Close

Stable States

A project log for Incandescent RAM

Use lightbulbs to store data!

eric-hertzEric Hertz 11/17/2018 at 20:118 Comments

Carrying on discussion from this project's front page from @DeepSOIC



DeepSOIC wrote 20 hours ago

Once upon a time, I thought, that there can be a lightbulb, that, if powered with a constant current source, will have two stable states. In one, resistance is low, and I^2*R is not enough to heat it up. And if it's hot, R is high, it dissipates more power, and sustains hotness.

I have never found a bulb that would do it though. I'm pretty sure, a thing somewhat like that can be achieved with paralleled leds, provided enough heat isolation of each led.

@roelh  wrote an hour ago

I tried this once (with series resistor instead of current source). Once connected, the brightness of the lamp came up very slowly. It might be used for a time delay ! But indeed, no two stable states.

A lamp has a positive temperature coefficient. But would the two-state effect be possible with an NTC resistor ?

Discussions

roelh wrote 11/18/2018 at 19:53 point

You guys know how to keep me busy... got me thinking again...  How many stable states can a two-transistor circuit have ? How about four ? No clock signal this time... Just resistors and 4 LEDs that show the state.

  Are you sure? yes | no

Eric Hertz wrote 11/19/2018 at 00:21 point

Hmmmm, I'mma have to see this one. Be sure to link it here. Props, btw, for your 1-transistor 'memory' a while back!

  Are you sure? yes | no

roelh wrote 11/20/2018 at 21:53 point

  Are you sure? yes | no

Eric Hertz wrote 11/25/2018 at 02:27 point

Slick

  Are you sure? yes | no

roelh wrote 11/17/2018 at 21:06 point

Indeed PTC devices are (or can be) two-state devices, like the polyfuses that are used for circuit protection (example PTS120660V005).

Problem with the bulb seems to be, that the filament is thermic isolated from outside world, by a vacuum around it. So when a very small current flows, the little amount of generated heat can not escape, and the filament will warm up, leading to a snowball effect. Perhaps cooling the filament would help ?

  Are you sure? yes | no

Eric Hertz wrote 11/17/2018 at 23:11 point

AHH. Indeed. Here I was thinking the isolation would cause it to be less-sensitive to such effects, finding its own equilibrium somewhere in P=VI. But, as you say, the heat can't escape. So, it'll just keep adding up. 

I guess, then, at some point equilibrium is found only when that heat can escape through other means; photons?

This suggests a highly non-linear resistance-power curve, highly dependent on time, etc! Especially at really low currents.

Weird.

  Are you sure? yes | no

Eric Hertz wrote 11/17/2018 at 20:30 point

My response, below, to the original comment grew too large for that thread. Great thoughts @DeepSOIC :

Interesting. It took me a few reads to grasp, but I think I get it. 

I vaguely recall having had similar ponderances long ago about... something, can't recall what.

The idea being that a device/component could satisfy Kirchhoff's laws two (or multiple?) ways depending on factors not typically considered. 

Here, a light being treated mathematically like a resistor, while really being more of a light/heat-producing thermistor, causes the circuit to have multiple stable solutions depending on the bulb's history. Yet, part of that, "history" is the actual "stable" state it's *presently* in, which is highly sensitive, then, to external factors; a cosmic ray might apply just enough extra heat to cause "thermal runaway" eventually causing the bulb/circuit to switch from one once-stable state to the next.

Two of these devices in parallel, driven by a constant-current would split the current 50/50 until a "cosmic ray" hits one, as it heats slightly its resistance increases, the other gets more of the current, causing it to heat as well. One might expect that to balance it back to 50/50, but the "damage has already been done", instead of balance we might get teetering/oscillation with ever-though-slightly increasing heat, eventually causing a switching of states, (plausibly from "working" to "blown")

For individual bulbs, though, there are multiple (long-term) stable states, no? E.G. if a 3V bulb is driven at 2V, it won't heat as much, so will be dimmer, and with a lower resistance. But, of course, at a different current.

That said, your idea still seems plausible, and if so, could allow for, e.g. "refresh" of an entire row of bulbs simultaneously by tying them in series and driving a constant current through. THAT'd be quite something!

And maybe more plausible being that these refresh pulses are just that, pulses, which might be too short to allow thermal runaway.

Hmmm... I needta get some paper!

Hmm, another thought: put a resistor in parallel with each bulb. Say a 'cold' bulb is 10ohms, and 100 when hot. A 50ohm resistor would draw current away from the hot bulb and push more current through a cold one. Erm, no... that's the opposite of our goal. but a parallel PTC... hmmmm

LOL wikip;edia: PTC thermistors 'latch' into a hot / high resistance state: once hot, they stay in that high resistance state, until cooled. The effect can be used as a primitive latch/memory circuit, the effect being enhanced by using two PTC thermistors in:;;; series, with one thermistor cool, and the other thermistor hot.[7]

  Are you sure? yes | no