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#20576 - 11/22/08 10:20 PM Whole-house power monitoring, need op-amp help
David Skinner Offline
newbie


Registered: 01/18/03
Posts: 15
Loc: Hensley, AR
I want to hookup 2 of these Current Transformers to my SECU-16/Ocelot... They output 0.3333VAC at rated max current, so I know I need a bridge rectifier to get DC, possibly a capacitor to even it out a bit, & an op-amp to multiply the output by about 12.. But I don't know much about designing an op-amp circuit, any pointers or design help?

Thanks!


Edited by David Skinner (11/22/08 10:28 PM)

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#20579 - 11/23/08 12:09 PM Re: Whole-house power monitoring, need op-amp help [Re: David Skinner]
JeffVolp Offline
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Registered: 01/26/03
Posts: 185
Loc: St. George, Utah
You will need a bit more than a bridge rectifier and an amplifier to get accurate results. Diode forward voltage drop runs between about .3 and .6 volt at low current, depending on whether it is a Schottky device or a standard signal diode. Double that for a bridge rectifier. So, feeding the output of the current transformer into a bridge rectifier would not work. Even putting the gain stage before the rectifier would leave you with a significant error.

You probably want to use a precision rectifier to eliminate the error caused by the diode forward voltage drop. Gain can be set so that max current gives you 5V at the precision rectifier output.

Jeff
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#20590 - 11/26/08 10:17 PM Re: Whole-house power monitoring, need op-amp help [Re: JeffVolp]
David Skinner Offline
newbie


Registered: 01/18/03
Posts: 15
Loc: Hensley, AR
 Originally Posted By: JeffVolp
You will need a bit more than a bridge rectifier and an amplifier to get accurate results. Diode forward voltage drop runs between about .3 and .6 volt at low current, depending on whether it is a Schottky device or a standard signal diode. Double that for a bridge rectifier. So, feeding the output of the current transformer into a bridge rectifier would not work. Even putting the gain stage before the rectifier would leave you with a significant error.

Great point I hadn't considered!

 Originally Posted By: JeffVolp
You probably want to use a precision rectifier to eliminate the error caused by the diode forward voltage drop. Gain can be set so that max current gives you 5V at the precision rectifier output.


Done a lil reading on these, most helpful being: Precision Rectifiers.
But this still leaves the signal kinda ripply, how do you get a stable signal for the non-continuous sampling ADC in the SECU-16? Or is that even an issue?

Thanks!

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#20592 - 11/27/08 05:47 AM Re: Whole-house power monitoring, need op-amp help [Re: David Skinner]
Guy Lavoie Administrator Offline
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Posts: 6548
Loc: Montreal, QC, Canada
To smooth out the signal, you would want to add a filter capacitor across the output of the rectifier. Something like 1uF should be ok.
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#20593 - 11/27/08 06:24 AM Re: Whole-house power monitoring, need op-amp help [Re: Guy Lavoie]
JeffVolp Offline
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Registered: 01/26/03
Posts: 185
Loc: St. George, Utah
That is a good reference on precision rectifiers.

If you use the full-wave version, be sure to add a resistor in series with the "smoothing capacitor" to isolate it from the low-impedance output of the op-amp. 10K and 1uF only gives a 10mS time constant, which won't work that well to filter the 120Hz ripple. Increasing the capacitor to a 10uF tantalum (with 10K to the rectifier output) should give you a pretty smooth signal to feed into the SECU16.

Jeff


Edited by JeffVolp (11/27/08 06:46 AM)
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#20594 - 11/27/08 07:45 PM Re: Whole-house power monitoring, need op-amp help [Re: JeffVolp]
David Skinner Offline
newbie


Registered: 01/18/03
Posts: 15
Loc: Hensley, AR
You guys are great help, thanks!

Ok, so I'm not seeing where the gain-control resistors go in these precision rectifier circuits & that doc says its recommended to amplify very low signals first. So I guess I should add a standard op-amp circuit b4 the precision rectifier?

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#20595 - 11/27/08 10:00 PM Re: Whole-house power monitoring, need op-amp help [Re: David Skinner]
JeffVolp Offline
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Registered: 01/26/03
Posts: 185
Loc: St. George, Utah
I have only used the configuration shown in Figure 4. The gain is set by the feedback resistor across the second op-amp (R5).

That current sensor should provide a low source impedance. You could add a gain stage in front of the precision rectifier to increase accuracy at very low levels, but just the circuit as it exists should do the job after adding some filtering to the output.

While I would have to study it further to be sure, I don't think you can use the circuit in Figure 6 for anything other than a gain of one. So, you would need another op-amp as a gain stage.

Jeff
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#20602 - 11/29/08 09:19 PM Re: Whole-house power monitoring, need op-amp help [Re: JeffVolp]
David Skinner Offline
newbie


Registered: 01/18/03
Posts: 15
Loc: Hensley, AR
 Originally Posted By: JeffVolp
I have only used the configuration shown in Figure 4. The gain is set by the feedback resistor across the second op-amp (R5).

Doh, didn't notice that one does have the gain resistors, thanks!

 Originally Posted By: JeffVolp
That current sensor should provide a low source impedance. You could add a gain stage in front of the precision rectifier to increase accuracy at very low levels, but just the circuit as it exists should do the job after adding some filtering to the output.

Cool! Do you invision much/any tuning being required or is this gonna be pretty straight forward?

I've since run into these Split Core Current Sensor w/ 0-5VDC Output, about $20 more than the ones above. Very temping for no electronic fiddling!


Edited by David Skinner (11/29/08 09:22 PM)

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#20604 - 11/30/08 11:14 AM Re: Whole-house power monitoring, need op-amp help [Re: David Skinner]
JeffVolp Offline
active contributor
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Registered: 01/26/03
Posts: 185
Loc: St. George, Utah
It all depends on how you value your time. Personally, I would opt for the 5VDC output unit.

Jeff
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X-10 automation since the BSR days

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#20634 - 12/06/08 11:38 PM Re: Whole-house power monitoring, need op-amp help [Re: JeffVolp]
David Skinner Offline
newbie


Registered: 01/18/03
Posts: 15
Loc: Hensley, AR
Ya, I think the 5V (or 0-10v w/ a voltage divider at SECU-16) devices are the way to go...
But, I still learned much about op-amps durring this, thank you!

David

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#20684 - 12/25/08 06:20 PM Re: Whole-house power monitoring, need op-amp help [Re: David Skinner]
rje Offline
newbie


Registered: 06/12/08
Posts: 10
Loc: Ontario, Canada
David

Depending on how precise you want your measurements I would look at using the Analog Devices True RMS to DC converter chips, AD737 is one. This should mate up well with the CT you were originally looking at and give you a true RMS value for current and then use the same chip for the voltage measurement.

As with vinyl music and 5 1/4 floppies the voltage and current sine waves are not the same as they used to be with all the switch mode power supplies in a home today.

The other thing to watch is the dynamic range for the Ocelot A-D converter, if you are at home wathcing TV with a few lights on you will only have 3-4 amps drawn but if the AC kicks in, the electric dryer is on and the stove is on that could go up to 70 amps or more.

The other problem is that the Ocelot is only an 8 bit converter, nothing against ADI, I have an Ocelot system running measuring in house temp & humidity, so with that narrow range the 8 bit converter is ok. But with other manufacturers bringing out low cost USB units that have 14 bit converters plus digital I/O and counters the ADI folks must upgrade the Ocelot system to keep customers.

Randy

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