A capacitive dropper, or transformerless power supply, has no galvanic isolation from the mains. There is no transformer anywhere in it. The whole circuit — the capacitor, the rectifier, the smoothing capacitor, the regulator, the load and every point people casually call the "5 volt rail" or the "ground" — floats at mains potential. Touching any part of it while it is connected can kill you. An earthed oscilloscope probe on what looks like its ground shorts the mains and destroys the instrument. People have died building exactly this circuit from figures they got off a web page, and that is the reason this page is written the way it is: it computes the published reactance and current relations from figures you supply, and it does not tell you that any design is safe, compliant, or ready to build.
Arb Digital publishes this as a reference calculation for people who already understand mains work and are checking somebody else's numbers. It is not a build guide. There is no parts list here, no recommended circuit, and no "known good" values: every quantity is an input, because publishing values would amount to publishing a mains circuit for people to copy. If you are looking at the physics of capacitors rather than a mains circuit, the capacitance calculator derives capacitance from geometry and dielectric constant, the capacitor charge time calculator handles DC charging through a resistor, and the motor capacitor calculator reports the electrical consequences of a nameplate motor capacitor value. None of those three is a mains supply design, and neither is this.
What This Calculator Computes, and What It Refuses To
From your supply voltage and frequency and your capacitor value it computes the capacitive reactance, and from that plus the series resistance it computes the RMS current the dropper branch can pass. It then reports the dissipation in the series resistor, the dissipation the shunt clamp device must absorb when your stated load takes less than that current, the peak voltage the capacitor is charged to, the energy stored in it, and how long a bleeder resistor of the value you enter takes to bring the capacitor below 60 volts after disconnection.
It refuses to do several things on purpose. It will not suggest a capacitor value for a target current, name a part or a voltage rating, or produce a schematic. It will not tell you whether your figures meet any standard, because that determination is made by test, on hardware, by people qualified to do it. The result area repeats the isolation warning on every calculation.
How to Use It
- Enter your supply voltage and frequency. Current scales directly with both, so figures evaluated at 230 V and 50 Hz do not transfer to a 120 V, 60 Hz supply.
- Enter the capacitor value and the series resistance. The published guidance permits only an X-rated X1 or X2 safety capacitor in the dropper position, and the series element is a fusible or flameproof resistor whose job is to survive inrush at the worst-case switch-on instant.
- Enter the clamp voltage and your load current. The difference between the available current and the load current is what the shunt clamp has to absorb continuously, including when the load is switched off.
- Enter the bleeder resistance. Then read the discharge time. Setting it to zero shows what the tool reports when no bleeder is present, which is that the capacitor stays charged indefinitely.
- Treat every output as a figure for a qualified engineer. Nothing here is an approval or an indication that a circuit is fit to energise.
The Formulas: Reactance, Current and Dissipation
A capacitor in series with an AC supply presents a reactance XC = 1/(2πfC) in ohms, with f in hertz and C in farads. Because the reactance is purely imaginary it combines with the series resistance as a magnitude, Z = √(R2 + XC2), and the RMS current the branch passes is I = V/Z. Dissipation in the series resistor is I2R. The shunt clamp device carries whatever the load does not take, so its dissipation is the clamp voltage multiplied by the difference between the available current and the load current. These are the standard AC relations set out in the OpenStax treatment of simple AC circuits.
Two further figures matter for the hazard rather than the function. The capacitor is charged to the peak of the supply, Vpk = √2 × VRMS, and it stores an energy ½CVpk2. With a bleeder resistor Rb in parallel, the stored voltage decays exponentially with a time constant τ = RbC, so the time to fall from the peak to a stated voltage Vt is t = τ ln(Vpk/Vt). This page reports the time to reach 60 volts because that is a commonly cited threshold in product-safety practice; reaching it does not make a circuit safe to handle.
