240v mains to logic output?

Jan Panteltje wibbled on Sunday 31 January 2010 20:04

CAPS lock stuck dear?

No sane person uses a neon anymore[1], though for some strange reason the still get bundled in with screwdriver sets.

And in that case, the fuse is going to do what exactly? Unless you are Mercury Man you will not be presenting a low impedance fault which is what the fuse is there to handle.

PMT? You seem to be very irritable...

One resistor, no. Why argue about a 20mm fuse anyway. There are no cons save a tiny bit of space and the pros are as stated, the one in 10,000 chance that something does fail, it will offer some additional protection.

What is idiotic about paying attention to wiring fragile stuff to high energy circuits, especially stuff that may have a co-presence with SELV circuits? Why do my microwave and amplifier have internal fuses despite there being a perfectly good one in the plugtop. Perhaps you'd better ring the manufacturers and tell them they're idiots for wasting 50p on unecessary fuses?

Depends on how it's mounted. A resistor simply smokes out, or opens if you take a fusible version. Many things are connected sans fuses, many wall warts, door bell transformers, and so on. A fuse is usually a good thing but by way of clever construction you can make sure that a short to ground is next to impossible. Of course it is never totally impossible as some might, for example, drop a 2-pound hammer onto it. Then again, that could short the input terminal of a fuse to ground as well and then ... *PHOOMP* :-)

Open a gas furnace or something similar. You'd be surprised about the number of fuses in there for its ittle transformers and electronics. Ours is a reputable name brand (Lennox) and the number is AFAICT zero.

On a sunny day (Sun, 31 Jan 2010 20:48:06 +0000) it happened Tim Watts wrote in :

THERE ARE CONS THE CHANCE OF THE FUSE DROPPING OUT OF THE HOLDER AND SHORTING SOMETHING LIKE SOME OPTOCOUPLER PINS IS WAAAAAAA_A^n__AAY BIGGER THEN BOTH

You forgot to say, "I AM PANTEUS" ;)

Cheers

Phil Hobbs

If the source side can deliver up to 100 A during a short, do you expect that the 20 mm fuse is going to reliable break it ?

Of course all wiring and PCB tracks up to the fuse must have a sufficiently low resistance and large current carrying capacity in order to reliably and quickly blow the previous fuse (typically 10/16 A fuse in 230 V world).

If you are going to use a fuse, make sure it will be able to break any current that the source can deliver or limit that current with a series impedance etc. before the flimsy fuse.

Paul Keinanen wibbled on Monday 01 February 2010 04:56

See below.

Indeed. Something like this;

(1.5kA rupture capacity with a fairly small I2t).

It might be overkill and I agree that some devices may not have secondary fuses. But one should bear in mind that those devices (should!) have been type tested, which is a luxury that usually doesn't happen with DIY units. Also, having seen what happened to a laptop supply where the neutral disconnected on a 3 phase installation (it got somewhere in excess of 300V RMS presented to it and smoked, in the hot and fire starting way, perhaps some of the cheap end manufacturers should consider doing so...

All things considered, I think I'm still happier with a fuseable element with known characteristics - unless the resistor in question has a stated rupture capacity and one can be sure that it's not going to cause other problems when it does go bang.

Cheers

Tim

Jan Panteltje wibbled on Sunday 31 January 2010 22:14

Stop stressing out sweetie, buy some of these:

I'm sure your current ones are too tight. Your colleagues will thank you.

On a sunny day (Mon, 01 Feb 2010 08:01:42 +0000) it happened Tim Watts wrote in :

Normally, on a small PCB like that, in case of a resistor short, something that never happens anyways, the tracks will evaporate. As the tracks will be designed for a few mA at most. Fuse enough. The only shorted resistors I have ever seen were composite carbon. I think it was spelled 'Vitrohm'. Those used to crack open and go O.C. too.

Vitrohm resistors and Wima capacitors, in old TV sets you better replace them all...

Modern resistors of those high values consist of a ceramic tube with a spiral of metal vapoured (word?) on it. Shorting is next to impossible, OC is very difficult with a metal film.

On a sunny day (Mon, 01 Feb 2010 08:09:14 +0000) it happened Tim Watts wrote in :

How would that help against a shorted resistor?

