Line Voltage Sensing

Sorry, what i meant was i'm suppose to get a scaled down replica of the mains voltage waveform.

Can you post a link to a schematic?

--Mac

"Lord Inexorx"

** That is a stupid and dangerous arrangement.

If the neutral connection ever comes loose, your IC will be subjected to 230 volts AC and likely destroyed.

The better and foolproof way is to use a small step down transformer - toroidal for best waveform accuracy.

Or at least, a differencing stage PROPERLY designed to accept 230 volts AC input with and the desired voltage reduction ratio.

....... Phil

Sounds like common mode voltage, you need 2 voltage dividers, one for each input of the op amp. A much better way is to use an ac input opto coupler.

Make sure the schematic shows how the power supply to your amplifier is connected, grounded, etc. with respect to the power line it is monitoring.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
------------------------------------------------------------------
Trust the computer industry to shorten the term "Year 2000" to Y2K.
It was this kind of thinking that got us in trouble in the first place.
        -- Adrian Tyvand

On Sat, 11 Mar 2006 13:27:00 +0800, Lord Inexorx wrote:

Your diff amp needs common mode rejection for the full amplitude of the source, and whatever it might be floating on.

To illustrate, the following SWCadIII schematc and waveforms are on alt.binaries.schematics.electronic.

For galvanic isolation, as mentioned elsewhere, a transformer or optically isolated method should be considered.

RL

Version 4 SHEET 1 880 680 WIRE -256 176 -256 160 WIRE -256 384 -256 256 WIRE -176 384 -256 384 WIRE -160 160 -256 160 WIRE -160 256 -256 256 WIRE -32 160 -80 160 WIRE -32 160 -32 144 WIRE -32 224 -32 160 WIRE -32 256 -80 256 WIRE -32 320 -32 256 WIRE -16 144 -32 144 WIRE 0 320 -32 320 WIRE 64 224 -32 224 WIRE 64 256 -32 256 WIRE 96 208 96 96 WIRE 96 448 96 272 WIRE 160 320 80 320 WIRE 160 384 -96 384 WIRE 160 384 160 320 WIRE 176 144 64 144 WIRE 176 240 128 240 WIRE 176 240 176 144 WIRE 192 320 160 320 WIRE 192 352 192 320 WIRE 272 240 176 240 WIRE 304 240 272 240 WIRE 304 320 192 320 WIRE 400 96 96 96 WIRE 400 128 400 96 WIRE 400 320 304 320 WIRE 400 320 400 208 WIRE 400 448 96 448 WIRE 400 448 400 400 FLAG 192 352 0 FLAG -32 144 N1 FLAG -32 256 N2 FLAG 272 240 n3 SYMBOL Opamps\\\\2pole 96 240 R0 SYMATTR InstName U1 SYMBOL res -64 144 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R1 SYMATTR Value 1E7 SYMBOL res -64 240 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 1E7 SYMBOL res 80 128 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 1E5 SYMBOL res 96 304 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 1E5 SYMBOL res 320 336 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R5 SYMATTR Value 1E3 SYMBOL voltage 400 112 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL voltage 400 304 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value 12 SYMBOL voltage -256 160 R0 WINDOW 3 -84 -60 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value SINE(0 140 60 0 0 0 20) SYMBOL voltage -80 384 R90 WINDOW 0 -32 56 VBottom 0 WINDOW 3 32 56 VTop 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V4 SYMATTR Value SINE(0 65 85 .1 0 0 30) TEXT -342 506 Left 0 !.tran 0 .3 .05

Two dividers, yes, but with their low end tied to the measurement- circuit ground. Inexorx says he's seeking a replica of the AC line voltage, and an opto-coupler wouldn't be accurate. Nor do I think a transformer is necessary, provided certain precautions are taken. The AC power line can have short spikes to many thousands of volts, and it's important that the resistive divider circuit handle these without endangering the equipment or any users. Because the subject is complex, I'll not take the time to here to explore it, except to say that even a simple version should have many components.

