Just a quick query. A regular incandescent light bulb running off of 120V AV could also run off of the same 120 mains power but converted directly to DC, yes?

 

Yes of course but at half the brightness I guess..

 

Well yes, but remember nothing in this world is 100% efficient so you would loose some power in the convertor.

If you used a simple bridge rectifier, you would drop a couple of volts and the bulb would be slightly dimmer.

This is all OK because an incandescent bulb is a resistive load.

The situation would be very different with a reactive load, such as a flourescent light.

 

One good feature is that with the DC voltage, a dimmer can use it's full operating range to dim the bulb, whereas with AC the dimmer needs to be turned more than half way before the bulb comes on.

With DC the voltage remains 117V throughout the dimmers full range, thus varying the current. With AC the Voltage becomes variable while the current remains constant and you need a minimum of 50V to turn the light on.

Overall the power is obviously the same for a given light intensity of the bulb, but the range of light intensity variation is greater with the DC.

How would one go about simply cutting the voltage in half, say to 60V but allow the full current necessary to operate the bulb in the AC realm??

 

Can't do that. The filament won't go to full brilliance without full voltage. Gotta have the energy to push electrons through the load.

If you never want more than partial output, place two bulbs in series. I think you'll find them too dim to be useful like that, though.

As to dimmers - if it uses a thyratron, the device will simply latch on and pass full current with DC. A good dimmer will go from a dim glow to full output - why can DC have a greater range?

 

How would one go about simply cutting the voltage in half, say to 60V but allow the full current necessary to operate the bulb in the AC realm??

You don't.
Power in Watts = Voltage x Current.
also= Current(squared) x Resistance
and= Voltage(squared) / Resistance

 

As Wookie said indirectly

Half the voltage is a quarter the power.

If you genuinely want half power use the configuration I think jj was talking about.
Instead of a bridge rectifier a single rectifier in series with the bulb will give full activation voltage, but block alternate half cycles of current.

Alberto, you obviously missed my point about reactive loads.

 

You don't.
Power in Watts = Voltage x Current.
also= Current(squared) x Resistance
and= Voltage(Squared/Resistance)

Yeah, another brain out-gassing! Should have stuch to ohms law as in my dimmer AC/DC observations above.

 

That rectifier will only pass half the ac waves + its own voltage drop(negligible)= Abt half the voltage out and brightness + More lethal DC volatge

 

That rectifier will only pass half the ac waves + its own voltage drop(negligible)= Abt half the voltage out and brightness + More lethal DC volatge

Not sure what you are referring to with respect to "That Rectifier". The rectification I had in mind was full bridge which does not cut the voltage in half, see figure 1. Figure 2, on the other hand does indeed cut the efective voltage considerably.

My question is now, although I have my own suspicions why, is why is 120VDC or even 60VDC more hazardous than 120VAC??

 

We have had some discussion about the dangers of both AC and DC voltages. Here is a link to one such thread - http://forum.allaboutcircuits.com/showthread.php?t=11268.

In some literature, DC voltages are presented as more dangerous because coming in contact with a lethal DC voltage is likely to freeze the muscles and prevent you from pulling away from the voltage point.

I would sat that electrical safety requires you to treat any bare wire as possibly carrying a lethal voltage, and to consider any voltage beyond 24 volts as potentially lethal.

In the home, you are much more likely to be shocked from 120 VAC. In the workplace, there can be all sorts of lethal voltages present. Arguing AC vs DC diverts attention from the point that electricity can be quite dangerous.

 

Ohh I referred to 'that rectifier' as a single diode... if its full wave, then its fine,, still losses will be there..
DC is dangerous than AC of the same voltage because AC has a varying magnitude and gives us pulses of energy when in contact.. But DC gives a fixed magnitude signal which is very much dangerous.. these happen only for voltages above 40v.. Just give a google.. you'll find out.

 

Beenthere,
Interesting reading at the site provided.

jj alukkas,
In other words, your comments regarding a single diode is almost a solution to reducing the voltage in half. Adding a smoothing cap would produce a crude sine wave but with only a positive amplitude, a rippled DC voltage at about 80VDC maybe?

It's a mute point as I will not be playing with it, nor do I have any practical application for it.

 

Some years ago there was a push to install button diodes on light bulbs to lengthen life, when I questioned sales man on availability of reversed polarity diodes no resp. Too many of one polarity would cause DC currents circulating in transformers. My outside night light uses LDR & triac for 60W bulb; life of bulb about 6 yrs.

 

In other words, your comments regarding a single diode is almost a solution to reducing the voltage in half. Adding a smoothing cap would produce a crude sine wave but with only a positive amplitude, a rippled DC voltage at about 80VDC maybe?

I'm quite sure jj didn't say that, any more than I did.

 

Nope, You can't even think about that unless you get a diode with a reverse breakdown voltage that high. And If it does work that way, we've got nothing to work on it with for a signal which is like it 0 shifted towards the negative region.

And how does such a signal increase the life of a bulb??

 

I hate to jump in this late in the game, but wouldn't a full wave rectified signal (with filtering) go for 1.4V of RMS on the DC output? This would mean the bulb would burn out, and there would be a higher current draw (while it lasted). RMS is root mean square, while full wave rectifiers tend to go for peak.