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kalee20 · 11th Oct 2018, 1:14 pm

If you measure the RMS voltage directly on the transformer, and then measure on the output of a bridge rectifier feeding a purely resistive load, the two readings should be exactly the same. In practice, the rectifier drop will make a small difference, but that's a detail.

If they're not, it could be that the meter is not a 'true RMS' meter. Or, it could be that there is an internal DC-removal thingy - maybe a capacitor to just AC-couple. But although I suspect that some meters do include that, I can't help asking why?? It reduces the usefulness - for example, I couldn't use such a meter to check heater volts in a chain using a diode dropper!

Note that switching the meter to DC and measuring the rectifier output jolly well SHOULD give a slightly different reading, because then it's reading average, not RMS.

11th Oct 2018, 1:14 pm	#27
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,077	Re: Full wave rectified RMS voltage If you measure the RMS voltage directly on the transformer, and then measure on the output of a bridge rectifier feeding a purely resistive load, the two readings should be exactly the same. In practice, the rectifier drop will make a small difference, but that's a detail. If they're not, it could be that the meter is not a 'true RMS' meter. Or, it could be that there is an internal DC-removal thingy - maybe a capacitor to just AC-couple. But although I suspect that some meters do include that, I can't help asking why?? It reduces the usefulness - for example, I couldn't use such a meter to check heater volts in a chain using a diode dropper! Note that switching the meter to DC and measuring the rectifier output jolly well SHOULD give a slightly different reading, because then it's reading average, not RMS.