missing op-amp chip in kill-a-watt?

[minerva]

@transcender: Please remember that the Kill-a-watt uses a transformerless power supply, and you have to be extremely careful if you're attempting any kind of interface to external electronics. It must be optically isolated, with an independent power supply on both sides of the optoisolators. Not sure if you already know this or not, I just wanted to make sure.

Basically, there is a 5V potential difference between the microcontroller's Vdd and Vss rails, but there is *not* a potential difference of 5 volts between the microcontroller's Vdd rail and mains earth - the 5V rail inside the kill-a-watt will in fact be at about 120VAC or 240VAC relative to mains earth. All the components inside are at the live AC mains potential relative to mains earth.

So if you just bring out a digital line - I2C or serial or whatever it is - from inside the Kill-a-Watt's microcontroller and connect it to another external microcontroller with a conventional power supply, that external microcontroller will see 120 VAC on that line, relative to its own DC ground. Which is a Bad Thing.

[shaheerulasar]

The products at Adafruit are great and tutorials very helpful. One such tutorial is the one for Tweet-A-Watt - It seems that P3 has changed their on-board chip, once again , to LW2902 instead of the LM2902N as mentioned on the tutorial. Here are some pictures below. Is there a way to get this chip working as there may be a difference in the pin config. For some reason I cant seem to find any spec sheets or rough schematic sheets for this chip to identify the pins on it. Could you help?

[riro424]

Spec sheet is on TI's website. http://www.ti.com/lit/ds/symlink/lm2902.pdf

Pin 1 is SW of El Paso on your KAW.

[shaheerulasar]

Thanks riro424 for sharing the link for the spec sheet. I really hoped there'd be one for the LW2902 as well. TI's website has no sign of that anywhere.

Does it matter that the Kill-a-watt has the LW2902 instead of the LM2902N chip. I could give it a shot with the on-board LW2902 but if I did without being sure that it was similar to LM2902N, I could potentially fry $20 (price for KAW ) - Is there an alternative way to establish their similarity?

Lols @ "SW of El Paso" but at this point, I'm not sure where North is
It would really be helpful if this way Is pin 1 is closest to R4, R7, R20 or R18?


note: blackend and underlined alphabet is just to make sure that there are no typos in the part number.

[tegrevboy]

Anyone else with some info on the new chip, and the options on using it with the tweet a watt.

[L.A.W]

It certainly has the TI logo on the topside, but no where in the datasheet does it list an "LW" for any package let alone a 14 pin SOIC. It has been a while, but the date code looks suspicious too.

I call it as a fake part. You can call TI and ask them to verify the topside marking, but if not pull it off and replace it with an LM2902.

Fun fact, "LM" stands for Linear Monolithic.

[zaqsaw]

I have the same board as shaheerulasar and I've soldered lines from pins 1, 7, 8, and 14 of the LM2902 (or LW2902?) though I've only gotten 2.3 volts from each one, even with a load. Does anyone know if this is what I'm supposed to read, or perhaps where I could find the voltage or current?

[pdp7]

I have the same board as shaheerulasar and I've soldered lines from pins 1, 7, 8, and 14 of the LM2902 (or LW2902?) though I've only gotten 2.3 volts from each one, even with a load. Does anyone know if this is what I'm supposed to read, or perhaps where I could find the voltage or current?

Hi zaqsaw, were you successful in getting it to work yet? Thanks!

[pdp7]

I have the same board as shaheerulasar and I've soldered lines from pins 1, 7, 8, and 14 of the LM2902 (or LW2902?) though I've only gotten 2.3 volts from each one, even with a load. Does anyone know if this is what I'm supposed to read, or perhaps where I could find the voltage or current?

I found I was having the same trouble too. Using at 2012 KAW with a surface mount LM2902. I would see a voltage vary between 2.3 and 2.5V. It wasn't until I removed the XBee from the XBee adapter that that the voltage rose up to 6.0V. Placing the XBee module back on the adapter, it then ran ok, send pulses every two seconds and the Python script on my laptop was receiving the data ok.

If I remove power and the big capacitor discharges too far, then I have to remove the XBee module in order for the cap to charge back up again.

So I believe this must be related to the smaller cap which is supposed to hold the XBee reset line for a few seconds after power up to keep this from happening... but I'm not sure where the issue is. Is it in my assembly technique? Or has the design requirement changed since Adafruit developed this 2009? (I know chances are it's my assembly that is at fault

For reference, here's videos:
http://www.youtube.com/watch?v=e6yt0_Ca ... Je&index=1

and photos:
https://plus.google.com/photos/11754200 ... banner=pwa

Any tips/advice would be appreciated... Thanks!

[pdp7]

turns out it was just my fault - I had the cathode of the cap soldered into the wrong hole. After correcting, it's all working correctly. I just need to figure out how to stuff it all inside.