My Next Project. Phase 2

This morning (Yes, it's 1:26 AM here...that's morning) as I stood sipping my ice water in the kitchen, immuminated only by my Arc AAA and a lolely "Night Light" I decited that it was time to start research on my next project.

I have to start with a simple question, harder questions to follow.

Don't worry about anything else related to this project right now, but here's my first question. If I have a nightlight that takes 7W and I plug it into a 120 V outlet why doesn't it blow up? Is the nightlight drawing 120 V? If I put a 2V bulb in a flashlight with 2 AAs in it the 3V of energy frys the bulb...I know one is DC and one is AC but as far as the bulb is concerned aern't they the same?

Furthermore I can put a 20 watt or 50 watt or 100 watt bulb in my light socket and they're all plenty happy...

I'm confused somone straighten me out. I know this is all much simpler that I am making it and I know that the explanation is simple I just don't know it

We're on phase 2

http://www.candlepowerforums.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=14;t=000399#000005

Okay, a few simple relationships exist amongs
P- power (in watts)
I- Current (in Amps)
R- Resistance/Impedence (in Ohms)
V- Voltage (in, uhh, Volts)

Start with Ohms law:
V=I*R
And then:
P=I*V

Move'em around, rearrange things all you want. You can get P=I^2*R if you want.

So, your '120V' 100W bulb has...
100=I*120 (P=I*V)
5/6 = I
120=5/6 * R (V=I*R)
R = 24*6 = 144 Ohms

So, for a bunch of 120V lamps, all they have to do is have different resistances and they will have different wattages.

Clear?

Doh! Figured it'd all come back to Ohms law...just couldn't figure how

Ok...thanks. So yes all the bulbs draw a full 120 volts but at different currents...got it.

Now, next question, on household current does everything you plug in have to draw 120 V? I know how AC/DC adaptors and step down adaptors work but can something draw (At the outlet) like 15V AC or is this ALWAYS performed through step down?

...this is all leading to something...really, just want to do some learning first...

Do you guys only tweak flashlights? Haven't gotten much insight on my Turn Signals topic, perhaps it was in the wrong forum? http://www.candlepowerforums.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=3;t=002830

Yes, if you plug something into a 120V AC wall outlet you will get 120V +/- 10%. If the circuit you plugged in is designed for lower voltage it will fry.

I've seen strings of LED christmas-tree lights that plug straight into an AC outlet. Put 40 3V LEDs in series, no problem; they'll be pulsing at 60Hz but you probably won't notice it. Dunno what that does to power factors since they're only drawing the positive half of the sine wave...

"Now, next question, on household current does everything you plug in have to draw 120 V? I know how AC/DC adaptors and step down adaptors work but can something draw (At the outlet) like 15V AC or is this ALWAYS performed through step down?"

Actually, nothing 'draws' 120VAC, voltage, or ElectroMotive Force, is the 'pressure' applied to force current to flow through a conductor.
The amount of current that flows, and hence the power dissipated, is dependant on the resistance (or impedance)of the load and the voltage applied.

Anything designed to be plugged into a 120VAC outlet will withstand that voltage. If a lower voltage is needed, then a step down transformer should be used. It's possible to build a low voltage supply using the reactance of a capacitor to drop voltage, but these are inefficient and dangerous. The only benefit from them is low cost.

Ok great. Here's what I'm going for...got some of those nightlights around the house, the type that turn themselves on ya know, and I was thinking for my next project, throw some 3mm whites and, maybe another color too just for fun, into one of these lights. Pulsing at 60 hz probably would be a problem, just thought (As I stood in the kitchen sipping my ice water with my Arc AAA and the nightlight) that the blueish immumination of LEDs would be much nicer than the yellow incandesent.

Now things get real exciting...if I can cram 2 or 3 3mm LEDs in there why not go for a Luxeon next time (or would that be to bright )

Dat2Zip made boards for this, and then quit as I understand. It's ok though because I think I could just stick a step down converter in there, probably too large though I donno.

The night light boards are temporarily been pushed way down the queue. I've got SMT fuses now rated for 120VAC, but, haven't had the time to test them.

