Light bulb not lit up by inductor

[LoveLaika]

Thanks so much. I think this is what I need, and it makes sense.

[vk6zgo]

Good Morning Lovelaika,  substitute an  analog voltmeter for your bulb and if you've got the meter on it's lowest range, you 'should' see the needle twitch, but just once.

Not with the OP's original values.
The inertia of the meter movement will prevent any twitch.

[vk6zgo]

   My problem is visualizing what happens during the knife switch travel time, which is a century compared to time constants.
   Moving that switch in 300 mSec. is approx. 300 X the 1 mSec. flash time.

   So my curiosity is about what happens when the circuit is just 'open' sitting there with magnetic field at max.
   I recall that you would use parasitic capacitance, to get a 'tau' describing decay.
You have decay of that magnetic energy, right ?
Even if totally open circuit, I believe it was the parasitic capacitance that aids in the drain-down.  I'm saying that occurs way, way before your knife edge does it's travel over to make contact.

As soon as the switch removes the supply, as the magnetic field can't be maintained without current from the supply, it starts to collapse, inducing reverse current into the coil.
It can't flow anywhere, there is a build up of voltage across the coil, which can be discharged through any load now switched into circuit.
With a very low inductance value, the voltage will be small to start with, & probably will decay rapidly.through the stray capacitance.

Storage in an inductor is a real thing---Google Kettering ignition.

[RJSV]

   Ahh;  thanks,  nice to ask my own basic physics question,  as I feel 'irritated' and obsessed...good physics explanation.

   Anyway that drain-down isn't going to leave much.  (I'm going to go search on that ignition.)

   OP:  Do you really want an incandescent (filament) type,  as LED component seems easier with less components ?

[coppercone2]

i think you need a bigger inductor so it explodes the light bulb

[LoveLaika]

Testing with an LED seemed to work a lot better than a bulb, as in, the LED lit up briefly while the bulb did not.

[RJSV]

   Could assume,  use the 30 volt power input, if you need to experiment with a filament.
For an LED, a low voltage probably 1.5 volt,
would do.   I'm never comfortable with leaving out a series resistor, like 59 ohms.
That drops LED voltage by roughly 500 mV for every 10 ma flowing (at peak).
Then,  I'd use the knife switch but only TWO of the terminals,  as the travel time is way, way too long...unless you try some ballistic method of super fast (and ridiculous) action.

   You get the one flash, with LED, and can put a storage scope onto the series resistor to convert to current flow for a reading of peak current.

   A filament has that messy change in impedance,  as it heats up.  Plus needing that 30 volt supply, then.

[RJSV]

Reply #12 has the good schematic,  thanks to Zero999 for posting that.

   I would add the 59 ohms series resistor, for limiting any excess LED current.

   Sorry about this taking time to resolve,  I've got a todo stack bigger than my own ego.

[Picuino]

Another practical circuit:
https://www.picuino.com/circuits/?ctz=CQAgjCAMB0l3BWcMBMcUHYMGZIA4UA2ATmIxAUgpABZsKBTAWjDACgwM9wVvjCe3TgIEQYNKLDDYMdQpHmFMcjEhiQkYePEmQUFOLrBrYatgDcQTGkOL7r3G1Gc0qYfVU-QEbAO5WnMDtaWw8-EH5wDAFIoLD-BxDBJMhw90chXgiBVP8wGglhKNFoqDTSoqK8T3KBauyQetyG+sjI1LAlJLik+v0AEwYAMwBDAFcAGwAXJgmGfucxI3CnVrqa-0Dg2ODUgHtkRucADzwtVhA0SSX9LUWqenUNKxw2IA

You must use an LED with the same maximum current as the relay coil (or a combination of LEDs in parallel that has a maximum current equal to or greater than the relay coil current).

[RJSV]

Thanks.  But I'm only partially following the oscillating circuit.
   What's the longer time period,  130 milli-secinds ?   That seems like the relay 'buzzing' at around 8 hz,  which would be about right.  Of course that's not a very robust oscillator,  as the moving contact just barely gets into an 'open' state,  but not certain if that is any problem.

   The short time periods makes sense, also,  but I'm having difficulty relating the two, looking at that pulse waveform shown.
   I think you just would get an illuminated LED similar to the solar garden light situation.

   

[SteveThackery]

But I'm only partially following the oscillating circuit.

Me too. I cannot see why that circuit should oscillate UNLESS the lamp is a device showing negative resistance - perhaps a neon lamp.  But neon-basednoscillators normally just use a capacitor.

[Zero999]

But I'm only partially following the oscillating circuit.

Me too. I cannot see why that circuit should oscillate UNLESS the lamp is a device showing negative resistance - perhaps a neon lamp.  But neon-basednoscillators normally just use a capacitor.

I can confirm the relay circuit will oscillate. Relays have hysteresis. It's a Schmitt trigger oscillator. I made one when I was a child, which I used as a shocker. I didn't put anything in parallel with the coil, just the terminals to zap people with. It's an old circuit, which has been used as a boost converter many years ago.

[RJSV]

Yeah, I mean,  (just so other readers are following this),  you can work out the behavior, mechanically:
   With voltage to the coil, that relay contact should start opening up the circuit,  but that's going to cut the power to itself, soon.
But, mechanically there will be delay, as the contact is on a springy metal,  which also has helped make good switch contact.
I'd guess 10 or 50 mSec. to pull off so contact is broken.  (That's the schmitt-like hysteresis).

   So, whatever that time takes, the LED is so much faster, you will get a pulse and the waveform has 6 uSec. duration.
But then we have to wait, a bit, until the relay contact spring brings the contact back.  That is, back to connect as the normally closed position it sits in,  when un-powered.
Thats some more milliseconds, maybe 100.


   I'd suggest, get rid of the oscillating relay, that could be all over the map anyway.
(Can I use the relay from my scooter ?  No.)

   Use your knife switch,  instead of the normally open relay contacts.  Then just open it, once.

[RJSV]

Oops,  I meant the normally open relay  contact.

[Zero999]

Oops,  I meant the normally open relay  contact.

The circuit will only work with a normally closed relay contact in series with the coil.

[SteveThackery]

Oh, damn! I didn't realise it was a relay!  What an idiot.  I was fooled because the animation does not show the relay contacts moving.

Sorry - bit of a brain fart there.

[Zero999]

Reply #12 has the good schematic,  thanks to Zero999 for posting that.

   I would add the 59 ohms series resistor, for limiting any excess LED current.

R1 already limits the current through the coil and therefore the LED.