ac power with a 12 volt switch

[marwin]

I know this is probably a stupid question, but can I run a 110v lighting fixture with an illuminated 12v switch? Please forgive my ignorance.

 

[crust]

You probably can if the switch contacts are rated high enough and the switch is one of those with distinct wires for the lamp. If you want the lamp to light as well it is a bit more difficult. You would need to step down the voltage to the lamp to light it. It's probably easier to get another switch. If the lamp is integrated electrically into the switch then no -- at least not without changing the bulb. But at that point it is easier to get a 110V lighted switch.

 

[stevez]

You should not use a switch that is not rated for the application. You could use it, it might work but you will have exceeded the rating of the device which might result in disaster.

If you had a source of 12 v power you could operate a relay with your 12 v switch. The relay would have to have a 12 volt coil and contacts with sufficient rating to handle the intended load.

Note that "rated for the application" usually means that the rating meets or exceed the amounts expected for the application. Example: if your lighting load were 2 amps a relay with contacts rated for 10 amps at 120 volts would be ok - so would one rated for 20 amps at 250 volts. If an AC motor is being powered the contacts should be horsepower rated.

I hope this helps.

 

[Phasor]

If you want the lamp to light as well it is a bit more difficult. You would need to step down the voltage to the lamp to light it

Not really very difficult - you just need to use a capacitor as a voltage dropper for the lamp. ie, connect the switched active to one capactitor leg, the other capacitor leg to one side of the lamp, and the other side of the lamp to neutral (assuming you have a neutral available at the switch).

And yes, you will need the type with seperate lamp leads to do this...

I'd be guessing that your lamp requires about 100mA - if so, then you require a 2.4uF capacitance - though 2.2 is the closest standard value. If the lamp requires some other value of current, you will need to adjust the capacitance accordingly.

But at that point it is easier to get a 110V lighted switch.

I agree with crust on that point.

 

[Noggin]

With the mentioning of ratings, what do voltage ratings mean? Why does radio shack sell 250v fuses? The darn thing would blow WAY before that anyway, its the current that matters....

Now on something like a transistor I could understand it, you might not want 250v across the thing.... and maybe in a switch you could have arcing.

But seriously, why rate some things with a voltage when it seems that it should be rated as something else?

 

[Optikon]

With the mentioning of ratings, what do voltage ratings mean? Why does radio shack sell 250v fuses? The darn thing would blow WAY before that anyway, its the current that matters...

It is not only magnitude of current that matters. It is current and time. I recall a quote I read once: "Anyone who thinks that a 1 Amp fuse will blow at 1 Amp has never been involved in the fuse selection process."
And it is true.. Fuse selection is not a trivial process. Depending on the particular fuse characteristics, that 1 Amp fuse may finally blow after it had 1 Amp flowing for 10 minutes! Each fuse is different - check the manufacturers data very carefully. Some are quick some are slow. Typically, if you need your fuse to blow immediately at current X, then you will likely pick a fuse that has a fast trip time at something less than X.

If the current is sufficiently limited, then the fuse won't blow. The ratings have alot to do with how the fuses are tested because they are expected to work at the voltages they are rated for. So, if I manufactured a fuse and found that it could work reliably at up to 500V why would I test and rate it for say 150V? I would lose all those customers who needed a 500V fuse. Of course, it could be installed in lower voltage environments as well.
These voltage ratings are generally the highest possible voltage that the fuse can work reliably (no arcing, reduced life etc..). And they also have much to do with the fuse material.

Now on something like a transistor I could understand it, you might not want 250v across the thing.... and maybe in a switch you could have arcing.

But seriously, why rate some things with a voltage when it seems that it should be rated as something else?

In many cases, somethings have ratings where it does not matter so much what they are specifically as long as you know what they are and what everything else is measured against (Example, opamps with data pertaining to various supply voltage ratings)

Other times, the ratings come from limitations of the material and assembly of the devices (Example, power mosfets with voltage ratings high enough to avoid avalanche breakdown of the semiconductor junctions)

Sometimes it has to do with convenience / practicality (Example, some ceramic capacitors rated for 1000V may actually be just as reliable at 1426.2V but who will test them at that voltage? Answer: No one. Test and hence, rate them at 1000V, call the extra 426.2V a safety margin and call it a day.)

Sometimes it has to do with all the above as well as a little bit of marketing. Example, hair dryers - voltage rating = 125VAC. That happens to be what we buy from the power company. Well, the motor and heating element can be run just fine on 140VAC but since everyone is buying this appliance to be plugged into the wall, they rate at 125VAC (not counting high line conditions which is already built into the rating)

To really understand why different things have seemingly unrelated ratings, you really need to study the manufacturers data. But you can rest assured, there are very good reasons the ratings are what they are.
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[Noggin]

I understand much of what you said, but I was coming from a different angle than that.

A bit more specific:

A fuse from radio shack may be 4 amp 250v rated. I understand that there may be some kind of safety margin, and that it won't blow instantly at 4 amps (like car stereo fused, there's a 100 amp one that takes 700 seconds to blow at 100 amps).

Anyway, if you have the fuse in series with a 100 ohm fuse and you use a 250v source, you'll get 2.5 amps current. Its the power that makes the fuse blow, so the fuse will have to dissipate 2.5^2 * R.

If the fuse is in series with a 1000 ohm resistor and a 2500v source, you'll get 2.5 amps of current. Again, the fuse will be subjected to the same power dissipation.

I'm still in school, and the only thing (pretty much) that I've been subjected to is "ideal" situations. Is there something I'm not taking into account? Its something I should probably know, because I don't choose fuses based on current and voltage, only current. Perhaps that voltage rating is the voltage across the fuse that causes it to blow in approximately 1 ms.... I don't see how this can be called "safety rating" but more of a "comfort rating" for someone who doesn't quite understand electricity (unless I'm the one that doesn't understand something!)

Of course, it could be installed in lower voltage environments as well. These voltage ratings are generally the highest possible voltage that the fuse can work reliably (no arcing, reduced life etc..). And they also have much to do with the fuse material.

So even if its not dissipating the power, the voltage its delivering/passing (don't know a better word for it) reduces the life of it if it is not rated for it?

 

[crust]

Understanding this has digressed a bit, the voltage ratings on the fuses are a safety margin. The NEC says something like fuses rated at 600V or less can be used for lower voltages. The idea as I understand it behind the voltage rating is that when used at or below that rating, when the fuse blows, there is no chance of arcing over the blown section of the conductor. There is also a diff between AC/DC fuses. The AC waveform switches rapidly and will therefor extinguish the plasma that forms in the fuse. However, the DC one will continue to flow and could continue to conduct. DC fuses sometimes have bent paths so that the plasma goes in the direction of the arc and the fuse breaks the circuit.

BTW Phasor, I like the cap idea ... i had not considered that.