So a transformer may be the easiest solution. If Inexorx needs an accurate output, he can lightly load the transformer with an RC to control high-frequency error and still enjoy a wideband low-voltage replica of the line voltage. For example, let's consider a Signal Transformer 241-3-24. This is a small 2.4VA split-bobbin type with excellent input-output isolation, but with the usual losses that we see in small, cheap transformers. We can analyze a simple model* to estimate its errors.

primary L_m, 4.6 H (low-level L-meter) primary L_m, 11.2 H (meas with 120Vac) primary Rdc, 372 ohms secondary L_ell, 25.1 mH secondary Rdc, 23.8 ohms open-circuit turns ratio, 0.254

First we'll consider the transformer's primary circuit, where its roughly 11H of (nonlinear) magnetizing inductance has a reactance of j4.27k ohms at 60Hz, which with 372 ohms of series resistance and 120V causes a 28mA magnetizing current at a calculated lag of -90+5 = -85 degrees to the 120 60Hz ac voltage. The transformer primary's R-L calculated vector loss is 0.4% at 60Hz, dropping to 0.09% at 120Hz, and less at higher harmonics. The 5-degree phase lag at 60Hz drops to 2.5 degrees at 120Hz and less above that.

Second, we'll consider the scene at the transformer's secondary. We can add a 100pF output capacitor, which with the 25mH leakage inductance will limit the high-frequency response to 100kHz, plus a 15k load resistor to damp the 100kHz L-C resonance. Multiply the primary's series copper resistance by 0.254^2 and add this to the secondary's resistance to get the transformer's Zout = 47.8 ohms. This means our 15k load will drop the output voltage by about 0.3%, which is fine given we don't know the turns ratio any better than than anyway.

Lord Inexorx will end up with an accurate voltage transformer up to 100kHz, with a small 0.4% drop-off at 60Hz (or an above-200Hz increase of 0.4%, if he prefers to think of it that way). This small error could be corrected with a few more parts.

• Data from leakage-inductance threads on s.e.d., January, 1998. Try these threads, Message-ID: Message-ID:

--
 Thanks,
    - Win

In article , Winfield Hill wrote: [....]

Another wat to go is to use a large value resistor to feed current into a transformer working as a current-xformer. This lets you use a mechanically smaller transformer at the cost of having to burn off a little power in the resistor.

--
--
kensmith@rahul.net   forging knowledge

There is an (audio) application showing this on the Lundhal transfomer site

martin

"Winfield Hill"

** All is sweetness and light until you factor in the effect of the "magnetising current " devil.

The I mag waveform is non-sine and hence NOT linearly proportional to input voltage - its reaches peak value close to each zero crossing causing significant distortion of the AC supply waveform appearing at the secondary.

With a low VA E-core, as mentioned by Win, this distortion is easily visible on a scope & amounts to circa 10% 3H.

The Imag current peak also causes a 15 to 20 degree lag in the zero crossings seen at the secondary - buggers up any zero crossing detector.

Also, the *actual* voltage ratio of the small E-core will vary with AC supply voltage - by at around 2 or 3% over the whole range.

All these nasties are easily avoided by using a toroidal type where Imag is Lilliputian.

......... Phil

Hello All,

Thank you for all the smart replies but i think i have left out something. I'm trying to do this for a 3-phase supply source. So the transformer solution though the best but would severely stretch the budget and the pcb real estate. Thanks all for sharing ur ideas & knowledge. Appreciate it.

Please forgive me for my ignorance but where do i get swcadii or a viewer for the aforementioned code. I've tried the newsgroup u have mentioned but the post is a replica of ur previous post here.

[snip]

Allan

In article , Lord Inexorx wrote: [snip]

The transformer solution is still the best bet though. Have a look at using 1:1 (600:600 ohm) telecommunications transformers, (as used in modems, etc) connected across the lower-R in your attenuator. Cheap, small, 200Hz to 4KHz, and low THD.

I have used these for low-level 400Hz. The only problem is that they do have some sensitivity to external magnetic fields....... lay them out at right-angles to each other, and fit the whole pcb assembly into a steel box.

A quick, (easy to implement, but more expensive), solution is 3-off LEM LV 25-P isolated voltage transducers.

--
Tony Williams.

"Tony Williams"

** Mad idea.

Forget it.

Ordinary AC supply transformers are far cheaper and ACTUALLY intended to operate at AC line voltage and frequency.

....... Phil