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From: Spehro Pefhany
Subject: Re: Flash a string of LEDs
References: <email@example.com> <firstname.lastname@example.org> <email@example.com> <3DEF8EED.firstname.lastname@example.org>
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Date: Thu, 05 Dec 2002 17:57:58 GMT
NNTP-Posting-Date: Thu, 05 Dec 2002 12:57:58 EST
On Thu, 05 Dec 2002 17:37:02 GMT, the renowned Fred Bloggs
>Right- and this doesn't make sense to me that you should take a source
>with significant AC on it, clean it up, and then use it to produce AC
>again. I have been simulating the circuit shown below with good results.
What component values have you been simulating with? This circuit
also has a theoretically stable state near the trigger point, as
well as possible latch-up in the "on" condition as you mention.
Is there a way to use an off-the-shelf PUT such as a 2N6028?
The main issue with that part is the turn-off, plus you need a
poorly characterized low current zener for the trigger since
there is no reference voltage available.
> view in a fixed-width font such as Courier.
> RECT AC---+-------------+------------+
> | | |
> --- | |
> --- 25U | |
> | | |
> / ___/ Q1 |e
> RS ZD // \ \|
> / --- 2N4403 |---+
> \ | /| |
> | | | |
> +-------+ | | |
> | | +------------+ |
> --- | | | |
> / \ | | | |
> --- ~~ | | | |
> | | |/ | |
> --- +---| 2N4401 | |/
> / \ | |\ +---| 2N4401
> --- ~~ | e| Q3 | |\
> | | | | e| Q2
> --- / | / |
> / \ RB1 | RB2 |
> --- ~~ / | / |
> | \ | \ |
> | | | | |
>There is very strong positive feedback action with the exception of
>difficult to quantify action at turn-off. The SCR is triggered by the
>voltage charge-up of the 25uF discharging through the zener, so that the
>trigger point and peak currents are well defined. Rs sets the discharge
>current level and RB1 gives the circuit immunity from timing capacitor
>leakage. RB2 is selected so that the zener current is somewhere at the
>knee region but below the peak output capability of the rectified AC-in.
>This is easily done with the stated 50uA source capability at full
>charge. Complexity comes in at turn-off when the capacitor has fully
>discharged. The Vbe,min/RB2 must be large enough to cause sufficient
>AC-coupled ripple current through the 25uF to the Q3 base circuit to
>shunt the Q2 base drive and induce turn-off, which is quite rapid and
"it's the network..." "The Journey is the reward"
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