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'[EE]: My first circuit using comparators and seeki'
2010\12\10@001559 by Justin Richards

face picon face
part 1 2325 bytes content-type:text/plain; charset="iso-8859-1" (decoded quoted-printable)

This is my first project using Comparators (and Eagle) and as such
seeking feedback. In particular I am interested in the Do's and Dont's
and suspect that I may need to add additional capacitors perhaps but
decoupling caps only seem to appear on 74 and 4000 series logic IC's.

The circuit is designed to protect a vehicles 12 volt battery from
excessive discharging when camping for example.

Tests on my vehicle 3.0 liter diesel indicate that it will still start
when the battery has drained to a level of 11.55 Volt.

Lower than this level and it tends not to start.

Therefore my requirements are:-

1. Minimal standby current.

2. Remove power from the load if the level drops below 11.6 volts
(adjustable) for more than 20 secs (to eliminate false trips due to
engine cranking and initial in rush current of portable fridge etc.)

3. Re-instate power to the load only if the upper level of 12.8
(adjustable) volts is reached. i.e the charging system is up and
running.


I have seen other examples on the web but they don't meet all the criteria.

The circuit (see attached)  function description is as follows:-
Note: LM339 U1 is the actual comparator in use, and not a LM324 as
shown in the diagram. A MTP305 is used as the output MOSFET Q1.

The 6.2 volt zener sets the reference voltage on both inverting inputs
on the 2 comparators.

The first comparator sets the high voltage reset level and the second
sets the low voltage cut out.

The diode D3 ensures that the feedback of the low voltage cutout is
only affected when the high voltage reset is active.

The feedback loop around the low voltage comparator is designed to
provide plenty of hysteresis and only interested when the low voltage
threshold is reached.

D2 ensures that once C1 is charged that it does not quickly discharge
when comparator 2 has detected low level and provides the 20 second
delay.


I tried to design the hysteresis around just one comparator in such a
way that it would handle both the low level cutout and the high level
reset but this proved difficult so ended up using 2..

I also dismissed the use of 16f628 PIC as I would expect the power
drain to increase with the additional components i.e linear regulator,
PIC etc.

All criticisms, recommendation etc welcome.

Cheers Justin


part 2 4843 bytes content-type:image/png; name="lv.png" (decode)


part 3 181 bytes content-type:text/plain; name="ATT00001.txt"
(decoded base64)

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2010\12\10@004658 by Josh Koffman

face picon face
On Fri, Dec 10, 2010 at 12:15 AM, Justin Richards
<spam_OUTjustin.richardsTakeThisOuTspamgmail.com> wrote:
> This is my first project using Comparators (and Eagle) and as such
> seeking feedback. In particular I am interested in the Do's and Dont's
> and suspect that I may need to add additional capacitors perhaps but
> decoupling caps only seem to appear on 74 and 4000 series logic IC's.

I haven't had time to study this in detail, but be sure you're
actually supplying power to your ICs. On many of the ones included in
the library the power pins don't automatically show up. Use the
"Invoke" command to help make them appear.

Josh
-- A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
        -Douglas Adams

2010\12\10@010918 by Justin Richards

face picon face
My understanding from the manual was what ever supply pins were on the
IC I had to ensure that I actually used supply symols/devices from the
Supply library that matched. In this case the IC had a V+ and a V- so
I "used" a V+ and V- and device from the supply library and connected
the nets.

When I run ERC it gives me a thumbs up.

Cheers Justin

On 10 December 2010 13:46, Josh Koffman <.....joshybearKILLspamspam@spam@gmail.com> wrote:
{Quote hidden}

>

2010\12\10@011337 by Josh Koffman

face picon face
On Fri, Dec 10, 2010 at 1:09 AM, Justin Richards
<.....justin.richardsKILLspamspam.....gmail.com> wrote:
> My understanding from the manual was what ever supply pins were on the
> IC I had to ensure that I actually used supply symols/devices from the
> Supply library that matched. In this case the IC had a V+ and a V- so
> I "used" a V+ and V- and device from the supply library and connected
> the nets.
>
> When I run ERC it gives me a thumbs up.

Cool, then you should be ok. I usually have a small area on the
drawing where I put all the power pins, just so I'm sure.

I also sometimes have multiple power nets (different voltages
usually), so this is a way to make sure I'm connecting to the correct
one.

