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To the best of my knowledge, Vrms is used to express an AC signal as the equivalent DC signal that would produce the same power dissipated in a resistor R; I believe this is best calculated using a thermistor to measure the heat given off from the resistor.

Now, if an analog input device were to specify that it can accept a 200 Vrms signal, wouldn't it follow that it can happily accept 200 VDC as well since Vpk-pk of the AC signal must be at least as large as 200V to achieve the 200 Vrms rating?

If the above statement is correct, then it follows that the opposite is not true; that is to say a device with an input spec of 200 VDC may not necessarily be happy with a 200 Vrms signal as that AC signal might have peak voltages over 200V which could damage the input circuitry.

Somebody please edify me

Edit

As requested, here is the device in question. It is a National Instruments 9255 C-Series module. Apparently, it actually supports up to 300Vrms but the same question applies. Also, here is the datasheet.

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  • \$\begingroup\$ Your logic is correct. Just in case, what is the device in question (manufacturer and model)? \$\endgroup\$
    – Nick Alexeev
    Commented Mar 9, 2013 at 1:21
  • \$\begingroup\$ If the Vrms limit is specified independent of frequency, perhaps. However, if it is specified at a particular frequency, there is a possibility that the impedance could be enough lower at DC to result in an overload. \$\endgroup\$
    – Chris Stratton
    Commented Mar 9, 2013 at 1:26

2 Answers 2

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The first part of your logic is often correct. However there are some instances where it is false. For example, supposing the input is a transformer, then it may be quite happy with 200 V AC input, but would burn out if you apply 200 V DC because its impedance at DC (ie: its resistance) is very low. This certainly applies to power transformers, for example those in power supplies (PSUs of the non-switching variety). While there are probably not many devices whose signal input stages are transformers, there certainly are some.

The second part of your logic (" the opposite is NOT true") is sometimes false, sometimes true (ie: your conclusion doesn't follow). Some devices happy with a 200VDC input may be happy with 200VAC input, some not.

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  • \$\begingroup\$ As a case in point of AC voltage measuring device via transformer: openenergymonitor.org/emon/buildingblocks/… To be sure, this is a DIY version, but I chose it because the schematic clearly shows that a DC input would be fatal. \$\endgroup\$
    – gwideman
    Commented Mar 9, 2013 at 1:36
  • \$\begingroup\$ Thank you for your answer. Looking at the datasheet that I posted above, on page 18 it specifies a DC coupled input with a 1Mohm input impedance. Given those two characteristics I think it's safe to say this device will tolerate a DC input. There is, however, some info on filters that it automagically seems to calculate the pass and stop bands for and I'm not exactly sure how that will play out with a DC signal. \$\endgroup\$
    – SiegeX
    Commented Mar 9, 2013 at 1:50
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Taking your last point to one extreme:

A waveform that generates high peak voltages, for example a Marx bank, but a much smaller Vrms value would not be safe for a dc circuit with Vrms = VDC. As in most cases I think it will depend on the circuit and the waveform that you want to feed to it.

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