Load Sensor Continued

[baggo]

After much head scratching and scribbling on bits of paper I decided that sticking with a DC sensing voltage was going to make life very difficult. Some of you had suggested using an AC voltage instead so I decided to give this a try. In theory a simple capacitor across the cfl's should provide a current flow across the shunt to activate the MAX472.

A simple square wave oscillator was built using a 555 timer chip and the output fed to the inverter output relay. The 555 runs at about 250 Hz.

By playing about with capacitor values I found that the smallest capacitor to reliably operate the sensor was 330n. Eliminating the 1k resistor in the oscillator output enabled a 220n to work but I wanted to keep the resistor to isolate the oscillator in case of shorts etc. I could probably replace it with another capacitor which may be better. Note that these capacitors must be X2 rated i.e. they must have a working voltage of at least 250 volts AC. Current consumption has obviously increased due to the addition of the 555 but is still pretty negligible- 46 mA switched and 18 mA in search mode.

At the moment the circuit seems to be working OK in the test setup so now it's out with the PCB equipment and make a proper board for it. It also needs a DC to DC converter to provide an isolated supply from the inverter battery but that should not be a problem.

[drdongle]

Why is the reset pin (4) tied to one of the AC lines ( I presume the neutral/ground)

Carpe Vigor

Dr.D

[baggo]

Pin 4 of the 555 is actually connected to +12 volts which is in turn connected to the AC neutral of the inverter output. The supply for the 555 and the MAX472 is common to both. I will post a full schematic asap.

I'm afraid the way I am doing the circuit drawings is not very satisfactory and the images do not come out clear enough. I'm using PCB123 for drawing the schematics, printing them out, and then taking a digital photo and posting a jpeg. I can export the schematic into Autocad as a .dxf file but then I've still got the same problem of getting a decent image to post. I probably need a better schematic drawing package. Any ideas here would be appreciated.

John

[drdongle]

I see that you also have a +/- dual power supply, I wonder why and am also concerned that 18 volts is usually the safe top end voltage for a 555 timer. A dual supply also provides an unnecessary complication to the system.

Dr.D

[baggo]

Sorry, that's an error on my part. The -12 volts should be ground. There is only a single 12 volt supply.

[drdongle]

It occurred to me later that you might be trying to generate a bipolar square wave.

Carpe Vigor

Dr.D

[arc]

Hi John,

You have several option if you export the dxf to AutoCAD.

This, of course, depends on which version of AutoCAD you have, but if it's Version 14 or later, should be no problem.

One option is to use the Add-a-Plotter wizard (in AutoCAD) to add a raster "printer", select Raster File Formats (as the Manufacturer) and under Model select JPEG.

If you select a large paper size, (eg. 22x34) or create a custom paper size for this printer, it will produce more pixels for better resolution when you resize the JPEG.

Another option is to export the AutoCAD file directly to a bitmap, bmp or wmf (or jpeg in later versions) using "Save-as". If need be, convert the resulting file to a jpeg using any one of a number of file viewers. I use IrfanView, Free image viewer/converter for Windows. http://www.freedownloadscenter.com/Multimedia_and_Graphics/Graphics_Viewers/IrfanView.html]

Hope this helps,

Dave

[baggo]

Thanks for that Dave, I'll give it a try

John

[arc]

John,

No prob. I use AutoCAD nearly every day doing Architectural Drafting.

I'm not to circuit board minded, but have made a couple of circuit boards for controlling my truck's oxygen sensor output and one for a steam injection system for it.

If this load sensor works out for you, keep us posted and I may try building one.

I appreciate your sharing the design.

Dave

[RatOmeter]

You can also get the schematic shown in its window, hit ALT-PrintScrn (which copies just the contents of that window to the clipboard).  The open MS Paint and paste it in there (press CTL-V).  Then in Paint, do a File->Save As in your favorite file format (GIF is a good choice).