ive got a string of LED christmas lights here, and have discovered i can substitute the tiny 2 AA power supply with one 18650 and it'll run fine?
@pearshapes I have to advise against doing that, though, unless it's a protected 18650. A bare lithium cell hooked up like that runs the risk of over-discharging.
@Rosemary a protected 18650? the ones i have have protection circuitry i believe
@pearshapes Make sure of that, if you can. While overdischarging isn't as risky as overcharging, especially in terms of fire potential, it can still cause problems.
@Rosemary yeah, it's the little lip on the positive terminal right? there's an IC in there?
@pearshapes I'm not sure exactly how they arrange them. I would look up the part number if there's one printed on it.
@Rosemary you think i should just lump a thermistor on it or st if i ever consider running it for a long time? lol
@pearshapes Unfortunately, you need more advanced control than that. If a lithium cell ever discharges below about 2.5 volts, it will never be able to charge back to its full capacity again. It doesn't matter how slowly it discharges to that point.
@pearshapes If you're not concerned about being able to recharge the battery though, you should be okay to use it. Just know that you'll be irreversably damaging the battery.
@Rosemary maybe... a zener diode rated for 3v?
@pearshapes If you put that in, you won't have enough voltage left to power the lights.
@Rosemary so, what, does the zener just absorb the power? i dont understand
@pearshapes Could you explain what circuit topology you're thinking of? Draw a schematic or something?
@Rosemary i'm no electronics engineer! i'm just tinkering
i'm a software developer and i pulled a multimeter out of a bag i found while cleaning then clipped some components off of a busted power supply's board and went "huh, zap zoop do funny thing with needle"
@pearshapes Right, but I'm trying to understand exactly where you intend to put the zener diode, so I can explain what happens with it.
@Rosemary just like. slurp that thing straight into the circuit and cause a small explosion
@Rosemary like, in series with the battery
@pearshapes Ah, yes, the thing about zener diodes is that the voltage across a reverse-biased zener diode is fixed to its zener voltage. The first-order approximation is to think of it like a voltage source.
@Rosemary so if i were to put a 18650 on it at 3.7v, it would throttle the voltage in the circuit to 3v and just dissipate the rest of the power? thats suboptimal but at the same time it kinda is what i want to achieve, since i wanna run a 3v-rated set of christmas LED lights off of a 18650 without fucking it lol
@pearshapes Not with the zener in series. In that configuration, it would have 3v across it, so the rest of your circuit would be left with 0.7v.
@Rosemary so it... absorbs voltage? or like. what does it do
i'm confused
can i use a transistor and a zener to keep the output above 3v
@pearshapes If you want 3v on the output with less than 3v on the input, you need much more advanced circuitry.
What the zener diode does is divert current through itself until it reaches a balance; the excess voltage is dropped in the resistor.
@pearshapes If you were to plot current vs voltage for a zener diode, you'd have a near vertical line at the zener voltage. The diode will absorb any amount of current to keep the voltage across the diode at the zener voltage.
@pearshapes Felthry wants to front now, so I'll not be around for a little while, but if you want any further help, it should be able to help you just as well as I can. Sorry for the poor timing!
@Rosemary its ok, i should be fine.. im just a little confused and tired atm
@pearshapes If you want a voltage regulator, you can use this circuit: http://falstad.com/circuit/e-zenerref.html
But keep in mind that if the voltage drops below three volts, say to 2.7V, the rest of your circuit still sees that 2.7V. The zener diode and resistor combination only bleeds off excess voltage.