Worked example, matching the defaults. At 230 V and 50 Hz with 0.47 µF, XC = 1/(2π × 50 × 0.47×10−6) = 6,772.6 Ω. With 470 Ω in series, Z = √(4702 + 6,772.62) = 6,788.8 Ω, so I = 230/6,788.8 = 33.88 mA. The series resistor dissipates 0.033882 × 470 = 0.539 W. If the load takes 15 mA at a 12 V clamp, the clamp absorbs 12 × 18.88 mA = 0.227 W. The peak voltage is 325.3 V and the stored energy is 24.9 mJ. A 470 kΩ bleeder gives τ = 0.221 s, so falling from 325.3 V to 60 V takes 0.221 × ln(5.421) = 0.373 s. Every one of those figures was worked out by hand before the code was written and the page reproduces them.
Why There Is No Isolation, and What That Actually Means
A capacitive dropper deletes the barrier. The capacitor is a series impedance, not an isolator, and current flows from the mains through the capacitor into the circuit and back out to the neutral or the other pole. Every node in the circuit is therefore connected to the supply through a low-impedance path. The voltage between two nodes in the low-voltage section may be 5 or 12 volts, but the voltage between either of them and true earth is mains voltage, and it is the earth-referenced voltage that drives current through a person. This is why the phrase "low-voltage output" is meaningless in this topology unless the words "referenced to mains" are attached to it.
Where This Topology Must Never Be Used
The published limitation on capacitive droppers is narrow, and it is worth stating without hedging. This topology has no place in anything with an exposed connector of any kind. It has no place in anything with a USB, serial, network or other data link, because that link carries mains potential straight out of the enclosure. It has no place in anything with a metal enclosure a person can touch, or a metal shaft, button, probe or fastener that reaches the outside. It has no place in anything a person handles during normal use. It has no place in anything you build for someone else, sell, give away, or install in a building where another person will encounter it.
What is left is the narrow case the topology exists for: a permanently enclosed, fully sealed, non-user-accessible load inside a product that is designed, insulated and tested as a mains-connected appliance throughout — a timer inside a sealed wall switch, a controller potted inside an appliance, a load that is part of the mains-connected assembly and never presented to a user. Even there, the entire assembly is treated as live mains equipment from end to end, with the insulation, spacing and enclosure requirements that go with that, and it is qualified by test rather than by calculation.
The Parts That the Published Guidance Treats as Mandatory
An X-rated safety capacitor, X1 or X2. Safety capacitors for mains-connected positions are covered by IEC 60384-14, Fixed capacitors for electromagnetic interference suppression and connection to the supply mains. X-class parts are constructed and tested to fail open rather than short. A general-purpose film or ceramic capacitor of the same nominal value and voltage rating is not an equivalent part: it can fail short-circuit, which puts the full mains across the downstream circuit, and it is a documented fire and shock hazard in this position. The voltage rating alone tells you nothing about this behaviour.
A bleeder resistor across the dropper capacitor. The capacitor is charged to the supply peak, and when the plug is pulled at the wrong instant it stays there. The energy figure this page reports is enough to deliver a serious and potentially lethal shock at several hundred volts, and it is why the discharge time appears in the result grid. A bleeder is a required part, and its own failure mode — going open-circuit silently, so nothing appears wrong until somebody is holding the board — is why the discharge is verified by measurement rather than assumed.
A fusible or flameproof series resistor. At the instant of switch-on the capacitor is uncharged, so it presents no impedance at all, and the inrush current is limited only by the series resistance and the supply. If that resistor is a general-purpose part it can fail as an open flame or a short. The published requirement is a resistor built to fail safely, and a mains-rated fuse in addition.
Enclosure, spacing and clearance. Because the whole board is live, the entire assembly needs the creepage and clearance distances, insulation coordination and enclosure that mains equipment requires. Those distances depend on working voltage, pollution degree and material group, and they are set by the product-safety standard that applies to the equipment class, not by anything on this page.
Where This Page Stops
More importantly, mains-connected equipment is governed by product-safety standards, and compliance with those is established by testing at a competent laboratory on real hardware, not by a reactance calculation. A qualified electronics engineer specifies mains circuits and a compliance test house verifies them. If you are working on the low-voltage electronics that a properly isolated supply would feed, the LED resistor calculator, the voltage divider calculator and the resistor power rating calculator cover that ground, and every one of them assumes a supply that is isolated from the mains. Any work on or near mains equipment also requires isolation and lock-out before anything is touched, in the manner described in the US OSHA control of hazardous energy standard, 29 CFR 1910.147.
