now here's a nifty little New Year puzzle for you...
what happens when the switch S1 connection changes from position 3 to position 2?
try & predict without building first!
i'd be interested to hear what a Sim makes of this
cheers
sandy
(Background:
i thought it was about time i experimentally tested my view of the old standard: 'charge two caps in parallel, feed back into the source as two caps in series'
i was surprised to observe something rather unusual with this config
oh, and i confirmed that the battery discharges, btw - but that's NOT unusual - with conventional batteries!!)
OK, here's a quick stab:
1) When S1 = 3, C1 will charge to 1.24V. C2 is previously charged to 1.24V through D1.
2) LED should be OFF.
3) When S1 goes to 2, C1's voltage may get clamped down somewhat by D1, and the battery will receive some energy back due to C1 and C2 being in series with their voltage > 1.24V.
4) Re-charging will cease when C1 + C2 has discharged to roughly 1.24V.
5) The LED should never illuminate.
.99
hi poynt
in my view, that's a good - and entirely reasonable - call
i'm glad to hear that someone else has the same expectation of what should happen
strangely, the circuit seems to have something extra up it's sleeve
any other takers - or further ideas?
regards
sandy
(in case anyone's wondering why i designed the circuit exactly this way - i didn't;
i noticed something odd by accident and tailored my original 'parallel to serial cap' circuit to focus on this new behaviour)
What I described above assumes S1 is a perfect switch.
As this is rarely the case with mechanical switches, there will be "switch-bounce" between the common contact (1) and the position (2) contact. As such, the L1 inductor may certainly kickback a number of times resulting in the LED illuminating for a brief moment.
.99
Perhaps sweeping the switching frequency, there can be a magic one (lol) that (either by resonating somehow L1 and combination of C1 and C2 or hitting a favorable time constant value) may enhance voltage level in any capacitor a little? (IF any increase shows up, voltage level in C1 is clamped by LED to 3.2V max at switch position 3.)
Diode D1 should be low barrier Schottky or Germanium to minimize its forward drop loss (it misses from the 1.24V C2 could charge up).
Looking forward to your finding too.
Gyula
all good points, gentlemen
when is a switch 'not a switch' - nearly always, it seems
and as you both suggest, it's the dynamics of the system that produce the interesting emergent features
and the interesting emergent feature of this system, as you've probably guessed by now, is that the LED does indeed flash
not only that, the LED flashes when S1 gets switched to short-out the LED
for there to be any inductive kickback present, our 'switching' would have to interrupt current flow through L1 associated with C1
ok - so let's pull the 'ideal switch' apart and use flying leads, to give us a little more control over the 'switching' action
C1 appears to fully charge in less than a second and C2 takes a few seconds
(C1 charges to 1.24V, D1 is Schottky so C2 charges to approx 1V)
if we leave S1 in position 3 long enough, we can charge C1 fully and we can ensure also that C2 is fully charged - then there should be no current through L1 to interrupt when we disconnect S1 from position 3
let's also leave S1 totally disconnected for a few seconds - this should also ensure that any inductive kickback in the coil, in the charging phase, has passed
(i think we're being over-cautious here, but that in itself shouldn't be a problem)
ok, so now that current has ceased to flow through L1 - let's connect S1 to position 2 (S1-2)
what are we connecting? we're establishing a short-circuit across the LED
(which as Gyula pointed out, has previously had the effect, together with D1, of ensuring that the voltage on C1 couldn't exceed around 3V)
so, by connecting S1 to S1-2, we're starting to establish a series connection for C1 & C2 (max. possible combined voltage is approx 2.4V) - we're also starting to clamp C1 with D1 (which will reduce combined voltage to approx 1.2V)
if the S1-2 connection experiences 'contact bounce' then one of two things can be happening at any instant:
a) S1-2 is closed:-
LED is shorted;
C1 is partially discharging shunted current via D1;
if the instantaneous voltage on C1 + C2 > 1.24V
then some current will also flow back into the battery
b) S1-2 is open:-
LED is not shorted;
if the instantaneous voltage on C1 + C2 > 1.24V
then some current will flow back into the battery
obviously current will only flow through the LED in its forward-biased direction, so the only circumstance when the LED might light is when the combined voltage on C1 + C2 > (1.24V + Vforward of LED)
Vforward of a white LED is likely to be >2V, so the combined voltage on C1 + C2 would need to be > 3.24V, and for a sufficient time to make a visible flash
we know that the combined voltage on C1 + C2 is approx 2.24V - which does not appear to be sufficient to light the LED
interesting
therefore some inductive kickback must be occurring in order to achieve a sufficiently high voltage to overcome the combined battery & Vforward voltages - but it can only be occurring on the recharge, not the charge cycle
so as poynt suggested in his update, a 'real' switch can have much more interesting effects than an 'ideal' one
a switch which we believed to be shorting a component out of a circuit can, under certain conditions, actually be causing the component to be significantly active (for a short period)
Quote from: nul-points on January 15, 2011, 04:40:54 PM
all good points, gentlemen
when is a switch 'not a switch' - nearly always, it seems
i wont play in your thred .. i will add this
when a mechanical switch isint a switch it is a ........ ?
lol
a transmitter ... and a reciever ..
;D
william