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'[EE] voltage monitors'
2008\07\31@005142 by Doug Metzler

picon face
I've got a solar charging solution that runs a small circuit.  It uses a
single 1.2v NiMH battery and my goal is to maximize the NiMH cycles.  



I got one of these slick little voltage supervisory chips:



www.mouser.com/Search/ProductDetail.aspx?qs=%2fugrpAKHX8rrqC4E3QUSBg%
3d%3d



That triggers at 1.1v - thus shutting off my circuit as soon as the NiMH
battery gets to the steep part of the discharge curve.



The supervisory drives a 2N2222 to pull the circuit to ground.



The problem is that as soon as the device triggers and shuts down the
circuit the voltage in the battery climbs back up to 1.2v and the circuit
turns back on, pulling the battery down below 1.1 so it turns off, and
repeats this probably a dozen times before finally settling below the
threshold



Is there a simple analog circuit I could wrap around the 2222 to keep it
pulled down for several hours even if the supervisory turns on and off?



Keep in mind that I don't have a uController available.  (although if I
could find a very inexpensive 1.2v controller that might be a solution).



Thank you,



DougM



2008\07\31@011821 by Forrest Christian

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I'd switch the 2n2222 for a 2n7000. This will eliminate the B-E drain
through the 2n2222, and permit the use of a simple r/c network on the
gate of the 2n7000 to achieve the delay.   Plus, it should be more
efficient without the B-E current.

I'd probably start with the open drain output version of the supervisory
circuit, and connect it from the gate to ground.   Put a capacitor in
parallel (also from gate to ground).   Add a high-value pull up resistor
to +V.

That way, when it triggers, it will yank the gate to ground, turning off
the 2n7000.   When it "untriggers", the capacitor will slowly charge
through the pullup until it reaches the gate threshold, at which point
the 2n7000 will turn back on.    

-forrest

Doug Metzler wrote:
{Quote hidden}

2008\07\31@012547 by Bob Blick

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Probably need something more modern with a lower gate threshold than the
'7000 since isn't this all running on 1.2 volts?

Cheerful regards,

Bob

Forrest Christian wrote:
{Quote hidden}

2008\07\31@012742 by Apptech

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{Quote hidden}

Data sheet

       http://www.onsemi.com/pub_link/Collateral/NCP304-D.PDF

One way is to add hysteresis to the sensed voltage.

A simple way (that possibly works) is to add a series
resistor to the sense IC input / power supply.
When operated it's current drain will rise, thereby
decreasing the input voltage. Consult data sheet and add an
appropriate R.

Too large and it won't rewake well when voltage rises.

For extra points and more complexity you could have a
voltage divider at the input with the transistor adding to
the loading. More points from using a time delay (aka
capacitor) to make the extra load fully or partially time
dependant.

Just the series R will probably help.



       Russell


2008\07\31@014613 by Forrest Christian

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Yeah, you're right...     I think my brain was stuck on the 2n2222 B-E
voltage drop for some reason, and not the 2V-ish typical threshold of a
2n7000.   I'll claim sleep deprivation as my excuse (only ~4-5 hours
last night).

That said I'm sure Zetex (and others) have a suitable low-threshold fet
which should work for the application as described.

-forrest

Bob Blick wrote:
{Quote hidden}

2008\07\31@123953 by doug metzler

picon face
I looked for a fet with a lower gate voltage and wasn't able to find one,
but I'll renew my search.  That definitely was the solution I was after
because of the transistor drain that you mentioned.  Thinking about it the
cap and large resistor solution might be a little tricky since with no drain
on the battery the voltage is likely to come back up to 1.2 and stay there.

Maybe there's a problem with my whole approach - I guess I really need
something that will shut off when the supervisory kicks, and not turn on
again until it detects a positive flow from the solar panel to the battery.

I'll go back to the drawing board and think about this some more - all
suggestions are welcome.

Thanks!

DougM


On Wed, Jul 30, 2008 at 10:46 PM, Forrest Christian <spam_OUTforrestcTakeThisOuTspamimach.com>wrote:

{Quote hidden}

> -

2008\07\31@134525 by Apptech

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>I looked for a fet with a lower gate voltage and wasn't
>able to find one,
> but I'll renew my search.

Digiket etc

FDN337 N Channel
FDN338 P Channel

& Zetex do some.

& the lovely CES2310 - but probably Asia sources only.

Try a series resistor in the supervisors input though.
And, you can add as much load as you want to the output to
increase the hysteresis as you wish.
MAY need a small cap at superviror input when R used.

MADE UP EXAMPLE ONLY

Supervisor Iq = 10 uA.
Superviros Ion = 100 uA.
Transistor load = 100 uA.

Using a 1k input resistor drops

- 0.01V at 10 uA
- 0.2 V with 100 uA IC + 100 uA transistor.

If you decided you wanted more drop with output on you could
either

- Raise Rin to say 3k3
or
- Add a load resistor to output, or lower base drive R.

Adding input R takes less current from battery when low.


       Russell



2008\07\31@144611 by doug metzler

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Thank you Russell, I'm going to order some FDN337's and play around with
this, but I am going to go back and look at my approach in the context of
the whole circuit and see if I can figure out some way to snap it off and
not have it turn back on until the solar panel starts generating current.
If I can nail this it will help an another larger circuit that I am
currently controlling using a 12F683.

you guys are great!

DougM

On Thu, Jul 31, 2008 at 10:42 AM, Apptech <.....apptechKILLspamspam@spam@paradise.net.nz> wrote:

{Quote hidden}

> -

2008\07\31@155943 by Apptech

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> Thank you Russell, I'm going to order some FDN337's and
> play around with
> this,


> but I am going to go back and look at my approach in the
> context of
> the whole circuit and see if I can figure out some way to
> snap it off and
> not have it turn back on until the solar panel starts
> generating current.

Try the series input resistor that I suggested and see how
it meets your needs.
That will "snap it off" by a controlled amount.

The IC will see a drop in input of

   (Ioperating - Ioff) / Rseries

when the IC turns on.
If that is more than the battery "recovery" voltage then it
will stay off.

When the battery charges again it will turn back on.

A method of detecting charging is to place a transistor Vbe
junction across the diode in series with the panel. Some
extra biasing may be needed.


       Russell





{Original Message removed}

2008\07\31@162452 by Enki

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       The FDN337N seems to not have the usual internal Zener between Gate
and Source for protection.
       Discovered that hard fact after 12 of them stopped working without
any reason.

       Mark Jordan


On 1 Aug 2008 at 5:42, Apptech wrote:

{Quote hidden}

> --

2008\07\31@165823 by Marcel Duchamp

picon face
Did you notice the Vgs-max spec?

It's rather low: +/-8 volts.  And that is the *ABSOLUTE MAXIMUM* number.
 Something to watch for here would be ringing on the gate from trace
inductance feeding the gate capacitance.



Enki wrote:
>        The FDN337N seems to not have the usual internal Zener between Gate
> and Source for protection.
>        Discovered that hard fact after 12 of them stopped working without
> any reason.
>
>        Mark Jordan

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