I definitely want to make this a safe item, since 120VAC has enough power to burn down the house if you know what I mean.

I am really concerned with anything that plugs into the household outlet.

If I'm lucky, I might be able to get it restarted before xmas or sooner. I haven't decided yet whether to drop it or continue making more. Don't let Darell see this post. I know he wants more.

-WayneY

So do I have any options? Your boards did more than just convert electricity--correct?

I need a step down, or some big resistors...I can't afford nor can I cram 40 LEDs into these lights

Originally posted by lux0:
....Anything designed to be plugged into a 120VAC outlet will withstand that voltage. If a lower voltage is needed, then a step down transformer should be used. It's possible to build a low voltage supply using the reactance of a capacitor to drop voltage, but these are inefficient and dangerous. The only benefit from them is low cost.

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<font size="2" face="Verdana, Arial">Actually....

An AC supply made by using a capacitor to create a voltage divider rather than a resistor is actually very efficient. If done correctly, they are not dangerous. There was a long discussion a while back (search for AC power or something similar). To summarize:
</font><ul type="square">[*]<font size="2" face="Verdana, Arial">The only watts actually consumed by the circuit are from running the LED, the small waste in the rectifier and protection resistors. Probably on the order of (4V LED + 1V rectifier)*(20mA) + (500ohms*(20mA^2)) = 0.3 watts (!!!)
</font>[*]<font size="2" face="Verdana, Arial">The capacitor must be self healing and rated for AC powerline use - I'd chose a voltage rating of AC voltage * 3 at least.
</font>[*]<font size="2" face="Verdana, Arial">A circuit like this must have low value current limiting resistors and a protection fuse to be truely safe.
</font>[*]<font size="2" face="Verdana, Arial">This type of circuit is vulnerable to damage from poor power quality (noise) and must never be connected to a lamp dimmer.
</font>[*]<font size="2" face="Verdana, Arial">The Sauce light washer and many other commercial products use this type of circuit for it's low cost and high efficiency.
</font>[/list]<font size="2" face="Verdana, Arial">I have two nightlight shells I'm converting now. Why? Mostly because I don't like the color or energy usage of incandescents, don't like orange neon, electroluminescent aren't bright enough and fade after a year or two, and it's fun!

Originally posted by lux0:

Anything designed to be plugged into a 120VAC outlet will withstand that voltage. If a lower voltage is needed, then a step down transformer should be used. It's possible to build a low voltage supply using the reactance of a capacitor to drop voltage, but these are inefficient and dangerous. The only benefit from them is low cost.

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<font size="2" face="Verdana, Arial">COmpletely missed that paragraph until it was quoted

I have two nightlight shells I'm converting now. Why? Mostly because I don't like the color or energy usage of incandescents, don't like orange neon, electroluminescent aren't bright enough and fade after a year or two, and it's fun!

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<font size="2" face="Verdana, Arial">Exactly. I have 2 I am (hopefully) going to canabalize...one I am going to put 3mm LEDs in, it goes in the bathroom. The other I want to put a Luxeon in it goes in the kitchen. Underdriven Luxeon nightlight that only turns on at night--very efficient

I can't underscore what php_44 said. Any capacitor for AC powerline use has to be *explicitly* rated for such use. I have a small bag of them here and they are *covered* with different agency approval stickers.

Most people don't worry about when a component fails--they don't have to, nothing bad will happen in most cases. For a flashlight, the worst that can happen is that all of the circuitry frys and the batterys get shorted--which can lead to a small melt down, sure, but *nothing* like what can happen if you short out an AC plug!

When a normal capacitor fails (they tend to get metal ions from their contacts migrating into the 'active' area of the device making them more and more resistive (lower resistance, that is) with time--this heats them up. Once they get hot enough, something melts and the contacts can short together. *Zot* Self healing capacitors are designed to let the material in the active zone fry without causing the contacts to short--thus failing 'safe'. Go to a good electronics supply store some time. Go look at the resistors--they make some specifically designed to fail 'safe' and not flame out.

