Convert AC RMS to DC

[Mickel]

I'm trying to measure 240V AC with a data acquisition box that will
measure up to 10V. So I used a transformer to take it down to 6.3V AC
and then a bridge rectifier and cap to make it DC. The voltage isn't
what I expected because I'm get 240V AC = 8.58V DC. I was expecting
240V to equal 8.9V (6.3 x 1.41). Is there something I am doing wrong?
Should I have some sort of small load on this?

Thanks in advance,
Michael

 

[Grant]

On Sun, 08 May 2011 14:44:05 +1000, Mickel <mickel@nospam.com> wrote:

I'm trying to measure 240V AC with a data acquisition box that will
measure up to 10V. So I used a transformer to take it down to 6.3V AC
and then a bridge rectifier and cap to make it DC. The voltage isn't
what I expected because I'm get 240V AC = 8.58V DC. I was expecting
240V to equal 8.9V (6.3 x 1.41). Is there something I am doing wrong?
Should I have some sort of small load on this?

No, if you want an rms to dc converter that's what you use, not
the peak rectified transformer output.

Thanks in advance,
Michael

 

[Phil Allison]

"Mickel"

I'm trying to measure 240V AC with a data acquisition box that will
measure up to 10V. So I used a transformer to take it down to 6.3V AC and
then a bridge rectifier and cap to make it DC. The voltage isn't what I
expected because I'm get 240V AC = 8.58V DC. I was expecting 240V to equal
8.9V (6.3 x 1.41). Is there something I am doing wrong?

** There are two diode forward voltage drops involved. Plus - all
transformers have "regulation factors" - means the output voltage is only
accurate to specs if the input voltage is as specified and the specified
load is applied.

Should I have some sort of small load on this?

** Yep or the voltage will not follow mains variations as you require. Then
you can calibrate your transformer set up by measuring the incoming AC and
the resulting DC and use the ratio in all computations.

Be aware, what you are measuring is the really the PEAK value of the AC
supply and assuming that the rms value follows it by the usual formulae.


..... Phil

 

[Phil Allison]

"Grant"

No, if you want an rms to dc converter that's what you use, not
the peak rectified transformer output.

** You want the OP to kill himself do you ??

Using a small 6.3 volt tranny is an ideal way to make the job of interfacing
a PC to the AC supply safe.

PLUS: the OP has posted NOTHING to suggest he is after a highly accurate
" true rms " value so your smart alec comments are way off the mark.



..... Phil

 

[Mickel]

On 8/05/11 3:40 PM, Phil Allison wrote:

"Mickel"

I'm trying to measure 240V AC with a data acquisition box that will
measure up to 10V. So I used a transformer to take it down to 6.3V AC and
then a bridge rectifier and cap to make it DC. The voltage isn't what I
expected because I'm get 240V AC = 8.58V DC. I was expecting 240V to equal
8.9V (6.3 x 1.41). Is there something I am doing wrong?

** There are two diode forward voltage drops involved. Plus - all
transformers have "regulation factors" - means the output voltage is only
accurate to specs if the input voltage is as specified and the specified
load is applied.

Should I have some sort of small load on this?

** Yep or the voltage will not follow mains variations as you require. Then
you can calibrate your transformer set up by measuring the incoming AC and
the resulting DC and use the ratio in all computations.

Be aware, what you are measuring is the really the PEAK value of the AC
supply and assuming that the rms value follows it by the usual formulae.

Thanks Phil, for my purposes I think this will work quite well. I only
really need to see the variations.

.... Phil