Josh
-- A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
        -Douglas Adams

2010\12\10@014002 by Mark Rages

face picon face
On Thu, Dec 9, 2010 at 11:15 PM, Justin Richards
<EraseMEjustin.richardsspam_OUTspamTakeThisOuTgmail.com> wrote:
{Quote hidden}

Possible problems with your circuit:

1) the FET will turn on kinda slowly (RC = 0.01).
2) the FET will never turn off.
3) if the FET does turn off (D2 leakage?) it will turn off very
slowly.  While it is partway on, it will dissipate power.  Smoke may
be released.
4) the trimpots will be extremely "tweaky".  To prevent this, put
fixed resistors in series with each end so that they will adjust over
only a few volts range.
5) low-side switching is a questionable idea in an automotive
environment.  Many loads have a metal chassis which is connected to
negative ground.  If the load is touching the negative grounded car
chassis, the electrons will happily take this path and bypass the
switch altogether.

As an example of an alternate approach I sketched this up, 100% untested:

http://i.imgur.com/dKXND.jpg

Meets requirements except for turn-off delay.   You could slow down
the circuit with a cap across R2, forming an RC low pass filter, where
R=(R1||R2||R3).
-- Mark Rages, Engineer
Midwest Telecine LLC
markragesspamspam_OUTmidwesttelecine.co

2010\12\10@022316 by Richard Prosser

picon face
On 10 December 2010 18:40, Mark Rages <@spam@markragesKILLspamspamgmail.com> wrote:
{Quote hidden}

>

2010\12\10@031616 by Justin Richards

face picon face
Thanks for the feedback.

I have breadboarded the design and found
>
> 1) the FET will turn on kinda slowly (RC = 0.01).

It appears to turn on quickly but I see your point.

> 2) the FET will never turn off.

It turns off after about 20 secs.

> 3) if the FET does turn off (D2 leakage?) it will turn off very
> slowly.  While it is partway on, it will dissipate power.  Smoke may
> be released.

Yes, this is a concern and considering buffering the output of the
stage that feeds the MOSFET with another comparator as Richard has
suggested.

> 4) the trimpots will be extremely "tweaky".  To prevent this, put
> fixed resistors in series with each end so that they will adjust over
> only a few volts range.

The trimpots are 20kohm that require many turns to make small changes
so they appear to behave nicely.  I was using a normal 3/4 turn type
trimpot and it was as you say tweaky.

> 5) low-side switching is a questionable idea in an automotive
> environment.  Many loads have a metal chassis which is connected to
> negative ground.  If the load is touching the negative grounded car
> chassis, the electrons will happily take this path and bypass the
> switch altogether.
>

This is a concern, thanks for pointing it out.  I am also new to
MOSFETS but assume a switch to P-Channel will fix this.

> As an example of an alternate approach I sketched this up, 100% untested:
>
> http://i.imgur.com/dKXND.jpg
>
> Meets requirements except for turn-off delay.   You could slow down
> the circuit with a cap across R2, forming an RC low pass filter, where

This does look like a nice solution but TL431's does not seem to be
listed at my usual component supplies.

Justin

2010\12\10@063406 by ivp

face picon face
> Yes, this is a concern and considering buffering the output of the
> stage that feeds the MOSFET with another comparator as Richard
> has suggested

It will be better to use the analogue voltages for controlling the driver
and have the FET driven digitally from it. That way you are putting the
FET in just two known states, without intermediate drive voltages

> The trimpots are 20kohm that require many turns to make small
> changes so they appear to behave nicely.  I was using a normal
> 3/4 turn type trimpot and it was as you say tweaky

You'd find that a large portion of the travel won't be used, and multi-
turn trimpots are more expensive (if that matters). Perhaps just a 1k
range is adequate, so a 3/4 turn 1k trimpot between two fixed resistors
would give you plenty of control

> This does look like a nice solution but TL431's does not seem to be
> listed at my usual component supplies

xx431 are common components in power supplies. If you have an
old PSU around you might find a couple of  TL, MC, UA etc

> I also dismissed the use of 16f628 PIC as I would expect the power
> drain to increase with the additional components i.e linear regulator,
> PIC etc

I'd not rule out a PIC. If you need to, choose one with a big (32k)
WDT pre-scaler and SLEEP it, performing an A2D on wake-up.
Power consumption could be just a few uA, less than an LM339
for example, and the wattage drop across an LDO regulator would
be small

All things considered though, a comparator circuit will do the job.
An LP339 would reduce overall consumption

Joe

*
*
**********
Quality PIC programmers
http://www.embedinc.com/products/index.ht

2010\12\10@063823 by RussellMc

face picon face
Was the pun in the title intended.

