Why does the unloaded voltage read high?

A small transformer’s secondary rises by about the regulation percent when nothing is connected. That higher RMS, after a capacitor charges to the peak and the diodes drop a volt or so, is the DC a meter reads with the load off.

What does ripple do to a device that expects clean DC?

Ripple is the peak-to-peak swing on the capacitor. The valley is the lowest voltage the device sees each cycle. A lock, camera, or controller that needs a minimum DC can fall out of spec at the valley even when the average still looks fine.

Why can a 24 VAC transformer read in the low 30s DC?

A meter on the secondary reads RMS. A capacitor charges to the peak: 24 × 1.4142 = 33.94 V. A silicon bridge drops about 1.4 V, so the DC with nothing connected is about 32.5 V. That is not 24 VDC.

AC to DC Conversion

What DC a low-voltage AC secondary produces through a rectifier and filter, and the RMS that produces a required DC.

AC to DC conversion calculator

Meters read RMS. A capacitor charges to the peak. A rectifier costs one or two diode drops on the way through. Inputs stay in the URL.

Starting points

Inputs

Direction

Conversion direction

AC to DC: what DC a secondary produces. DC to AC: what RMS secondary produces the required DC.

Circuit

Rectifier

Diodes in the current path: two on a bridge, one on center-tap or a single diode, none when AC is straight through.

Filter

Capacitor smoothed charges to the peak. Unfiltered is the average a meter reads on DC.

Diode type

Silicon 0.7 V per diode. Schottky 0.3 V per diode. Fixed values, not current-dependent.

Line frequency

Transformer regulation

Typical small-transformer figure. Confirm against the nameplate. Used for the unloaded secondary rise.

AC source

AC secondary

5–48 V RMS secondaries used on access control, CCTV, intercom, and door hardware.

AC value

RMS is what a typical meter reads.

Results

DC at the device

32.54V

Circuit DC

Peak AC (sine)

33.94V

DC no load, capacitor smoothed

32.54V

Average DC, unfiltered

20.20V

Diode drop total

1.40V

Ripple and reverse

DC at unloaded secondary (26.40 VAC RMS)

35.93V

Warning

Transformer regulation is a typical small-transformer figure. Confirm it against the nameplate.

Worked example

24 VAC RMS through a silicon bridge and a filter capacitor: 24 × 1.4142 = 33.94 V peak, minus 1.4 V of diode drops, is about 32.54 VDC with nothing connected. That is why a 24 VAC transformer feeding a rectifier does not read 24 VDC.

How to use

Set the direction. AC to DC uses a secondary voltage. DC to AC uses the DC the device needs.
Set the rectifier, filter, diode type, and line frequency. Those stay the same in both directions.
Read the headline as DC at the device, or as the RMS secondary that produces the required DC. Supporting numbers are labeled with the condition they belong to.
Standard secondaries (12, 16, 18, 24 VAC) list the DC each one produces under the same circuit. None of them is marked as the answer.

Assumptions

Sine-wave input. Peak and average figures are for a sine, not a square or clipped waveform.
The transformer is treated as ideal except for the regulation percent entered. That percent is a typical small-transformer figure to confirm against the nameplate.
Diode drops are fixed (0.7 V silicon, 0.3 V Schottky) and do not change with current.
Ripple uses the standard I / (f C) approximation. It is close for small ripple and less so when Vpp is large compared with the DC.
Scope is transformer secondaries, rectifiers, and supplies in the 5–48 V range used on access control, CCTV, intercom, and door hardware. Not line-voltage service work.
Results are estimates. Manufacturer data and a meter on the installed circuit govern.

FAQ

Why does the unloaded voltage read high?: A small transformer’s secondary rises by about the regulation percent when nothing is connected. That higher RMS, after a capacitor charges to the peak and the diodes drop a volt or so, is the DC a meter reads with the load off.
What does ripple do to a device that expects clean DC?: Ripple is the peak-to-peak swing on the capacitor. The valley is the lowest voltage the device sees each cycle. A lock, camera, or controller that needs a minimum DC can fall out of spec at the valley even when the average still looks fine.
Why can a 24 VAC transformer read in the low 30s DC?: A meter on the secondary reads RMS. A capacitor charges to the peak: 24 × 1.4142 = 33.94 V. A silicon bridge drops about 1.4 V, so the DC with nothing connected is about 32.5 V. That is not 24 VDC.

Disclaimer

Field estimate only. Manufacturer data, the authority having jurisdiction (AHJ), and current applicable code govern material and whether a circuit is energized. See the disclaimer.