Why this simple oscillator/flasher doesn't work?

[RussellH]

I'm trying to understand why this simple led flasher doesn't work. The concept is that the capacitor will charge enough (.6v) to turn on the transistor. The transistor will use up (discharge) the capacitor by switching collector/emitter turning the led on. The transistor will then switch off as the capacitor charges again.

On the breadboard with a small capacitor the led flashes once and turns off. I guess this is because the capacitor is fully charged and won't allow the DC current to pass through it anymore?

With a large capacitor the LED stays on all the time. I'm guessing this is because the capacitor is not fully charged so it allows the DC current to pass through?

Sorry I'm new to this and I'm trying to understand the concepts so I can hopefully design my own circuits later on.

 

[ericgibbs]

I'm trying to understand why this simple led flasher doesn't work. The concept is that the capacitor will charge enough (.6v) to turn on the transistor. The transistor will use up (discharge) the capacitor by switching collector/emitter turning the led on. The transistor will then switch off as the capacitor charges again.

On the breadboard with a small capacitor the led flashes once and turns off. I guess this is because the capacitor is fully charged and won't allow the DC current to pass through it anymore?

With a large capacitor the LED stays on all the time. I'm guessing this is because the capacitor is not fully charged so it allows the DC current to pass through?

Sorry I'm new to this and I'm trying to understand the concepts so I can hopefully design my own circuits later on.

hi Russell,

As you say the capacitor charges up and stays charged up.
A larger capacitor just takes longer to charge up.

You need a second transistor in a circuit that discharges the cap so it keeps repeating the flash

Have a look at this astable circuit.
**broken link removed**

Example:

 

[RussellH]

Hi Eric,

Thanks for the link. I actually have a book that shows a 2 transistor LED flasher but being the minimalist I am I was wonder why I couldn't discharge the capacitor using a single transistor Perhaps by using a feedback of some sort. After all the transistor is a switch so when it turns on I should be able to get an LED or something to discharge the capacitor no?

Here's one (bottom of the page called cheapo oscillator) but I don't quite understand what "NC" means and how this works.

http://members.shaw.ca/roma/twenty-three.html

 

[Hero999]

Or build a reverse bias oscillator.

http://www.geocities.com/capecanaveral/lab/5185/circuit2.html#SINGLE

 

[RussellH]

Thanks. What exactly is different about reverse-bias that makes the same transistor function differently?

Where can I find more info. Anyone have a source that breaks down circuit at the component level to tell you how and why the circuit does what it does. I look at circuits and I go...how did they ever figure that out?

 

[Nigel Goodwin]

Thanks. What exactly is different about reverse-bias that makes the same transistor function differently?

Where can I find more info. Anyone have a source that breaks down circuit at the component level to tell you how and why the circuit does what it does. I look at circuits and I go...how did they ever figure that out?

It's simply using the reversed biased BE junction as a zener diode, they usually zener about 7V or so?.

 

[Hero999]

It's not zener breakdown but avalanche breakdown.

The voltage on the capacitor builds up until the transistor suddenly conducts causing it to discharge through the LED until the current is so small it turns off after which is charges again; this cycle repeats over and over.

 

[Nigel Goodwin]

It's not zener breakdown but avalanche breakdown.

Depends what you want to call it I suppose?, but you can use the BE junction as a zener diode, about 7V or so - it's a fairly common application.

 

[blueroomelectronics]

It's not zener breakdown but avalanche breakdown.

The voltage on the capacitor builds up until the transistor suddenly conducts causing it to discharge through the LED until the current is so small it turns off after which is charges again; this cycle repeats over and over.

It looks similar to a white noise circuit I've seen somewhere.

 

[ericgibbs]

It's not zener breakdown but avalanche breakdown.

The voltage on the capacitor builds up until the transistor suddenly conducts causing it to discharge through the LED until the current is so small it turns off after which is charges again; this cycle repeats over and over.

hi hero,
You are correct its a Vceo, avalanche breakdown relaxation oscillator, nothing to do with Vbe zener operation, thats a different effect.

 

[Nigel Goodwin]

hi hero,
You are correct its a Vceo, avalanche breakdown relaxation oscillator, nothing to do with Vbe zener operation, thats a different effect.

I looked at the circuit wrong!

 

[ericgibbs]

I looked at the circuit wrong!

Hi Nigel,
It happens to us all.

 

[Roff]

hi hero,
You are correct its a Vceo, avalanche breakdown relaxation oscillator, nothing to do with Vbe zener operation, thats a different effect.

Notice, though, that collector and emitter are swapped. In an NPN, Vceo is specified with the collector positive with respect to the emitter. I think it's running as an inverted transistor. It's a (little known?) fact that many transistors have beta>1 when run in inverted mode. Using it this way allows you to use a lower supply voltage than if you had to break down the C-B junction (I have built an oscillator this way). In this circuit, base-emitter breakdown is still the mechanism that initiates current flow. Beta is what provides the negative resistance to make the oscillator.
If I'm full of crap, I'm sure someone will let me know.

 

[bananasiong]

I looked at the circuit wrong!

Did you mean emitter follower? Or some other thing else?

It's simply using the reversed biased BE junction as a zener diode, they usually zener about 7V or so?.

 

[avdrummerboy]

I can't get an LED flashing from either of these circuits

The first one is obvious why it doesn't work, the second on I don't know why it wont work for me.

 

[Torben]

I can't get an LED flashing from either of these circuits

The first one is obvious why it doesn't work, the second on I don't know why it wont work for me.

Hi av, and welcome to the forum.

The second one is a temperamental beast. You might find that it will work with one transistor but not the next one, even if they're the same part #. I've had good luck with 2N2222s. Try a few different transistors in there and eventually you'll find one that works.

The circuit relies on the fact that a reverse-biased transistor can sometimes be made to act like a tunneling diode (IIRC) which in essence gives a negative resistance.

This has been discussed here before, in this thread. I was going to link another old thread which discusses the circuit, but that turned out to be this thread.


Regards,

Torben

[P.S. This thread has been dead for almost 2 years.]