Be very careful working with AC lines. We'd like to keep all of the members of the CPF. Those of us who mess with this are trained to and are (well, those of us still *around*) *very* cautious.

Checklist to keep Saaby alive...

Self healing capasators
Fuse

What else?

Originally posted by Saaby:
Checklist to keep Saaby alive...

Self healing capasators
Fuse

What else?

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<font size="2" face="Verdana, Arial">Yes, one more - don't play with house current unless you fully understand every aspect of what you're doing. Fire/death/serious injury can result in less than a second.

When developing prototypes that operate on 120V power, most engineers carefully calculate every failure mode and result before proceeding. And then only using an isolation transformer first during initial testing.

Please take this warning seriously, 120V is not like working with batteries.......

Watch out for the problem of 'stuttering' in a nightlight with a photocell. I've seen them get into a rapid strobing for a few minutes and kill an incandescent. I hate to think what that kind of pounding would do to a LED. I'm sure that's why almost any fluorescent can't be used with a photocell or dimmer.

Use a ground fault interrupter (GFCI). Test it.

Put your LEFT hand behind your back before reaching into any 110v system with the tool in your RIGHT hand (so the path to ground does not go past your heart).

KEEP your left hand behind your back.

I learned that when I wired my own house -- hired a competent union electrician to plan it and start teaching me by working with me on it. We got through most of the city inspections before she departed -- she went off to medical school. The inspector kept saying "Beautiful job! beautiful" -- I overbuilt for safety at every step.

this house has flaky wiring, grounds I mean...Faught with a GFCI outlet for 2 or 3 hours in my room before we gave up...Some idiot built this house.

Maybe I'll hold off on the nightlight conversion, for now at least...

Don't see why a photocell flickering could ruin a LED. Sure it would wreak havoc with the curcuts that keep the LED from melting but in a theoretical situation where a photocell was hooked directly to a LED, as long as the maximum voltage never exceded the LEDs maximum it wouldn't be a problem. Only supplying half voltage?? LED doesn't turn on. Flickering on and off? LEDs don't seem to mind that.

Maybe I'll go for something a little safer, like attaching a LED module of some sort to a preexisting AC->DC converter. Yeah, that is sounding better all the time.

These were $1 Nightlights but upon attempting to disassemble I also decited they were designed to be disposable

Still not sure what I want to do though, I don't want to kill myslelf...that's about all I know.

Saaby,

If you're hell bent on a nightlight, I've got 2 boards here that Dat2zip made. You can one for $15 (I think that's what I paid). Nice safe way to light LEDs from 110 volts..........

lambda--the gesture is nice but I think I'm just going to wait around for version 2, I'm in no hurry and I've got plenty of other things to keep me occupied until it comes around.

It will come around right Wayne?

Originally posted by dat2zip:
Don't let Darell see this post. I know he wants more.

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<font size="2" face="Verdana, Arial">Heh. Don't wake Darell up, whatever you do!

Ryan just told me that I found this post all on my own, so that's the story I'm sticking with.

Yeah, when Xmas-gift time comes rolling around, I'll be knocking on your door again Wayne! I think I've finally maxed out on these around my house - until I add more outlets

No, no, no! This is good! LEDs have *no* problems with pulsing--well, there is a thermal cycling problem like *anything made out of matter* will have, but nothing *major*. I mean, what do you think most dimmable LEDs do? They're PWM'd--pulse width modulated. Pick some frequency above the human visual systems ability to perceive, and modulate the brightness of the light by changing the width of a pulse at that frequency. So, the light turn on and off hundreds or thousands of times a second. They run that way for decades....

So, this is good! Incandescent bulbs take time to heat up and cool down, so the 'strobing' is very slow, but with an LED for the light produciton, the turn on and turn off times are very short (in the terms of hundreds of nS or less). You'll get a free 'dimmer' effect when the nightlight turns on and off! Great! This is a feature!

Now, most night light assemblies with a photodetector in them will be designed for incandescents, so they'll have some circuitry in them designed to prevent the 'strobing'--which may keep you from getting the dimming effect. So, the rule is, guy the cheapest night light you can find--hoping that they cut costs by leaving that part out.