TL431 is possibly the most common IC on earth.
Also possibly the cheapest.
Widely available.
Add it to your collection of must have parts.
Available in 2%, 1%, 0.5% parts.
Typically about 20 mA and 36 V rated.
Comes into regulation at around 500 uA
Usually 2.5V reference.

Low voltage (1.25V) version typically 18V and lower mA.
Comes into regulation in 50-100 uA range.

Poor man's LM385.

Typically 10's of cents in US.
In China I pay under 2 cents each for low voltage (mostly superior
specs) version in modest volume.

An extremely useful device.
Essentially it is a precision programmable zener.
Use as shown, Anode to ground, cathode "up".
When gate / sense terminal is driven above reference voltage cathode
conducts current to anode. Cathode may fall as low as reference
voltage (actually slightly lower but don't try it at home).  When gate
voltage is below reference voltage cathode is high Z.
Can be thought of as an open collector comparator with a ref voltage
on in-, gate input to In+ and output to Cathode. Add a diode from gate
to cathode (diode anode to gate) and the model is quite good.

May be a reference source, low power shunt regulator, or part of a
more complex regulator etc.

Digikey typically 50c/1
9c / 3000
6c / 100k

China < 2c ?1000?

53 cents/1, 37c/10 here
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ZTL431AFCT-ND

SOT23 & TO92 versions and more.Zertex versions here

      http://www.diodes.com/datasheets/ZTL431_432.pdf

Beware pinouts and namings.
TL431 here - TL432 has different pintout.
But I use eg TLV432 which is low voltage but may not use same pinout.
Variation between manufacturers and people not following instructions
allows a good path to product recall. Ask me how I know :-(. (You HAVE
to be there!)


                 Russell











CAN be used as a low frequency smps if you must.

e


On 10 December 2010 21:16, Justin Richards <spamBeGonejustin.richardsspamBeGonespamgmail.com> wrote:
{Quote hidden}

>

2010\12\10@064420 by RussellMc

face picon face
>> I also dismissed the use of 16f628 PIC as I would expect the power
>> drain to increase with the additional components i.e linear regulator,
>> PIC etc
>
> I'd not rule out a PIC. If you need to, choose one with a big (32k)
> WDT pre-scaler and SLEEP it, performing an A2D on wake-up.
> Power consumption could be just a few uA, less than an LM339
> for example, and the wattage drop across an LDO regulator would
> be small
>
> All things considered though, a comparator circuit will do the job.
> An LP339 would reduce overall consumption

Using CMOS Schmitt gates plus a TL431 type comparator will allow
operation at whatever the reference current is. An LM385 allows under
10 uA from memory.

An eg CD40106 is probably sub 1 uA when not switching.

Time delay with Schmitt can have high R value for low current.

A single package of CD40106 or equivalent can produce results of a
complexity that most people would not believe. (Any technology
sufficiently advanced, or using hex Schmitt inverters is
indistinguishable from a rigged demo" IA(adapted) )


2010\12\10@091422 by Justin Richards

face picon face
> Was the pun in the title intended.
>
> TL431 is possibly the most common IC on earth.
> Also possibly the cheapest.
> Widely available.
> Add it to your collection of must have parts.
> Available in 2%, 1%, 0.5% parts.

I tend to only experiment with parts from my local (with 20km radius)
electronic stores Jaycar and Altronics.  I rarely do mail order as
they always seem to take a long time, have to pay a conversion fee,
dont get to see it on the counter before I buy etc.  There have been
exceptions but I have to be keen.

Both these stores stock LM339 in SMD $AU27.00 and DIP $AU1.95 but no
TL431. These stores also dictated the choice of N-Channel MOSFET which
was cheap and readily available unlike the P-Channel MOSFET which
appear to be rare and expensive.  I will recheck the junk pile as
suggested for TL431

2010\12\10@091510 by Spehro Pefhany

picon face
At 12:15 AM 12/10/2010, you wrote:
>This is my first project using Comparators (and Eagle) and as such
>seeking feedback.


