the latter, although due to supply considerations a 10μF capacitor is used. The high-value resistors R2 and R3 that are in parallel with capacitors C2 and C3 respectively are there for safety and they ensure that once mains power is disconnected these capacitors do not maintain a high-voltage charge.

Inrush current limitation

However a lot of material out there shows the use of under-100Ω resistors with a power rating of under 500mW, which I suspect is down to the short-lived nature of the inrush and the power drop when the circuit is in a steady-state. Since pretty much all the thermisters I could find have a “cold” resistance of 100Ω or less I suspect the choice of resistance is either focused more on steady-state losses rather than theoretical inrush, or the value is based on empirical experience. The notionally 50Ω thermister used in this circuit starts out at 62Ω but then drops to 20Ω immediately after power disconnection.

Emitter-follower circuit

A 22kΩ resistor was eventually used as the exact resistance is not really of any concern, so the selected component was based on cost rather than target value. Inrush current is not a problem as the discharged smoothing capacitor C2 will bypass this entire section of the circult during the initial powerup surge, but the NPN transitor is rated to withstand C2 being charged up to full peak voltage. The bypass capacitor C4 was included in the design to help smooth over small transcient changes in the voltage across the zener diode but in practice was left out of the final build of the circuit.

PCB fabrication

Although the release candidate of KiCad 7 was available I felt it best to stick with KiCad 5 for this project. In more recent times I have tended to only opt for a PCB if I need to use surface-mount components but such projects are ones I prefer to avoid these days, and most of the project ideas have lined up are biased towards using discrete components rather than integrated circuit chips. I expect KiCad 7 to be very different to KiCad 5 and at the moment learning a new package is outside my bandwidth.

Practical use of the circuit

Within this circuit the voltage drop across the zener diode is also the transistor's base voltage and at least for the lower loads, the difference between the base voltage and the output voltage is around 0.55 volts which coincidentally is the same as the transistor's V_be. The difference between the smoothing capacitor voltage drop and the output voltage also corresponds with V_BE which makes me suspect that the transistor was saturated so it is acting like a switch rather than an amplifier. The voltage regulation part of the circuit is a shunt regulator which is basically potential-divider made using a resistor and a zener diode, as summerised in the schematic below.

In this arrangment the resistor needs to be sized to allow for the minimum current draw I_z that the zener will operate with plus the base current required to allow the transistor to pass I_c to the output. The transistor has a quoted minimum gain hfe of 40 so a base current I_b of 4mA should allow a collector current I_c of at least 160mA, although the data-sheet does hint of a potential gain of 200+ in some circumstances that don't seem far off the values for this circuit.

The zener diode's mimimum operating current I_z is 1mA so the resistor need to be sized so that the residual voltage drop would corresponds to 5mA, and here 16 volts is assumed as supply V_cc and from this the zener diode's 6.2 volts is subtracted.

In reality the voltage drop across this 2kΩ was 37mV which corresponds to 18.5μA which is not even close to the zener diode's specified minimum current, and this voltage drop was the same with a 22kΩ resistor. However when the zener diode was removed the 11kΩ load resistor in place got toasted as something close to 220 volts was being dropped across it, so while the measured values seemed odd at least qualitatively the circuit was behaving as designed.

List of components

Below are the footprints used by each component since messing up the footprint selection was a major problem with the previous circuit. Note that the PCB pad numbers are a bit different since the PCB included an extra resistor not shown in the circuit design schematic. Finding the right footprint can be hard at times because their classification is not always obvious — for instance the thermistor footprint was listed under Varistors, and it was still a bit small for the thermistor used.

Remarks

The project in itself was an interesting experience but like many other recent projects this one was mostly completed months ago and things got in the way of bringing it to a conclusion. This write-up could have been better since I don't fully understand the quantitative side but it is time to move onto other projects.