electronics noob asks: i can fade a bunch of LEDs using PWM (and an FSR); is it possible to use a standard low power (6v, 0.5A) lightbulb and get the same response; the bulb lights up off 5v/gnd from the board, but i'd like to get it to fade using PWM, but it won't
Noob as well, but 500ma is far too much from an ardunio pin, you'll burn it out trying to run that. I can see it happening with a transistor but not from a pin directly. Obviously an incandescent bulb warms up and cools down differently to a LED but I don't see why a PWM signal wouldn't make it fade.
I don't see why a PWM signal wouldn't fade a bulb, despite the warm up and cool down times being much longer. Full pulse is no different than 5v DC and no pulse is 0v. Between them the bulb wont have time to cool down or warm up properly sure, but I can't see a reason why a 50/50 pulse wouldn't supply half the amount of joules to the bulb, making it half as dim.
The problem he seems to have is that while connecting to the 5v line will pull the .5A needed, any of the arduino pins won't produce anywhere near that current, more like a tenth of it.
It will work, with a transistor or other higher-current sourcing/sinking interface in place; the same type of control is used in "modern" lighting dimmer controls for household light (I put "modern" in quotes, because the push is to use compact flourescents and not incandescent lights - these dimmer controls only work with incandescents, so are they still considered "modern"? Idunno)...
Now - what I don't know is whether you can use PWM with an SSR (solid state relay). I seem to recall reading that you don't want to do that, but I am not sure.
The bulb will stay hot (glowing) long after (milliseconds, but that's a lot of PWM) the voltage is removed, which won't give the effect of dimming..
IIRC, the resistance of the bulb kinda "smooths" it out, and it heats up less resulting in a dimmer bulb. If I had a small DC bulb and socket handy and such, I'd give it a try myself...
The thermal mass of a filament is a fine low pass filter, all by itself.
The heating (and thus the brightness) is proportional to the average current through the filament, and PWM works fine as a way of changing the average current. How do you think a typical household lamp dimmer works (true, the "pulses" are somewhat oddly shaped, but it's still a form of PWM.)
Note that this is a somewhat different mechanism than the way LEDs are dimmed by PWM. With LEDs, you get shorter pulses of essentially full brightness, and your eye does the low-pass filter thing. With an incandescent you actually change the amount of light emitted.
The thermal mass of a filament is a fine low pass filter, all by itself.
I figured as much but since I have not sat down and tried it ...
If I had a small DC bulb and socket handy and such, I'd give it a try myself
I dont have a small dc bulb, but what I do have is a small xmas tree bulb that lights up full blast at 5 volts (who knows how much current it draws ... yet) I am pretty sure its a standard issue 120v jack squat wattage bulb, but it was in a string we bought last year so who really knows...
so I will get it a try my self, if nothing else but for SnGs
a small xmas tree bulb that lights up full blast at 5 volts (who knows how much current it draws ... yet) I am pretty sure its a standard issue 120v jack squat wattage bulb
xmas tree strings usually connect 30 to 50 bulbs IN SERIES, so that when you put 120V across the whole string, you get 2 to 4 Volts across each bulb... Sometimes the packages of replacement bulbs will list the actual per-bulb voltage (though why anyone would buy such a package, at about 10x the price per bulb of a string, I dunno...)
The thermal mass of a filament is a fine low pass filter, all by itself.
this is 100% correct, the burn / fade property of the filament acts like a low pass filter, averaging the applied current as heat / light
the xmas tree bulb I had measured a resistance of 40 ohms, ohms law puts that at 125ma dead cold, course resistance changes with heat
using a full sized 7805 voltage regulator with flimsy heat sink rendered it almost hot to the touch after a relativity short amount of time with a 9 v supply and 1 lamp, previous measurements suggest a few hundred constant ma with this non standard setup, so be careful, especially when hooking more than 1 lamp up at a time
Also since were drawing a bunch of current through a inductor a transistor was used to actually supply power to the single lamp, a 2222, which is rated at ~600 ma
Ok, so we know it works; we also know that in some manner we can scale it up to full sized bulbs, but the question is "what to control them with"?
SCR? Triac? SSR?
I want to say "for a beginner" that the SSR is the way to go; but if you had the experience (to wire everything up and make it safe), then one of the first two would probably be best (smaller package and such, but you would still need heat-sinking, likely)...
SCR, will only work with AC. (Yes you can use them with DC but not for dimming)
A triac is simply two SCRs wired back to back so it can handle AC in both directions.
Most SSRs are for use on AC only and contain a triac. However there are some SSRs that you can use for DC. These are basically FET switches and are probably the best for low voltage applications.
an SSR is frequently AC only, and switches at the zero crossings, meaning that you'd have to have (significantly) less than 60Hz PWM "frequency." The original question was about 6V, 500mA lightbulbs; for which many transistors would work fine. Or something like a ULN2803 (which is 8 darlington transistors in a chip; frequently used for driving motors and relays.)
To get going quickly, a motor shield would work (but it would get expensive for lots of lights.)