Enter V, C, and R to find discharge time, or V, C, and T to find the required resistance.
Bleeder Resistor Circuit
A bleeder resistor is connected in parallel with the capacitor in a power supply. After the supply is turned off, the resistor safely discharges the stored energy so the capacitor does not remain at a dangerous voltage.
The capacitor voltage decays exponentially. The bleeder resistor determines how quickly it reaches a safe level after power-off.
How a Bleeder Resistor Works
When a power supply is switched off, its smoothing capacitor can hold a dangerous charge for minutes or longer. A bleeder resistor sits permanently across the capacitor and slowly drains that charge once the supply is removed, so the terminals reach a safe voltage before anyone opens the equipment.
The capacitor and resistor form an RC circuit, so the voltage falls exponentially: V(t) = V₀ × e−t/RC. The product R × C is the time constant (τ), the time to fall to about 37% of the starting voltage. After five time constants (5τ) the capacitor is more than 99% discharged and considered safe. Rearranging gives the design formula this tool uses: R = −T / (C × ln(Vsafe / V₀)). The underlying voltage, current and power relationships come from Ohm’s Law.
Each quantity has its own unit: voltage in the volt (V), capacitance in the farad (F) (usually microfarads, µF), resistance in the ohm (Ω), time in seconds (s) and dissipation in the watt (W).
How to Use This Calculator
- Enter the capacitor voltage (V₀), the supply voltage the capacitor charges to.
- Enter the capacitance and pick the unit (µF, mF, F or nF).
- Enter either the resistance or the target discharge time. Give a resistance to find how long it takes, or a target time to find the resistance you need.
- Set the safe voltage threshold if needed (default 1 V), the level you consider safe to touch.
- Read the results, resistance, discharge time, time constant, initial current, peak power and stored energy, each with step-by-step working.
Worked Examples
Meeting a 1-second safety discharge. A 400 V bus holds a 470 µF capacitor that must fall to 50 V within 1 second. R = −T / (C × ln(Vsafe / V₀)) = −1 / (470 µF × ln(50 / 400)) = 1.02 kΩ. The time constant τ = RC = 1020 × 470 µF = 0.48 s, so 1 second is about two time constants.
Power trade-off. That 1 kΩ bleeder dissipates a peak P = V₀² / R = 400² / 1020 = 157 W at the instant of switch-off, falling quickly as the capacitor discharges. A larger 100 kΩ bleeder wastes only 1.6 W continuously while the supply is on, but takes minutes to discharge, the classic speed-versus-waste trade-off.
Stored energy. The same capacitor stores E = ½CV₀² = ½ × 470 µF × 400² = 37.6 J, enough to give a serious shock or arc, which is exactly why a bleeder resistor is fitted.
Why Bleeder Resistors Matter for Safety
A charged capacitor is a stored-energy hazard. Standards such as IEC 62368-1 and IEC 60950 require accessible capacitors to discharge to a safe voltage within a set time after disconnection, typically to 60 V or less within 1 to 2 seconds for mains-connected equipment. Sizing the bleeder with this calculator helps a design meet that limit. For background, see the Wikipedia overview of the bleeder resistor and the RC time constant.
Bleeder Resistor Calculator FAQ
Do I always need a bleeder resistor?
Any circuit where a capacitor charges above approximately 50 V and a person might access the internals should have one. Low-voltage circuits typically self-discharge fast enough through the load, but a bleeder is still good practice if the equipment can be opened for service.
Can I use a larger resistor to reduce power waste?
Yes, but discharge takes longer. Double the resistance and you double the discharge time. Check whether the longer time still meets your safety standard’s requirement.
What if I have multiple capacitors?
If the capacitors are in parallel, sum their capacitances and use the total. If in series, each capacitor needs its own bleeder resistor, or a single resistor across the entire string sized for the full stack voltage.
How does temperature affect discharge time?
Resistance drifts with temperature, which slightly increases discharge time. For safety-critical designs, calculate using the worst-case highest resistance and capacitance values at maximum operating temperature.
Can I use an active discharge circuit instead?
Yes. A MOSFET-switched discharge resistor activates only at power-off, eliminating continuous power waste. The resistor can be a much lower value since it only conducts for a few seconds.
What units does the calculator support?
Microfarads, millifarads, farads and nanofarads for capacitance; ohms, kilohms and megohms for resistance. Voltage and time are entered in volts and seconds. Results display in the most readable unit automatically.
Current Sense Resistor Calculator
A current sense resistor is a low-value resistor placed in series with a load on a PCB. Current flowing through it creates a small voltage drop — typically millivolts — that a sense amplifier IC (INA219, INA226, MAX9938, etc.) amplifies to a level a microcontroller ADC can read. The calculator above takes any two of the four core values and returns the other two instantly, with an optional amplifier gain field to show the output voltage your ADC will see.
