Today's Message Index:
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1. 12:04 PM - Re: Next Gen 9005 (Eric Page)
2. 03:56 PM - Re: Re: Next Gen 9005 (Robert L. Nuckolls, III)
3. 04:12 PM - Re: Re: Next Gen 9005 (Robert L. Nuckolls, III)
Message 1
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| Subject: | Re: Next Gen 9005 |
As a non-engineer/hobbyist I enjoy seeing these projects develop, as I always learn
something new. With that in mind, I have a couple of questions.
I realize the component values aren't established yet, but I'm not familiar with
the arrangement of the voltage reference. I presume that it's a TL431 or similar,
but I don't fully understand what it's doing in the circuit. Is it there
to provide a stable Vdd that's within limits for the op-amp (max 6V for the
MCP602, if that's what you're using)? Am I right that R116 (flag note 1) will
need to be something like 270 ohms to keep the 431's cathode current within
limits?
Frankly, the rest of the circuit's operation is a bit of a mystery to me as well,
but I'll take one bite at a time!
Thanks, Bob.
-Eric
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=505624#505624
Message 2
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| Subject: | Re: Next Gen 9005 |
At 02:04 PM 1/7/2022, you wrote:
>
>As a non-engineer/hobbyist I enjoy seeing these projects develop, as
>I always learn something new. With that in mind, I have a couple of questions.
>
>I realize the component values aren't established yet, but I'm not
>familiar with the arrangement of the voltage reference. I presume
>that it's a TL431 or similar, but I don't fully understand what it's
>doing in the circuit. Is it there to provide a stable Vdd that's
>within limits for the op-amp (max 6V for the MCP602, if that's what
>you're using)? Am I right that R116 (flag note 1) will need to be
>something like 270 ohms to keep the 431's cathode current within limits?
>
>Frankly, the rest of the circuit's operation is a bit of a mystery
>to me as well, but I'll take one bite at a time!
>
>Thanks, Bob.
Yes, it's an LM431, precision adjustable zener.
It's Vref is 2.495 Volts. So two equal resistors
in the voltage divider will configure a precision
5v zener to establish Vcc for the dual opamp.
The tap on the voltage divider also provides
the precision reference for the first op amp
configured as a comparator with a tad bit of hysteresis.
Yeah, we'll pick a current limit resistor which
in this case can be pretty big . . . the i.c.
Icc is low so the power supply rail doesn't need
to be driven so hard.
I've been using the LM431 in a lot of designs
as Vcc/Vref sources with only 4 components!
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
Message 3
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| Subject: | Re: Next Gen 9005 |
>Frankly, the rest of the circuit's operation is a bit of a mystery
>to me as well, but I'll take one bite at a time!
The comparator references at the (-) op amp input
and samples bus voltage trough a divider at the (+)
input. A resistor from output to (+) sets up
hysteresis. For the comparator, hysterisis will be
on the order of 500 mV at the bus.
Second op amp is an astable multivibrator with
timing established by the RC time constant of
the network between output and (-) input. The
multivibrator hysteresis will be about 1/3
and 2/3 of Vcc.
When the comparator sees voltage above setpoint
it's output will be HI which is conducted through
the diode to (-) of the oscillator causing it's
output to be clamped LO and holding the FET
in an OFF state.
As bus voltage drops below setpoint, the comparator
output goes LO and releases its grip on the oscillator
which then begins to illuminate the warning light at about
2.5 flashes per second.
This design lends itself to a real estate conservation
technique I've used for years. The 8-pin DIP opamp
has pins long enough to STRADDLE a row of four,
1206 case components underneath. Similarly, four
more can be lined up between the rows of pins on
the bottom. So within the footprint of the i.c.,
a total of nine components can be wired up. The
FET is a SOT23 device so this board should be
quite compact.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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