AC volts with multimeter

[Buzzy]

I just found out that my meter does not have enough resolution to measure mVac . But it can measure a mVdc with higher resolution. Should i use a bridge rectifier and convert ac volt to dc volt and then measure the dc volt ? Or do i need to spend money on another DMM ?

[jackinnj]

Easiest way of getting really acurate AC volts is with a true-RMS meter like the Fluke 8920A or the HP 3403 (or other HP instruments). But dealing with the situation at hand. Remarkably, the Fluke 8920A is sometimes offered for around $20 on EBay.

If you are going to go the way of rectifying the a.c. use a "precision rectifier" -- basically a pair of opamps which will swing rail to rail -- then you have to filter the resulting signal with a low pass filter -- almost to the level of DC.

http://www.tech-diy.com/precision_rectifier.gif

if you're ambitious and want truely "true" RMS measurements you should consider the Linear Tech LTC1966 or LTC1968. Linear sells a development board for $50.



The LTC1966 is more acurate, the LTC1968 has greater bandwidth:

[Buzzy]

The purpose of this is to measure voltage for t/s parameters. I don't want it to be super accurate. I just need something acceptable.

So there should be no problem using a bridge rectifier made of small diodes right ? Or would these give me some other problem ?

[jackinnj]

Your going to lose the junction potential of the diodes -- for a 1N4148 its about 600mV -- and you've got to draw some current across them to get it to work. further, you have to integrate the output -- i.e. use a capacitor -- which loads the circuit differently at each unique frequency.

This circuit appeared in the Modern Circuits Ref Manual

[Nordic]

Yep you will at least have to find some germanium diodes... like is used in analog VU meters...

[gareth]

Or just buy a Fluke 7XXX series which will be probably good enough for you

Edit..I use a calibrated 73III

[Buzzy]

I might as well skip the t/s parameter testing. So troublesome.