single transistor oscillator

On Thursday, October 24, 2013 3:05:57 PM UTC-4, Vivek N wrote:
> I tried building a single transistor LED flasher like this :
>=20
>=20
>=20
> http://cappels.org/dproj/simplest_LED_flasher/Simplest_LED_Flasher_Circui=
t.html
>=20
>=20
>=20
> I didn't have the recommended 2n2222 - I tried with a BC548, 2n2369 and 2=
n3055 - None of them work.
>=20
>=20
>=20
> I'm using a trimpot set to 2 kiloohms and a 470 microfarad capacitor. Ins=
tead of a 100 ohm resistor I put 3 superbright LEDs in series.
>=20
>=20
>=20
> I get 1.2 volts across the LED and 8.6 volts across the capacitor. My sup=
ply is exactly 12.8 volts
>=20
>=20
>=20
> Does this required negative resistance effect only work with specific tra=
nsistors?

No- you can make it work with any transistor. The circuit should work with =
the BC548. The problem is your 2K resistor is allowing too much current thr=
ough the transistor in steady state for it to shut off. With a 12.8V supply=
 and 8.6V across the capacitor, that makes for 12.8-8.6=3D4.2V across the 2=
kR for 4.2/2k=3D 2.1mA, which is too much. As a first cut, I would replace =
the 2k with a 20kR. Dunno anything about your superbrights so not going to =
speculate, but putting a 100R in series with them can't hurt. This should m=
ake it flash oscillate.

On Thursday, October 24, 2013 3:49:13 PM UTC-4, Jim Thompson wrote:
> On Thu, 24 Oct 2013 12:05:57 -0700 (PDT), Vivek N
> 
> <rep.movsd@gmail.com> wrote:
> 
> 
> 
> >I tried building a single transistor LED flasher like this :
> 
> >
> 
> >http://cappels.org/dproj/simplest_LED_flasher/Simplest_LED_Flasher_Circuit.html
> 
> >
> 
> >I didn't have the recommended 2n2222 - I tried with a BC548, 2n2369 and 2n3055 - None of them work.
> 
> >
> 
> >I'm using a trimpot set to 2 kiloohms and a 470 microfarad capacitor. Instead of a 100 ohm resistor I put 3 superbright LEDs in series.
> 
> >
> 
> >I get 1.2 volts across the LED and 8.6 volts across the capacitor. My supply is exactly 12.8 volts
> 
> >
> 
> >Does this required negative resistance effect only work with specific transistors?
> 
> >
> 
> >
> 
> 
> 
> It requires the transistor to avalanche... that is, be lucky.
> 
> 
> 
> It's crap circuit design.
> 
> 		
> 
>                                         ...Jim Thompson
> 
> -- 
> 
> | James E.Thompson                                 |    mens     |
> 
> | Analog Innovations                               |     et      |
> 
> | Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
> 
> | San Tan Valley, AZ 85142   Skype: Contacts Only  |             |
> 
> | Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
> 
> | E-mail Icon at http://www.analog-innovations.com |    1962     |
> 
>              
> 
> I love to cook with wine.     Sometimes I even put it in the food.

Load line of the collector circuit doesn't intersect the Ie/Vec curve below Ie breakdown sustaining current...