Core Formulas
R = Vsense / I — sense resistance from voltage and current
P = I² × R — power dissipation in the sense resistor
Vout = Vsense × Gain — amplified output voltage to ADC
Units default to PCB-level ranges: milliohms (mΩ) for resistance, millivolts (mV) for sense voltage, milliwatts (mW) for power. The optional amplifier gain field shows the output voltage your ADC actually sees, saving a manual multiplication step.
Sizing the Sense Resistor
Sense Voltage vs. Power Dissipation Trade-Off
A larger sense resistance produces a bigger voltage drop, which improves signal-to-noise ratio and makes the amplifier’s job easier. But it also increases power dissipation and steals voltage from the load. The goal is the smallest resistance that still gives a clean, readable sense voltage after amplification.
Most current sense amplifiers have a full-scale input range between 50 mV and 320 mV. Pick a sense resistance that places your maximum load current near the top of that range. For a 10 A load and an amplifier with a 100 mV full-scale input: R = 0.1 / 10 = 10 mΩ. Power dissipation at full load: P = 10² × 0.01 = 1 W — significant enough to require a properly rated resistor and adequate PCB copper for heat sinking. For detailed power analysis, use our Power Dissipation Calculator.
Worked Example — 5 A Load with INA219
The INA219 has a ±80 mV default full-scale range and a programmable gain amplifier. Target Vsense = 80 mV at 5 A:
P = 5² × 0.016 = 0.4 W (400 mW)
Vout = 80 mV × Gain (internal to INA219, output is digital via I2C)
A standard 15 mΩ or 20 mΩ sense resistor rated for at least 1 W covers this application with margin. The voltage burden on the load is only 80 mV — negligible in most supply rail designs.
High-Side vs. Low-Side Current Sensing
High-Side Sensing
The sense resistor sits between the power supply and the load, on the high-side rail. The amplifier reads the differential voltage across the resistor while its inputs float near the supply voltage. High-side sensing preserves the load’s ground reference, which matters for circuits that share a common ground plane. It also detects short-to-ground faults. The trade-off: the amplifier must handle a high common-mode voltage, so you need a dedicated high-side current sense amplifier rather than a generic op-amp.
Low-Side Sensing
The sense resistor sits between the load and ground. The amplifier’s common-mode voltage is near 0 V, so a basic op-amp or comparator can read it. Low-side sensing is simpler and cheaper, but it lifts the load’s ground by the sense voltage, which can cause issues in ground-sensitive analogue circuits. It also cannot detect short-to-ground faults since the fault current bypasses the sense resistor entirely.
Choosing a Sense Resistor — Key Specs
Tolerance and TCR
Current measurement accuracy depends directly on the resistor’s tolerance and temperature coefficient of resistance (TCR). A 1% tolerance resistor introduces up to 1% current measurement error before anything else in the signal chain. For precision applications, use 0.5% or 0.1% tolerance. TCR (measured in ppm/°C) describes how much the resistance drifts with temperature — important because the sense resistor heats up under load. Metal-element and metal-foil sense resistors offer TCR as low as 15 ppm/°C.
Package and Power Rating
Standard SMD packages (2512, 2010, 1206) handle 0.5 W to 2 W. For higher currents, use metal-strip or four-terminal (Kelvin) sense resistors rated for 3–5 W. Four-terminal packages separate the current path from the voltage sense pads, eliminating contact resistance error at the solder joints — critical when the sense resistance itself is only a few milliohms.
Kelvin Connection
At very low resistance values (below ~10 mΩ), the resistance of the PCB trace and solder joint becomes comparable to the sense resistor itself. A Kelvin (four-wire) connection uses separate pads for current flow and voltage measurement, so the amplifier reads only the voltage across the resistive element — not the trace resistance. Most dedicated current sense resistor packages above 1 W include Kelvin pads.
Common Sense Amplifier ICs
The sense amplifier bridges the gap between the millivolt-level sense voltage and the microcontroller’s ADC input range (typically 0–3.3 V). Popular choices:
MAX9938 — analogue output, fixed gain (25 V/V or 50 V/V). Vout = Vsense × Gain, read by an external ADC.
INA180 / INA181 — analogue output, gain options from 20 to 200 V/V. Good for high-side sensing up to 26 V common-mode.
When the amplifier gain is known, enter it in the calculator to see the output voltage directly. This tells you whether the signal fits within your ADC’s input range at full-scale current — and whether you are wasting resolution at low currents.
PCB Layout Considerations
Route the sense voltage traces (from the resistor pads to the amplifier inputs) as a differential pair, keeping them close together and away from noisy switching nodes. Connect the sense traces directly to the resistor pads — not to the main current-carrying copper pour — to avoid picking up voltage drops in the power trace. This is the PCB equivalent of a Kelvin connection. Keep the amplifier IC as close to the sense resistor as possible to minimise trace length and noise pickup.
Frequently Asked Questions
Last updated: March 2026