Sure. A few brief comments...

1]      The MOSFET should be kept either turned on hard or off hard,
        with short transition times, or it may not survive. Pay
        attention also to pathological cases. For example,
        if the engine is cranked for 30 seconds then the voltage
        might be low enough during that period to kill the MOSFET
        You may need a third threshold below which the MOSFET is
        turned off immediately (Under Voltage Lock Out). The BUZ21
        in your schematic is spec'd at 10V Vgs. The heavier the load,
        the more likely you are to kill the MOSFET during slow switching,
        especially if it's been running for a while with the load and the
        junction temperature is starting off at a high value.

2]      Your time delay is 100% controlled by leakage currents.. 30 seconds
        implies about 150nA. Stability is dubious under hash conditions.
        It wouldn't take much leakage the other way to keep the MOSFET
        on continuously. A special low leakage electrolytic is one way
        to do this reliably (0.002CV or less than 500nA leakage for a
        47uF cap at 5V). Then the leakage would be less than 15% of
        the timing current.

3]      A PIC circuit could be lower power.. your zener reference alone
        draws 1.2mA with 12V in (and isn't all that great a reference)
        and the LM339 draws another mA IIRC. A PIC16F628 plodding along
        with a 4MHz ceramic resonator (way overkill) uses less than a mA
        including two built-in comparators. The most popular automotive
        regulator is probably the LM2931, which draws about 0.4mA typically
        (with a light load*), so you're actually significantly better off even
        with the most common and suboptimal parts. The LM2931 gives you
        +60/-50V survival without additional parts. It's possible (but
        probably not very useful) to get down orders of magnitude further
        with a bit more effort.

        * The LM2931 is a typical bipolar LDO with a lateral PNP (low beta)
        pass transistor. It needs a capacitor on the output for stability,
        and also the quiescent current goes up significantly as the output
        current increases. At 100mA Iq could be >15mA!

4]      I suggest connecting the pot elements in series maybe with a small
        resistor between. The voltage ranges don't need to (and shouldn't)
        overlap. I think I'd tend to divide down the reference rather
        than the input. Maybe 6.7V on the top and 5.5V on the bottom,
        and divide the input down by a fixed 2:1.

5]      Vehicle electrical systems have transients that kill electronics.
        Some protection against something like +/-60V spikes would be a
        good idea.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
TakeThisOuTspeffEraseMEspamspam_OUTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

2010\12\10@095347 by Olin Lathrop

face picon face
Justin Richards wrote:
> I tend to only experiment with parts from my local (with 20km radius)
> electronic stores Jaycar and Altronics.

Then you're going to severly limit the electronic parts you can use.

Mail order is how it's done.  Get over your need for instant gratification
and learn to read data sheets so you don't have to physically see a part
before using it.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2010\12\10@100953 by RussellMc

face picon face
Oz presumably?

LM336
Jaycar ZV1624
Stupid price

You could consider opening an account at RS and Jaycar.
Cost nothing.
No minimum volume per time or per purchase.
Prices range from OK through terrible BUT
avaiability is usually superb.
Courier fee is modest but still a courier fee so pasys to but a few parts at
once.
VERY worthwhile.

Datasheet  http://www.diodes.com/datasheets/ZTL431_432.pdf

No counter needed :-)

http://newzealand.rs-online.com/web/
http://australia.rs-online.com/web/

MOQ 10 at $AU0.16 (a bargain).
TO92
http://australia.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=6619830


Element14 / Farnell

International front page (Oz was down)

http://www.farnell.com/

And finally ... some good and bad prices here.


http://shop.ebay.com.au/?_from=R40&_trksid=p3907.m570.l1313&_nkw=tl431&_sacat=See-All-Categories


Russel

2010\12\10@101954 by Justin Richards

face picon face
>
> Mail order is how it's done.  Get over your need for instant gratification
> and learn to read data sheets so you don't have to physically see a part
> before using it.
>
Hmmm, 7 weeks ago I bit the bullet and ordered Circuit Cellar, still
waiting.  They assure me it can take up to 8 weeks for delivery.

Also, shipping to Australia from many places in the US requires
something like $30.00 min postage for $1.00 parts.

