RMS means "root mean square". The mains supply we get is 230Vac RMS, and if connected to a heater, such a heater would give out the same amount of heat as if it were powered from 230Vdc.
It will be appreciated that an AC waveform because of it's nature has times when there is a maximum voltage, and times when there is no voltage. This applies whether or not the voltage is on a positive peak, or a negative peak.
If a heater is connected to an AC supply, then sometimes it is getting power, and sometimes not. Sometimes, it is getting a considerable amount of power, more than indicated by the application of 230 volts. Sometimes, it is getting no power at all. But measured over a period of time, it is getting the same amount of power as if it were getting a constant 230V from a DC supply.
From this, it follows that at the peak of a cycle, there is voltage in excess of 230 volts, and the mathematical relationship is that the PEAK voltage is 1.414 multiplied by the RMS voltage. (About 325 volts!). Where did 1.414 come from? Well, it happens to be the square root of 2!
Taking the 18V secondary on the transformer, this is 18Vac RMS, making the peak voltage 18 times 1.414 (25.452). As the rectifier will lose around 0.6 volts per diode, (and there are effectively two series diodes in a bridge whichever half cycle we are dealing with), then this "diode drop" must be subtracted, to give 25.452 - 1.2, so the "off load" voltage on the capacitor will be about 24.252 volts DC.
As transformers will have a lower voltage on load than when unloaded, a decent transformer will have a voltage slightly in excess of it's specification when no current is being drawn, hence the 24.252Vdc referred to above may be over 25Vdc.
The origin of some of the text on this page is from the RSGB with additions by the web master
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