You are sooooooooooooooooooooooooooooo confused!

all...

of metal vapoured (word?) on it.

I was under the impression that a fuse was always required for fire and user safety for any mains connected device?

I get my furnace serviced every year which includes an efficiency measurement. To take the measurement they drill a hole into the exhaust outlet which is vented outside to insert their probe. The ABS pipe is exposed to temperature variations which leads to condensation guess what happened? I should also mention that all they use is duct tape to cover the hole they drilled.

I started smelling a familiar burnt insulation smell throughout the house I'm sure everyone here knows the smell I'm talking about:.

To make a long story short condensation had lifted the duct tape up enough for a small trickle of water to track down the pipe and dripped on the main control board shorting it out. This melted the insulation on the wires to the board and left a nice big black mark on the board. The crawl space were the furnace is located was full of smoke too, lovely.

Fortunately I also have a gas stove and fireplace and I pay $200 a year for a service agreement. So it was all fixed at no cost to me. I guess I was lucky the house didn't burn down but from what you guys are saying fuses aren't used in furnaces so maybe they should be. A line to line short on the board definitely has the potential for a fire hazard wouldn't you say?

When you also consider that panel fusing or breakers for a furnace are slow blow type you can wind up with a lot of current flowing through the wiring like say behind your insulated walls before the panel fuse or breaker trips.

On a sunny day (Mon, 01 Feb 2010 09:25:29 -0500) it happened Hammy wrote in :

It is all a long story, and shows how stupido those service guys work. An other climate nut case at work!

If a fuse would have helped is not certain at all. I have had water in a230 V mains extension cable laying in front of the window. Window was open, and it rained, water came in, and the same funny smell. Some current, but no blowing of any (mains) fuses). So disconnected it it and replaced connector. Such things can always happen. When that board got wet, and where would the fuse have been? On the board likely, it would have shorted just as well *before* the fuse. And external fuse is always present, the big one. In my case with the opto, the circuit is in a very small match size PVC box, and pretty much water tight :-) Fuses would only increase the probability of problems there.

likely,

and pretty much water tight :-)

The argument not for a fuse because there is a resistor assumes that there is only a path for current through the resistor. My point was that that may not be the case obvious from my experience.

If a fuse is placed right were the wire comes into the board which is were it should be anyways only an ass places it several inches away the likely hood of a short caused by environmental conditions or debris ; occurring before the fuse is pretty remote. To eliminate this likely hood altogether get an inline fuse.

You can never eliminate all possible hazards but you should take reasonable steps to reduce the likely hood of them happening or at least to mitigate any collateral damage to people or property if they do.

Like you mention the workers were idiots but idiots aren't in short supply so for me I use fuses.:)

Oh and before you get worked up I'm sure your circuit is safe and with a resistor connected right to the line feeding the opto the likely hood of a short is remote. My comments were more in general.:)

But, it wants about 4 mA (peak) for reliable detection; from 250VAC, that means about a 75k resistor, dissipating most of one watt. The alternative of a 5M resistor feeding a 1:5 current transformer would dissipate

Why does six turns of wire on a lossy bit of iron push designers to denial?

On a sunny day (Mon, 01 Feb 2010 10:02:31 -0500) it happened Hammy wrote in :

likely,

and pretty much water tight :-)

Maybe you should then also use a fuse before the mains filter with Y caps as found in almost any piece of home equipment, like a PC? Unfortunately those are build into the connectors. The chances of such a cap failing are much bigger then a resistor failing. Now about nuclear plants and Tjernobyl, well OK that was done by an idiot PhD IIRC.

On a sunny day (Mon, 01 Feb 2010 10:28:14 -0500) it happened Hammy wrote in :

But I already wrote a reply :-)

An inductive current sensor is a simple, safe way to do this. See

John Nagle

On a sunny day (Mon, 01 Feb 2010 13:12:00 -0800) it happened John Nagle wrote in :

Oh, *current*, I do that with old tape recorder playback heads: ftp://panteltje.com/pub/play_back_head_current_sensor_img_1153.jpg

Just curious. What goes through that white wire you're sensing? The epoxy indicates that this is not a general-purpose sensor. What kind of sensitivity do you get before amplifying it with the LM324?