I like to see the component on the counter for a couple of reasons,

1. the first is that for some strange reason mail ordering part x
delivers part y.
2. Previous orders sent what was in stock and then back ordered the
critical parts.
3. If they dont have it in stock and I am at the counter I may be able
to come up with a substitute.
4. If a picture speaks a thousand words then a physical object yells more.
5. Gratification is what hobbies are all about.

Yes, I accept that it severely limits what parts I can use but it does
make one extremely resourceful.

2010\12\10@102434 by Justin Richards

face picon face
> You could consider opening an account at RS and Jaycar.
> Cost nothing.

I have used RS but I feel like I am a big inconvenience to them so I
shop where I seem to be welcome which is Jaycar and Altronics.

RS was where my critical parts were back ordered which I think at the
time was 74LS90 which everyone else had.

Justi

2010\12\10@110521 by Michael Watterson

face picon face
On 10/12/2010 15:19, Justin Richards wrote:
{Quote hidden}

I'm in Ireland
I find my local counter (Maplin, a sort of inferior Conrad/Jaycar) very expensive and ill stocked.

RS & Farnell sometimes.
But China is typically about a week.
Australia not much more. (Mini-kits, jaycar and Home Box office DVDs).
I also buy stuff off at least one list member here that sells online (Netherlands).
Some places I've used
http://www.techtir.ie/info/electronics-suppliers

Read descriptions of electronics parts on eBay VERY carefully and google them. I've had good success and saved much.
 Italy, France, UK, Thailand, Singapore, Turkey, Malaysia, Taiwan, Hong Kong, and China via Ebay.
(postage usually too high for USA).

I've used Mail Order since before there was an Internet. Now much easier. No need to write and buy catalogue first.



2010\12\10@110737 by Justin Richards

face picon face
> Sure. A few brief comments...
>
>         if the engine is cranked for 30 seconds then the voltage
>         might be low enough during that period to kill the MOSFET

I was not aware of this.  To be sure I understand, if the supply
voltage across the MOSFET in series with the load drops then there is
a risk of destroying the MOSFET. I am actually using a MTP3055E.
>
> 2]      Your time delay is 100% controlled by leakage currents.. 30 seconds
>         implies about 150nA. Stability is dubious under hash conditions.

This has now been rectified by using an additional comparator which
has the non-inverting section fed by a cap in parallel with a 1Mohm
resistor.

>
> 3]      A PIC circuit could be lower power.. your zener reference alone

Yes, the reference is not good, but I figured it did not need to be
just repeatable which I suspect it is.  Again for me the commonly used
LM2931 is a long way away.

>
> 4]  I'd tend to divide down the reference rather
>         than the input..

I have seen this approach but wasn't intuitive.  To me it was easy to
think of fixing a reference then setting thresholds relative to it.

>
> 5]      Vehicle electrical systems have transients that kill electronics.
>         Some protection against something like +/-60V spikes would be a
>         good idea.

I had considered this but not completely sure how to resolve it.  I
was considering an inductor in line with the 12V in, and a cap across
the 12volts in.  I can get 30V zeners and could use 2 in series to get
the 60V but are very low power.

Thanks to all for taking the time.  I have lots to research.

Cheers Justin

2010\12\10@112455 by RussellMc

face picon face
RS and Farnell usually show actual stock and actual location. I always
phone order and when the lady (so far) says they have X available it
has so far always been so. With Farnell & RS  you can see stock in Oz
and UK and with Farnell maybe also in US at ?Newark?

It doesn't matter if the people on the counter at RS act as if you are
a nuisance - although I've always found them OK regardless of order
size - small volume and rapid service at OK to terrible price and
great availability is their business model. Use it if it works for
you.

Farnell used to deliver here before 7am next day for orders up to 6pm
NZ time !!!! Now it's by early afternoon - still good.


Automotive spikes: Some regulators are designed  especially for an
auto environment. Data sheet will say. LM293x or perhaps LM29xx series
are one such line.

Supply - resistor - zener to ground - resistor - zener to ground. With
filter caps, works well for most spikes. Watch ground return paths.

TL431 is so good for the $ - especially 10 for $1.60 as before, that
using them as std may be a good idea. For regulators using them you
drive a FET or whatever and dimension it thermally as required



 Russel

2010\12\10@120546 by Spehro Pefhany

picon face
At 11:07 AM 10/12/2010, you wrote:
> > Sure. A few brief comments...
> >
> >         if the engine is cranked for 30 seconds then the voltage
> >         might be low enough during that period to kill the MOSFET
>
>I was not aware of this.  To be sure I understand, if the supply
>voltage across the MOSFET in series with the load drops then there is
>a risk of destroying the MOSFET. I am actually using a MTP3055E.

