78xx/79xx Linear Regulator Calculator: Ripple, Dropout, Heat and Noise
From the transformer secondary to a regulated output: ripple, dropout margin, heatsink sizing and residual noise of a three-terminal regulator.
The three-terminal regulator is the black box we all have in a parts drawer: two capacitors, three pins, and a fixed voltage comes out. It works almost every time, and when it does not, the cause is usually one of three things — at the bottom of the ripple the input voltage falls below the regulator’s minimum requirement, the package gets hot enough to trigger thermal protection, or the residual output ripple is too high for the circuitry downstream. This tool examines those three causes one by one, starting from the transformer secondary voltage.
The ripple calculation does not use the sawtooth approximation: the capacitor is simulated over each mains period, including the secondary resistance and diode drop, and steady state is found by solving the periodicity condition directly. It is the same engine used by the rectifier tool, and it also returns diode peak current and capacitor RMS current — the two quantities needed to select those components.
Output and regulator
The four values below come from the datasheet of the selected part and can be edited: if your device is from another manufacturer, or you want to work with a maximum value instead of a typical one, enter the value from your datasheet. Where the table has no verified value, the field is left blank and the tool tells you.
Input: transformer, rectifier and reservoir capacitor
Enter the RMS voltage of the winding section feeding the rectifier: with a bridge this is the entire secondary, with a center tap it is half the secondary, and with a dual supply it is half the secondary for each rail. Series resistance is the resistance of that winding section plus the wiring: on a transformer of only a few VA it may be several ohms, while on a 50 VA transformer it may be only a few tenths of an ohm.
Results at nominal mains voltage, maximum load and nominal capacitor value.
Why diode peak current is so much higher than output current
The capacitor recharges only while the rectified sine wave is above the voltage already stored on the capacitor: a narrow window, here around fifty degrees out of one hundred and eighty. All the charge consumed by the load during the rest of the cycle must pass through that window, so peak current is several times the DC current. That is why the current form factor is so high, why the secondary heats more than the output power alone would suggest, and why the capacitor must be selected for RMS ripple current as well as capacitance.
Increasing capacitance reduces ripple but narrows the charging window even further, so peak current rises. It is a trade-off, not a parameter to maximize without limit.
Dropout-margin check
This is the check that determines whether the design actually works. All worst-case conditions must be applied at the same time: minimum mains voltage, capacitor at the low end of its tolerance, maximum output current, and output voltage at the top of its guaranteed range, because that gives the highest required input threshold.
Worst-case input voltage over one mains period. The horizontal line is the threshold below which the regulator drops out of regulation.
Power dissipation and heatsink
The entire input-to-output voltage difference becomes heat in the regulator, multiplied by the current flowing through it: that is the price of simplicity. Here the worst case is the opposite of the previous check — maximum mains voltage, because that is when the voltage drop across the regulator is greatest.
Regulator dissipation versus average input voltage, showing the three operating points and the ceiling imposed by the cooling system.
Equivalent plate versus a real heatsink
The displayed area comes from an explicit model: ten watts per square metre per kelvin, counting both faces of a blackened vertical plate and including convection plus radiation. It is useful for understanding whether the requirement is roughly a ten-square-centimetre tab or a half-metre extrusion, not for selecting the final part.
The value used to buy a heatsink is its catalog thermal resistance, measured at a specified temperature rise — commonly seventy-five degrees — in still air. If the heatsink is inside a closed chassis, or mounted horizontally instead of vertically with vertical fins, its real performance can easily worsen by thirty percent. Choosing the next larger size is usually sensible.
Residual output ripple
The regulator attenuates the ripple reaching its input but does not eliminate it, and the attenuation becomes poorer as frequency rises. The residual output ripple combines with the regulator’s own noise.
Ripple spectrum lines before and after the regulator. Input amplitudes come from the simulation; attenuation uses the single-pole model described below.
How ripple rejection is modeled, and how much to trust it
The datasheet provides a single guaranteed figure at 120 Hz under specific voltage and current conditions. The full frequency response may be shown as a graph but not as a table. Here the rejection is kept flat at that value up to the corner frequency you specify, then reduced by twenty decibels per decade: this is a typical model for these regulators, not a manufacturer-specified curve.
For the first ripple component — 100 Hz with 50 Hz mains — the specified rejection figure is directly relevant and the estimate is solid. At higher harmonics it is only an estimate. If you need the real value, measure it: AC-couple the probe at the output, use the actual load, and observe the result.
Capacitors, protection and layout
What not to omit during assembly
- The 0.33 µF capacitor belongs at the regulator input pin, not half a metre of wire away at the reservoir capacitor: its purpose is to prevent high-frequency oscillation, and a long connection can defeat it.
- The output-capacitor ground and regulator ground should meet at a single point, and the load return should leave from there. A ground loop between input and output carries the diode recharge pulses with it, and those pulses are large and steep.
- The metal tab of a TO-220 is connected to the centre pin: on a 78xx it is ground, while on a 79xx it is the input. Two uninsulated 79xx regulators on the same heatsink create a short circuit.
- Thermal compound should be a thin film that fills surface imperfections, not a layer: too much increases thermal resistance instead of reducing it.
Raised output voltage with a resistor divider
Placing a resistor between the common pin and ground makes the regulator maintain its fixed voltage between output and common, so the output rises by the voltage developed across that resistor. It works, but the quiescent current also flows through the same resistor and is not tightly controlled; that contribution shifts the result.