DIY Kill-A-Watt

qubesquare

n00b
Joined
Jun 2, 2008
Messages
7
Is this circuit safe? What happens when something goes wrong and the load becomes a dead short?

Define safe. lol. For all it's worth, this circuit isn't worth the risk of injury one might suffer if something goes wrong. If you want a cheap and accurate watt meter just buy the "kill-a-watt" its only about 20 bucks.

For my project, I needed to figure out how much wattage a device was using. This was a crude way to do it but it worked for my specifications. I guess a fuse would be a smart addition to this circuit.
 

Mohonri

Supreme [H]ardness
Joined
Jul 29, 2005
Messages
5,751
If the load shorts out, the worst you can do is fry the opamp. Even if the opamp were to fail in such a way as to become its own short, it still would have a resistance of 2kOhms to whatever voltage comes down the line. Which wouldn't mean much, since the current would prefer to go through the .1Ohm resistor instead of the 2kOhms of the opamp. Even if the .1Ohm resistor were to burn out and create an open circuit (we're really multiplying probabilities here), you'd have 120VAC maximum across the 2kOhms, which drop you to about 60mA rms.

The probability of each of these things happening is very small, and the chance of all of them happening at the same time is extremely remote. And even if they all did happen, it still wouldn't be dangerous.
 

mikeblas

[H]ard|DCer of the Month - May 2006
Joined
Jun 26, 2004
Messages
12,775
Define safe. lol.
I'm asking from the engineering perspective. If it's not safe, then how would the design change to make it safe? It seems like it can't possibly be too safe, since the high voltage/high current section isn't isolated from the low-voltage section. At the derived output, there's also a high DC potential over ground, I think.
 

Mohonri

Supreme [H]ardness
Joined
Jul 29, 2005
Messages
5,751
Actually, there's no high voltage anywhere in that circuit. How? Because the inline resistor (the 0.1Ohm one) is on the neutral line. In other words, one side of that resistor is at 0V and the other is at maybe 1V, if you're driving a 1200W vacuum off it. Heck, you could short either side of that resistor to ground without any excitement, unless it's on a GFCI-protected circuit.

There's a possibility of getting high voltage on that resistor, but it would take a dead short between the hot and neutral lines. If that happens, you'll have much larger worries on your mind than whether this little low-voltage, low-current DC circuit is safe.
 

mikeblas

[H]ard|DCer of the Month - May 2006
Joined
Jun 26, 2004
Messages
12,775
Actually, there's no high voltage anywhere in that circuit. How? Because the inline resistor (the 0.1Ohm one) is on the neutral line.
Ahh, I see; it's just dependent on correct polarity. Thanks!
 

Mohonri

Supreme [H]ardness
Joined
Jul 29, 2005
Messages
5,751
Yeah, if the wiring in your house is incorrect, and the hot and neutral lines are switched, then it becomes a bit more dangerous, since that whole circuit will be at 120VAC with respect to ground.
 

Maxtr0sity

Limp Gawd
Joined
Dec 24, 2005
Messages
280
I just built the power meter setup shown by OP. Works great. All the parts are bought off Jameco for those interested. Someone should write an Instructable on how to do this.
 

lobo115

n00b
Joined
Aug 25, 2014
Messages
1
Hello, i know that it is a old thread but i want try it, but pictures are no availables :(:(:(:( , could you send it again, or tell me how do it?, thz and sorry for the inconvenience
 
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