No, if the gate-to-source voltage drops much below 10V (but more than
a small fraction of a volt). Your original circuit does not
necessarily do this, but it has other problems (slow turn-off),
and the cure will likely lead you down this path.

{Quote hidden}

I'd wager that 60V will kill the LM339. You should endeavor to
limit the transients to the ratings of the parts, 36V in the case
of an LM339.

A 22V 3,000 W TVS (basically a very solid zener diode) will have
a clamping voltage of <36V while clamping 85A. That's not bad,
but it is not guaranteed to survive the double battery test. A
24V 3,000 W TVS will clamp to 38.9V at 77A, above the rating of
the LM339, but will survive a double battery.

Depending on the circuit, a tighter constraint may be the Vgs(max)
of your MOSFET, typically between 10V and 20V maximum, depending
on the type.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
RemoveMEspeffspamTakeThisOuTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

2010\12\10@121350 by Richard Pytelewski

flavicon
face
I used to just use parts on-hand and the project would turn our so-so.  Get
the right part(s) for the right job and mail-order a few extras and expand
your parts stash.  Long-term, you'll be glad you did.... FWIW

Rich

{Original Message removed}

2010\12\10@121628 by Richard Pytelewski

flavicon
face
Spiro:

My MacAfee SW continues to alert me when you send e-mail to the PIC list.
It says:
---------------------------
McAfee SiteAdvisor Warning

       This e-mail message contains potentially unsafe links to these
sites:
       trexon.com
---------------------------
FYI....

Rich

{Original Message removed}

2010\12\10@124309 by RussellMc

face picon face
> Depending on the circuit, a tighter constraint may be the Vgs(max)
> of your MOSFET, typically between 10V and 20V maximum, depending
> on the type.

Yes. FET gate maximum voltage is usually a fairly real spec. Therating
is based on gate oxide thickness - break it down and the FET expires.
Usually a simple clamp (zener) on the gate will allow as much drive as
you need while staying within Vgs limits.


2010\12\16@045448 by Justin Richards

face picon face
>
> As an example of an alternate approach I sketched this up, 100% untested:
>
> http://i.imgur.com/dKXND.jpg
>
> Meets requirements except for turn-off delay.   You could slow down
> the circuit with a cap across R2, forming an RC low pass filter, where
> R=(R1||R2||R3).
> --

I acquired a TL431 from an old PSU and experimented with this example
http://i.imgur.com/dKXND.jpg but could not get it to provide any
hysteresis, i.e the turn on level was the same as the turn off level.

I have however, incorporated the TL431  into my previous design.  As
someone mentioned it does provide a very good reference and far
superior to the zener.  Russel mentioned that they only require 500uA
for regulation but these seem to require 10mA.  I will need to read
the manual again and experiment more here.

The circuit checks out ok with an 8 amp resistive load.  Nothing seems
to get hot and the levels once set are reasonable consistent
regardless of the load which appeared to be an issue when using the
zener.  But I am not 100% certain this was due to the zener.  I think
it may have had to do with the supply rails shifting with the large
load.

Thanks for all the assistance.

Cheers Justin

2010\12\16@141429 by Mark Rages

face picon face
On Thu, Dec 16, 2010 at 2:54 AM, Justin Richards
<justin.richardsEraseMEspam.....gmail.com> wrote:
>>
>> As an example of an alternate approach I sketched this up, 100% untested:
>>
>> http://i.imgur.com/dKXND.jpg
>>
>> Meets requirements except for turn-off delay.   You could slow down
>> the circuit with a cap across R2, forming an RC low pass filter, where
>> R=(R1||R2||R3).
>> --
>
> I acquired a TL431 from an old PSU and experimented with this example
> http://i.imgur.com/dKXND.jpg but could not get it to provide any
> hysteresis, i.e the turn on level was the same as the turn off level.
>

PNP + R3 is meant to provide hysteresis.  What is the voltage across
R3 when the circuit is off?  When it is on?

Regards,
Mark
markrages@gmail
-- Mark Rages, Engineer
Midwest Telecine LLC
EraseMEmarkragesspammidwesttelecine.com

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