Arb Digital builds reference calculators and documentation for electronics and engineering sites — the kind that earn links because the maths is correct, the assumptions are stated, and the hazards are not glossed over.
Browse the free tools Talk to Arb DigitalCommon Mistakes to Avoid
- Calling any part of the circuit low voltage. The potential difference across the load may be small, but every node sits at mains potential relative to earth. That is the voltage that drives current through a person.
- Connecting an earthed instrument. An oscilloscope ground clip on this circuit is a short across the mains. Isolation of the instrument or of the circuit under test is the only way anyone measures a live mains-referenced circuit, and that is specialist work.
- Substituting a general-purpose capacitor for an X-rated one. Voltage rating is not the property that matters. X-class parts are qualified to fail open; ordinary film and ceramic parts can fail short, which is a fire and shock hazard.
- Omitting the bleeder, or assuming it works. The capacitor holds a lethal charge after disconnection, and a bleeder can fail open silently. Discharge is verified by measurement every time, not assumed.
- Using the topology anywhere it can be touched. Exposed connectors, data links, touchable metalwork, handheld products and anything built for another person are all outside what this topology can be used for.
Related Free Tools From Arb Digital
For the physics of a capacitor rather than a mains circuit, the capacitance calculator derives capacitance from plate, coaxial or spherical geometry. For DC charging through a resistor, use the capacitor charge time calculator, and the capacitor energy calculator for stored energy. The motor capacitor calculator selects nothing either. On the isolated low-voltage side, the voltage divider calculator, the LED resistor calculator and the resistor power rating calculator all assume an isolated supply. Browse the free online tools hub for the rest.
Frequently Asked Questions
No. There is no transformer and no isolation barrier of any kind. Every node in the circuit, including the low-voltage rail and anything labelled ground, is connected to the supply through the dropper capacitor and sits at mains potential relative to earth. It is lethal to touch while connected.
The capacitor presents a reactance of one divided by two pi f C. That combines with the series resistance as the square root of the sum of their squares, and the RMS current is the supply voltage divided by that impedance. The current is proportional to supply voltage, frequency and capacitance.
Because X1 and X2 class capacitors, covered by IEC 60384-14, are constructed and tested to fail open-circuit in a mains-connected position. A general-purpose film or ceramic capacitor of the same value and voltage rating can fail short-circuit, which places full mains across the downstream circuit and is a fire and shock hazard.
Because the dropper capacitor is charged to the peak of the mains and retains that charge after the equipment is unplugged. Stored energy at several hundred volts is capable of a lethal shock. A bleeder discharges it, but it can fail open without any outward sign, so discharge is verified by measurement rather than assumed.
Not with an ordinary earthed oscilloscope. The probe's ground lead would connect a mains-potential node directly to protective earth, which is a short across the supply that destroys the instrument and can cause an arc flash. Measuring a mains-referenced circuit requires isolation and is specialist work.
Anywhere it can be touched or connected to. That rules out exposed connectors, USB, serial or network links, touchable metal enclosures and hardware, anything handled in normal use, and anything built, sold or installed for another person. It is restricted to permanently enclosed, sealed, non-user-accessible loads inside equipment that is treated as live mains throughout.
No, and it cannot. It evaluates published reactance and current relations from figures you enter. It models no failure mode, no transient and no standard. Mains equipment is specified by a qualified electronics engineer and verified by testing at a compliance laboratory on real hardware.
This tool is provided for education and engineering reference only. It evaluates published reactance and current relations from figures you enter, and it does not design, approve, verify or endorse any circuit. Nothing on this page indicates that any set of values is safe, compliant or ready to build, and a capacitive dropper is never isolated from the mains. Mains-connected equipment must be specified and reviewed by a qualified electronics engineer and verified against the applicable product-safety standards by a competent compliance test house before it is energised.
