Current sensing circuit help

[(*steve*)]

FFS!

THE JUNCTION OF THE TWO ZENERS IS THE CENTER OF YOUR SPLIT SUPPLY FOR THE OP AMP

DON'T CONNECT THE 'KING MAINS TO THE LOW VOLTAGE SIDE.

AAAAAAGH!

So that circuit is correct so far. Remove the ground connection from the mains and place it between the zeners. It allows you to get rid of the C1/C2/R7/R8 splitter from your previous circuit.

 

[Yuda]

Ok Ok... I moved the ground connection like you said. The simulation is working in this configuration. I am able to (somewhat) zero out the output and get roughly 1.5 volts out with a 0.1 watt load. I will try to build this circuit tomorrow night...

I am sorry that I am slow on the up take, I am still learning the basics. I get that it may be frustrating to explain a circuit to someone that does not quite understand everything, and trust me, I know that I do not understand everything, but that is why I came here to this site... You clearly know way more than me which I am grateful for, however, from here on out please explain everything to me as if I know nothing so that way I am able to learn, and you are able to only explain something once. I apologize if I have angered you for my lack of intelligence in regards to basic circuitry...

 

[(*steve*)]

The thing you have to realise, is that EVERY point in that circuit can kill you.

If you have an isolation transformer, use that. If not, have someone with you who can call an ambulance and perform CPR on you until it comes.

 

[Yuda]

I always understood that a transformerless power supply was dangerous... I am always cautious when working with one. I am going to build the above circuit tonight and I will let you know how it goes. The only thing I am pretty sure that I dont have any 7 volt zener diodes. Will the above circuit work at 20VDC? I have a bunch of 10V zeners.

 

[Yuda]

Question, would putting another ceramic capacitor in series with my neutral (much like how C1 is hooked up on my Hot) protect my circuit from accidental flipping of the mains?

 

[Yuda]

I tried to build the circuit but I need to order more zener diodes (I ran out of 7V Zeners). I have a question though. In the simulation I am grounding between C2 and C3 and I am also grounding D1 and D2 by the transformer, are these ground points strictly for the simulation to run properly, or do I need to also ground these points in real life? If I do not need to ground these points in real life do I just have to solder a wire from the center of C2 and C3 and the solder the other end to D1 and D2?

And if I do need to ground these points in real life what can I ground those points to? My outlet in my garage is a 2 prong outlet so I do not have a ground connection and like you said before, I can NOT use my neutral to ground those points.

 

[(*steve*)]

Two 10V zeners will be fine.

Nothing (other than the isolation provided by a transformer) will make it safe if there is any access to the circuit.

The resistor in series with the capacitor is there to act as a fuse for when the capacitor fails. A resistor in the neutral line makes it less safe because if it fails, the circuit is live.

 

[Yuda]

Two 10V zeners will be fine.

Nothing (other than the isolation provided by a transformer) will make it safe if there is any access to the circuit.

The resistor in series with the capacitor is there to act as a fuse for when the capacitor fails. A resistor in the neutral line makes it less safe because if it fails, the circuit is live.

Ok I will not make those changes then. Also how about my question regarding the grounding points?

 

[(*steve*)]

And your circuit should have NO connection to mains earth. The circuit floats above ground. Any earth connection will effectively be a short circuit to ground and cause some amount of fault current. If you have earth leakage breakers, such an earth connection will likely trigger them.

 

[(*steve*)]

If you are posting while I'm answering, I probably won't see the post until after I reply.

No need to remind me, you'll probably be doing that as I reply to the other message.

 

[Yuda]

My bad I didnt know if it posted sorry.

 

[(*steve*)]

And I just noticed that you say to using a ceramic capacitor.

I'm not sure if any ceramic capacitors are appropriately rated to be connected across the mains (which this one effectively is).

You should be using a Y1 or Y2 class metalized film capacitor in this application if you want to be reasonably sure it will fail open circuit.

 

[Yuda]

I will double check the capacitor I am using, I am almost certain it is a ceramic capacitor and if I remember correctly it was rated for 250VAC but If it isn't I will make sure to change it. Thank you for pointing that out!

 

[Yuda]

https://www.mouser.com/productdetail/81-rder72e105mue1h3a

That is the part number for the ceramic capacitor I am using. I thought it was rated for 250VAC but it is actually rated for 250VDC.

 

[(*steve*)]

This is an appropriately rated capacitor for 110V mains.

 

[Yuda]

Thank you for taking a look at that capacitor, I ordered all the parts I am going to need to build the circuit but I just wanted to make sure about that ground connection I asked about. Do I just need to solder a wire from the center of C2 and C3 and solder the other end to D1 and D2 and not worry about a ground connection like I have in the simulation because like you said, the whole circuit floats above ground?

 

[Yuda]

I got the parts today and I built and tested the circuit but the circuit is not working. The output of the op amp is always high (about 12 to 13VDC). I tried adjusting the gain with no success and I also tried adjusting the trimmer to zero out the output and still, the circuit did not work. I tried using various different ct transformers to see what would work, I noticed that if I used a CT transformer that had a lower secondary resistance the output of the opamp dropped a bit but it still stayed at around 6 volts. With a low resistance secondary on my CT I was able to zero out the output using the trimmer (close to zero but not quite zero, roughly 23mV) but then the output would do nothing when I applied a load to the 120vac mains. I tried plugging in a 100watt bulb, a phone charger, a shop vac with no change on the output. I do not know what to try at this point, do you have any ideas?

 

[(*steve*)]

Show us the actual circuit you constructed

 

[Yuda]

Ok when I get to my work bench tomorrow I will take a picture and post it.

 

[(*steve*)]

But also post the circuit diagram.