Testing neutral-to-ground voltage of switching mode power supplies

[Kingblert]

I am attempting to create a sine wave signal from 0V to 12V at 40 Hz and up to 1.4A, with original power input 120VAC 60 Hz wall power. I've come up with the circuit below, which I'm pretty sure works in theory but the implementation is not working. I can get the correct high voltage (12V) at the output by tuning the feedback resistor, but the op-amp output never goes to zero. I suspect poor understanding on my part of the DC return (neutral) lines compared to each other or an absolute reference. I had assumed that the neutral outputs of my DC power supplies would be the same, probably the same as wall neutral, and within a few volts of ground. This is not the case.

The op-amp is set up in a non-inverting amplifier configuration, and the gain should be correct to give a 0-to-12 V output based on a 0-to-5 V input. But this requires that the AC signal source and the op amp are working from the same "zero".
The AC signal is generated by an Arduino Uno running in PWM mode, followed by an RC low-pass filter. This part works: tests out good at 40 Hz and with the right voltage at V+, when measured from the Arduino neutral, which is shared with the low side of the capacitor in the RC filter. I've shown the series 2.5V DC and 2.5Vpk-pk AC power supplies here as a stand-in for the Arduino-LowPass combo. I'm pretty sure the details of the signal source don't matter, because the op-amp output never went close to zero even when the output of the Arduino was commanded to steady-state 0.
The voltage input to the Arduino is supplied by a 9 VDC wall wart two-prong switching-mode power supply.
www.digikey.com/product-detail/en/phihong-usa/PSM03A-090-R/993-1234-ND/4031879
The op-amp is a Texas Instruments LM675.
www.ti.com/lit/ds/symlink/lm675.pdf
The LM675 requires a minimum 16 VDC power source (Vcc to Vee). I selected a DC K1796 from Kynix:http://www.kynix.com/uploadfiles/pdf0125/K1796.pdf
Hot-to-Neutral DC voltage of both power supplies tests great, but the circuit didn't work: the output of the op-amp would never go close to zero. I eventually tested neutral-to-ground (wall power ground) on both power supplies, and found that there is a 6 Vrms on the 9V supply neutral (and hot), and a whopping 65 Vrms on the 18V supply neutral (and hot). Frequency of both is somewhere in the 500 Hz - 800 Hz range (my meter wouldn't settle). Additionally, the 9V output neutral measures 0 VDC relative to ground, but the 18V output neutral measures +3 VDC. So clearly they aren't working from the same zero. In addition to screwing up my circuit, the 60 Vrms would require a different and stronger approach to safety in this circuit than I had been planning on when I thought the highest voltage would be +18V relative to ground.
It is possible that there is measurement error, especially for the 18V DC output relative to ground and the frequencies of the DC power supply outputs. I'm only using a multimeter with a frequency capability, I don't own an oscilloscope.
My question(s):
1.Is this something inherent to the switching-mode power supply technology, and was it unreasonable of me to expect the neutrals of the power supplies to be similar the wall neutral? Do I need to switch the power supplies I'm using? I note that Arduino can accept a 16-18V as its input power, but it is strongly recommended to limit to 9-12 V.
2.What do I need to change to achieve the intended output? Can I tie the two neutrals together electrically without harm to any components? Would that fix the problem? Should I be connecting anything in this circuit downstream of the DC power supplies directly to ground?
3.The load of this circuit will probably be a two-prong household lamp with a 12 V LED bulb. How can I ensure the neutral output (DC return) is safe?
I did see this previously posted question, but either it wasn't close enough to answer my question for my circuit, or I'm not educated enough to understand how it applies. Other previously asked questions mostly have to do with household AC wiring.

 

[Pick2]

First , you need to use the proper terms to avoid confusion:
Ground is mother earth , third planet from the sun , drive a rod in the ground and connect up a wire to it.
Neutral is one current carrying leg of your 120 VAC that , at some point ,is connected to Ground
In your circuit , you can call it Vee , 0V , or circuit common , but don't call it Neutral or Ground even if , at some point it is connected to them. It is confusing. if it's not carrying current from your 120 VAC , don't call it Neutral.

"and a whopping 65 Vrms on the 18V supply neutral (and hot)."
Your PS has a transformer which electrically isolates the AC input from the DC output. That is why you read 65 VRMS between those points. they call it floating because there is no connection. When you connect up the meter you ARE connecting those points thru the high resistance of your meter which usually reads about half of the AC.

The OP amp has a "totem pole" output. One transistor to Vcc to pull the output high and one transistor to Vee to pull the output low. That transistor will keep the output from getting to Vcc or Vee , typically within 0.6 Volts of either rail. You would need a negative Vee to get the output to 0 volts. You would typically use -5 Vee to get the output to 0 volts.

( I will send you the Bill for this advice , because I've been trying to get rid of him for ages )

 

[serpretetsky]

Sorry if this is a bit late. Like Pick2 mentioned, the DC outputs are floating (they have no voltage coorelation with the AC inputs, and certainly no voltage coorelation to the outputs from a different supply). This is common to many power supplies (im actually not certain how common this for switching supplies, but it is certainly true for any supply that has a transformer). Yes, you can connect the negative terminal of all of your DC supplies together (the arduino negative terminal and the opamp negative terminal). That will give you a common reference that all of your voltages will be based off of. Your op-amp might still not hit 0 because it's too close to the rail, but it should be much closer.

I don't know what kind of 12v led bulb you are using, but typically you just connect the negative terminal to the same common negative all the supplies are connected to. I say I don't know what kind of 12v bulb you are using because if you are using bare LEDs this is typically not a very good circuit. Typically you want to add resistors in series with bare LED's before you drive them.