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From: Fred Bloggs
User-Agent: Mozilla/5.0 (Windows; U; Win 9x 4.90; en-US; rv:1.0.1) Gecko/20020823 Netscape/7.0
X-Accept-Language: en-us, en
Subject: Re: Flash a string of LEDs
References: <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <3DEF8EED.email@example.com> <firstname.lastname@example.org>
Date: Thu, 05 Dec 2002 18:17:33 GMT
NNTP-Posting-Date: Thu, 05 Dec 2002 10:17:33 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Spehro Pefhany wrote:
> 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?
RB1=10-100K and RB2=4.7k with a 160uA peak input current source driving
a FWB as well as sims using a voltage source with series R. Rs set for
20-100mA peak currents.
> 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.
Actually it does not, and this is because the input AC cyclically goes
to zero so that the 25u discharges into the zener with this discharge
current reverse biasing Vbe,Q3. So turn-on is guaranteed.
> 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.
Actually, it is not that poorly defined. Using a 15V zener and RB2=4.7k,
the circuit fires at 14.85V. The higher zener (avalanche) voltages have
sharper knees as you may recall. Turn-off is THE issue here. Q3 has an
overwhelming beta-advantage in stealing Q2's base drive. There is a
complicated relationship between the ratio of the Q3/Q2 base drives and
the peak output capability of the AC input.
My latest, although not best, sim on file is:
Please view in a fixed-width font such as Courier.
| | |
--- | |
--- 25u | |
| | |
| ___/ Q1 |e
| ZD // \ \|
| --- 2N4403 |---+
| 1N5245B| /| |
| 100 | | |
+-/\/\--+ | | |
| | +------------+ |
--- | | | |
/ \ | | | |
--- ~~ | | | |
| | |/ | |
--- +---| 2N4401 | |/
/ \ | |\ +---| 2N4401
--- ~~ | e| Q3 | |\
| | | | e| Q2
--- / | / |
/ \ 10k | 4.7k |
--- ~~ / | / |
| \ | \ |
| | | | |
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