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From: richard on 16 Dec 2009 13:24
DaveC wrote:
> A full-wave rectified 32vac voltage without filtering or smoothing
> components. Just a bridge rectifier connected across a transformer 32v
> secondary (no center taps) without a load connected to the output of the
> bridge.
>
> What will my DMM read if it's in DCV mode when connected to this output?
>
> Thanks,
> Dave
>

2*sqrt(2)*32/3.14159265=28.8101221484

Integral from 0 to pi of Sin(theta)/pi = Average voltage of Sin(theta) = 2/pi
therefore 1/sqrt(2) rms -> 2/pi
therefore 1 -> 2*sqrt(2)/pi
therefore 32 -> 2*sqrt(2)*32/pi
From: John Larkin on 16 Dec 2009 22:34
On Thu, 17 Dec 2009 00:59:02 +1100, "Phil Allison" <phil_a(a)tpg.com.au>
wrote:

>
>"Jasen Betts"
> DaveC
>>
>>> A full-wave rectified 32vac voltage without filtering or smoothing
>>> components. Just a bridge rectifier connected across a transformer 32v
>>> secondary (no center taps) without a load connected to the output of the
>>> bridge.
>>>
>>> What will my DMM read if it's in DCV mode when connected to this output?
>>
>> with no capacitors and a cheap average-reading DMM about 28V
>
>
>** So more expensive DMMs read *OTHER* than average values on their DC
>volts ranges??
>
> Do tell.
>

Howzit going, Phil?

A DC-coupled true RMS meter would read different from an averaging
meter.

But in real life, diode leakage and capacitance and transformer
inter-winding capacitance would probably result in an unpredictable
reading.

John


From: Phil Allison on 17 Dec 2009 02:01

"John Larkin"
>
>>** So more expensive DMMs read *OTHER* than average values on their DC
>>volts ranges??
>>
>> Do tell.
>
> A DC-coupled true RMS meter would read different from an averaging
> meter.

** Dogs bark and cat go meow.

But a DC voltmeter is always average responding.


> But in real life, diode leakage and capacitance and transformer
> inter-winding capacitance would probably result in an unpredictable
> reading.


** I have a 4.5 digit meter with true RMS AC ranges (200kHz BW) that can
be DC coupled - a very handy feature.

But one does not use it for the job the OP described without adding a load
resistor, cos of the issues you mention.



..... Phil



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