Need help with a Power Failure Alarm Circuit

[CDRIVE]

Yes Kris, OptoCouplers were discussed in this thread 8 months ago. It appears to have been ignored.

I have mentioned this before but it's worth repeating. When a Bridge is directly connected to the mains the circuit can not be grounded. Doing so will short out a section of the bridge. On the other hand, because it can't be grounded, circuit common will sit -170V above earth ground!

Hey, I thought I said I was outa here?

Chris

 

[CDRIVE]

It took a while to find this. I drew it for another member in the UK where the mains are 220VAC. Your circuit common will be 1/2 this voltage but still lethal.

Note: Though I show a GND symbol on mains neutral it was done for the spice sim only. Mains Neutral is not considered earth ground though it's very close to its potential minus the voltage drop from current flowing through the Neutral conductor. Ground and Neutral are tied to the same node in your electrical distribution panel however.

Chris

Edit: Changed the word "common" to "Neutral" where referencing the mains.

 

[KrisBlueNZ]

All agreed Chris. Although, if you're using the capacitive reactance feed method and you put the capacitor in the phase line before the bridge, the circuit common (which is the best way IMO), the circuit common (negative bridge terminal) doesn't sit half way between Phase and Neutral (on average); it's much closer to Neutral.

Using an AC input optocoupler with a capacitor feed is a pretty tidy way to detect mains presence if there's a separate power source, as there is in this application.

 

[CDRIVE]

Kris I'm quite burned out right now. Please draw what you're describing regarding the bridge.

Thanks,
Chris

 

[KrisBlueNZ]

The graph shows the voltage on the "0V" rail relative to the Neutral rail for a capacitor-fed bridge with the capacitor in the Phase line.

I was just pointing out that the "0V" rail of the circuitry does not go far from the Neutral voltage. So although the "0V" rail is not grounded, and must not be connected to Earth, at least it is not as dangerous as the 0V rail of the primary circuitry of an off-line switching supply or a "half live" TV set.

 

[CDRIVE]

Kris, thanks for posting that. Last night I had the brain power of a Beagle. It's all clear now!

I still think he should use an opto. Which begs another question... Why haven't any IC manufacturers ever packaged a NE2 type OptoIsolator? I've seen little neons that were about half the size of an NE2 that may have been a smaller version of same or different model altogether. It's advantage would be extremely low power disapation. It would be far lower than any LED emitter. Currently the only option is making a discrete module and potting it.

Chris

 

[KrisBlueNZ]

Yes Chris, sorry, I wasn't recommending the line-powered design for this application. I was just pointing out that it's not as dangerous as a half-live circuit. I agree that an optocoupler is probably the best solution for this job.

Regarding neons in optocouplers, I guess there would be mechanical issues with the glass and vacuum. Also I don't think neons emit very much light compared to LEDs. A potted module would work, I guess, but I think the phototransistor current might be quite low because of the weak light. Have you ever tried making one?

 

[CDRIVE]

Regarding neons in optocouplers, I guess there would be mechanical issues with the glass and vacuum. Also I don't think neons emit very much light compared to LEDs. A potted module would work, I guess, but I think the phototransistor current might be quite low because of the weak light. Have you ever tried making one?

I haven't but I and possibly you participated in a refrigerator/freezer thread where the OP wanted to use one of two integral neons to trigger a timer or something. One neon was a mains indicator. The other turned on when the compressor was running. This was the one he was interested. I think I drew him a schematic using a PT that he taped to the neon. He reported success. I've searched my schematic files but I don't know what I named it. The NE2 model that I have was copied to Tina from my LTSpice library but Tina doesn't recognize it as a control light source for optos. This leaves the possibility that I sim'd it using a LED or lamp as a light source.

Chris

Chris

 

[CDRIVE]

I finally found it but it was not here on EP. I didn't realize it was that long ago. I don't visit there any longer. The EP clan is a much friendlier group and this includes our mods and administration. I don't think they drink beer over at ACC.

http://forum.allaboutcircuits.com/showthread.php?p=472797#post472797

Chris

 

[CDRIVE]

Reading through that thread the OP used an LDR to negate the NE2 flashing on +- cycles. The LDR is too slow to follow the perturbations. Still, it's surprising that the NE2 emits enough light to drive an LDR but it did.

Chris

 

[KrisBlueNZ]

I see. I guess as long as the circuit following the phototransistor or LDR is sensitive enough, it will detect a pretty weak light.

I subscribed to AAC as well, but I don't much enjoy it, compared to EP.

 

[CDRIVE]

Kris, that's because we're not aloof!

Chris

 

[KrisBlueNZ]

Yeah, they don't seem very friendly. They do have a good resident expert though. Ron H, I think his name is. Very knowledgeable.

 

[CDRIVE]

Kris, full reply is here..

https://www.electronicspoint.com/just-like-say-t262467.html#post1565600

Chris

 

[davenn]

thread closed

it has moved into the realm of too dangerous

Dave