9V AC adapter

Hi all,

I'm using a 9V AC transformer for the AC input for REAL power measurement but I'm getting -negative values, see below

-42 220.54
-41 220.50
-42 220.99
-42 220.34
-42 220.60
-41 220.87
-42 220.30
-42 220.86
-42 221.01
-45 221.02
-43 221.15
-43 221.07
-42 220.28

When I measure the voltage with a multi meter I get 246V AC and Amps=0.506A, so that should give me 124Watts, This is confined with the Radiant OWL http://www.theowl.co.za/meet.htm which reports 113W... The AC adapter, must the output be smoothed or something, I cannot seem to find a unit off the shelf, so that is why I'm using a transformer that outputs 9V AC, It is stable but I don't have any caps or resistors on the output, will that work?

Also, the 2nd value in the monitor, is that suppose to be the AC voltage that is measured by the 9V AC? I'm using only one CT, so I guess this below is correct in the sketch?

/*
 EmonTx CT123 + Voltage example
 
 An example sketch for the emontx module for
 CT only electricity monitoring.
 
 Part of the openenergymonitor.org project
 Licence: GNU GPL V3
 
 Authors: Glyn Hudson, Trystan Lea
 Builds upon JeeLabs RF12 library and Arduino
 
*/
const int CT1 = 1;
const int CT2 = 0;                                                      // Set to 1 to enable CT channel 2
const int CT3 = 0;                                                      // Set to 1 to enable CT channel 3
 

Sorry for all the questions :-)

 

 

Robert Wall's picture

Re: 9V AC adapter

The wrong sign is due to the phase of the voltage being wrong in relation to current. Either swap the polarity of the transformer/adapter connections, or reverse the CT on its cable.

See here: http://openenergymonitor.org/emon/buildingblocks/ct-and-ac-power-adaptor...

"The AC adapter, must the output be smoothed or something, I cannot seem to find a unit off the shelf, so that is why I'm using a transformer that outputs 9V AC, It is stable but I don't have any caps or resistors on the output, will that work?"

No, you must not smooth the AC. The shop http://shop.openenergymonitor.com/ sells the recommended AC adapter.

"I guess this below is correct in the sketch?"

It looks OK to me. If you read the sketch program, it outputs the real power (W) in line 62: Serial.print(emontx.power1);  and voltage down near the bottom at line 78:  Serial.print(" "); Serial.print(ct1.Vrms);

 

The accuracy of the current measurement below 1% (i.e. below 1 A, or about 250 W, with the standard 100 A CT) starts to deteriorate, so you should not expect to measure 100 W accurately.

esawyja's picture

Re: 9V AC adapter

Thank you, I will order a AC adaptor from the shop

bvwh's picture

Re: 9V AC adapter

Hi all

Can someone just tell me why an ac adaptor is needed instead of a voltage divider(resistors)? is this because of accuracy because the phase shift seems like an even bigger hassle?

Please can someone clarify this matter?

Thanks in advance!

richmc's picture

Re: 9V AC adapter

The AC adapter/transformer is to provide isolation from the mains for safety, with a potential divider if you were to lose the negative connection you would have 240V on the output (not good).

bvwh's picture

Re: 9V AC adapter

thanks

 

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