Understanding my multimeter - AC reading

[ethrbunny]

Im trying to track down noise in a circuit. When I put my meter on Vdc I get the appropriate 5V DC from a 7805. But when I put it on Vac I get 8V between the 9V line and the 5V line. All other combinations read 0. Why?

[niftydog]

analogue or digital multimeter?

Do the 9V and 5V supplys connect to the same ground reference?

Is the 9V line regulated?

What does it measure between the 9V supply and ground when set on Vac?

more info needed I think. I'm thinking that you're talking about a 5V regulator that's fed with 9V?

[ethrbunny]

1) digital
2) yes
3) yes
4) 0v
5) yes

I lack an o-scope to be able to properly explore this.

[niftydog]

ok, so you're measuring between the input and output of a 5VDC regulator.

What about if you switch the probes around? Still 8V?

I must say this is strange. There's no AC at the input, I'm assuming therefore no AC at the output, yet your DMM says there's AC between input and output.

Who knows... maybe it's causing the reg to oscillate or something. An oscilloscope might help, but it might also affect the circuit and cause it to stop oscillating!