Can someone explain the "AC plus DC" mode function on a DMM ?

[Milmat1]

 I have an old Fluke 87 (among others)  that I have used for many years. While researching info for a new DMM, Not a replacement but rather a complimenmt to my trusty old 87. I find that I really dont understand the functionality of the "AC+DC" mode on some newer DMM's.
What is it and when would it become usefull ?

[radiolistener]

I have an old Fluke 87 (among others)  that I have used for many years. While researching info for a new DMM, Not a replacement but rather a complimenmt to my trusty old 87. I find that I really dont understand the functionality of the "AC+DC" mode on some newer DMM's.
What is it

AC and DC are components of a complex signal.
AC Voltage means RMS Voltage for AC component (effective Voltage of alternating current).
DC Voltage means Voltage for DC component (constant Voltage offset).

Both components produce power dissipation on the applied load.
So, you're needs to sum them together in order to get effective Voltage for a total dissipated power.

AC+DC Voltage = sqrt([AC RMS Voltage]^2 + [DC Voltage]^2)

and when would it become usefull ?

it can be useful when you want to calculate real power dissipation on the load.

When power source has DC component (DC bias) it will leads to additional power dissipation on the load. DMM which measure just AC RMS Voltage don't take DC component into account. DMM which measure AC+DC Voltage take into account both components.

[mc172]

Attached is a picture of a 1Vp-p sine wave with a 1V DC offset. This is "1V AC(p-p) plus 1V DC". It should hopefully make sense when you look at the Y scale.

[Shock]

It's the square root of the squared DC plus squared AC RMS measurement.

Most useful on dual display meter where you can see the DC+AC and AC measurement at the same time, which would give you a proportional indicator of a problem with your AC signal. I think it's more useful for most people to use the individual DC and AC measurements on a dual display, then they can clearly see there is a DC offset on their AC signal or vice versa AC ripple on their DC signal.

[David Hess]

I have never had a use for AC+DC mode but to give an example, if you rectify AC to produce pulsing DC, and filter it with a capacitor, then you end up with a sort of sawtooth waveform which depends on the load current.  An average AC or RMS AC measurement will measure the ripple, and a DC measurement will measure the average DC level, but to measure the true RMS value, an RMS AC+DC measurement is required.

If a meter lacks AC+DC measurement, then it can be calculated from separate AC and DC measurements as described by radiolistener above.

[Milmat1]

Folks,
Thanks for all the great explanations.
 I pretty well understand the compkex waveform wheras you may have a sine wave with a DC offset. (Imagine measuring PSU ripple by switching to AC-COUPLED).

Where the DMM is set to AC we will only see the RMS value of the AC voltage. In this mode any DC compnent is blocked, Right.
So the AC+DC mode allows us to see the total "Effective" voltage ?
Correct ?

[bdunham7]

So the AC+DC mode allows us to see the total "Effective" voltage ?
Correct ?

Yes.  If you had a toaster that was fed different combinations of AC and DC, the AC+DC TRMS reading would be the accurate indicator of how hot your toast was getting.

[Milmat1]

THANK YOU !!