As his trip point voltages will be either well below the trip point
when the charging is off or well above the trip point when it's
charging, I didn't figure he'd need it. Thanks for the suggestion,
though.



On Tue, 28 Oct 2003 03:28:17 GMT, Woody wrote:

In article ,
says...
You need a circuit like the one on:
http://www.ncws.com/rcrock/lowbat.htm

If you replace the LM336 and its 3K resistor with a 10K little
thumbwheel potentiometer (volume control), hooking the wiper arm to
the minus input to the LM311 voltage comparator and the two ends
between battery + and - terminals, you can adjust the trip point all
over the place to suit your needs.

Replace the little alarm beeper with a small 12V relay that will turn
your big control relay on and off remotely mounted at the inverter's
DC power source. The little relay contacts, being isolated, will
prevent the big relay's coil from kicking high voltage into the IC
destroying it.

The LM311 is a voltage comparator IC, made for this service. Its
output (hooked to the piezo buzzer in the schematic) switches from off
to full on whenever the voltage at the + input crosses the voltage set
on the - input. This will click the relay ON any time the voltage
drops below your set point. If you power the big control relay from
normally closed contacts on the little relay powered by the IC, any
time the little relay is energized, the big relay will be
de-energized, turning off the fridge. I like this because if this
circuit fails for some reason, the failure mode is to leave the fridge
ON, not off, continuously....protecting your food in an emergency
scenario.

As this circuit draws virtually whatever the tiny control relay draws,
a few milliamps, there's no reason to ever turn it off. Just put its
source to the breaker the inverter is running from.

Once set by the variable control, it'll stay that way for years.

To adjust it is simple. Turn the charging on, and adjust the control
so the inverter runs. Turn the charging off and adjust the control
back until the inverter shuts off at the desired voltage. You can
adjust the shut-down point to anything you like.....leaving the
inverter on until the battery voltage drops to X volts.



On Sat, 25 Oct 2003 22:13:20 -0700, "R. A. Piziali"
wrote:

I have need of the following:

A relay that will handle a 15amp load and close at about 13.0-13.4vdc and
open at, say, about 12.7vdc. Is such a device available - if so where? OR what
can I put together to operate in such a manner??

------------------

The LM311 circuit described should do the job, but does not have ANY
hysterisis and could "chatter" (on-off). Also, I would filter the
supply voltage.

A resistor connected between the 311 output (right side) and the "+"
input will add hysterisis. That is, turn on at voltage 1 and turn off
at voltage 2, rather than a single on/off point in voltage. The value
will depend on the amount of hysterisis desired and the other component
values (8K2 and 10k resistors). The resistor value can be calculated or
determined experimentally... Probably somewhere from 10K to 1 megohm.

I would add a 0.01 Uf ceramic cap across the +- chip leads and a 50 to
100 Uf at the power feed, maybe some ferrite too. ...Keep the false
triggers at a minimum...

Instead of a small relay a medium sized transistor would reduce the
moving parts and drive the big relay. The NPN circuit below will invert
the "sense" (logic) of the circuit. Should drive most all relays is you
use a 3-8 amp pwr transistor. The diode (1N4004 or equal) "snubs" the
inductive kick-back from the relay coil.
...Inversion can be done by swapping +- inputs too.

| To +
|
X
X 1000 ohms Lg Relay coil
X | /C---------^^^^^---------- To +V
| 200 ohms |/ |--Diode-| Cathode
------/\/\/\/\------| NPN
B|\
| \E
------ to Negative (ground)


Another (non-inverting) method using a PNP transistor.

| /------ To + voltage
1000 ohms B |/ E
-----/\/\/\/\-------- |
|\ C Rly coil
| \-----^^^^^^---- To neg (ground)
| Diode |
Cath

If My ASCII art is trashed and you need to know, email me for a better
drawing. - where MyCall=KZ4AK.

73,
Woody


Larry W4CSC

"Very funny, Scotty! Now, BEAM ME MY CLOTHES! KIRK OUT!"