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From: Fred Bloggs
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Subject: Re: Flash a string of LEDs
References: <email@example.com> <firstname.lastname@example.org> <email@example.com> <3DEF8EED.firstname.lastname@example.org> <email@example.com>
Date: Sun, 08 Dec 2002 17:17:04 GMT
NNTP-Posting-Date: Sun, 08 Dec 2002 09:17:04 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? 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.
Okay- I have the SCR under control with up to 50:1 spread on
holding:trigger current levels. The criterion for deterministic
triggering with no possibility of stable state is that the circuit
current draw at trigger point be less than the minimum current available
from the charging source. I am seeing margins of 2:1 here at worst case
0oC analysis runs. The SCR technique works on either AC or DC- no noise
required- and it's fairly parts conservative compared to the astable.
> 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.
Yes- you should be able to go with a PUT also. The zener is NOT that
poorly characterized at low currents, if you take a look at some
detailed data sheets, you will see that the diode is characterized for
maximum leakage current at reverse bias approaching 0.8 x Vz, and this
is well below any reasonable trigger level. This should be enough to
achieve 10% in practice, and is good enough for a LED flasher.
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