Low Voltage Cutout

[[email protected]]

Hi

I have an access control system running off a float charged battery.
If there is a power outage of +-8 Hours, the battery drops to a level
lower than the access-control system can handle, and then it does not
recognise the tags (the readers actually lockup, but the maglocks stay
locked).

I was thinking of putting a 9v zener diode on the battery, running a
relay coil, that will cause the load to be disconnected when the
battery is at 9v. one problem i see with this is that when the load is
disconnected, the battery can 'bounce' up to above 9v again, which
will cause the relay to keep on chattering,.

Is my understanding correct, and is there an inmprovement that will
eliminate the chatter



BATTERY --------------------------|> ZENER DIODE------- RELAY COIL
|
---------------------------------------------------------- RELAY COIL

 

[MooseFET]

Hi

I have an access control system running off a float charged battery.
If there is a power outage of +-8 Hours, the battery drops to a level
lower than the access-control system can handle, and then it does not
recognise the tags (the readers actually lockup, but the maglocks stay
locked).

I was thinking of putting a 9v zener diode on the battery, running a
relay coil, that will cause the load to be disconnected when the
battery is at 9v. one problem i see with this is that when the load is
disconnected, the battery can 'bounce' up to above 9v again, which
will cause the relay to keep on chattering,.

Is my understanding correct, and is there an inmprovement that will
eliminate the chatter

If the load circuit is sure to pull the voltage down to zero quickly,
you can use the voltage on the load side to do the positive feedback.
The most extreme version would be to have a push button you need to
press to turn the system back on.

 

[ehsjr]

Hi

I have an access control system running off a float charged battery.
If there is a power outage of +-8 Hours, the battery drops to a level
lower than the access-control system can handle, and then it does not
recognise the tags (the readers actually lockup, but the maglocks stay
locked).

I was thinking of putting a 9v zener diode on the battery, running a
relay coil, that will cause the load to be disconnected when the
battery is at 9v. one problem i see with this is that when the load is
disconnected, the battery can 'bounce' up to above 9v again, which
will cause the relay to keep on chattering,.

Is my understanding correct, and is there an inmprovement that will
eliminate the chatter



BATTERY --------------------------|> ZENER DIODE------- RELAY COIL
|
---------------------------------------------------------- RELAY COIL

If it is a 12V battery, 9V is too low. You want to disconnect
the battery at somewhere between 10.5 and 11 volts - higher
is better for the battery.

To prevent the "bounce syndrome", hold the relay energized
through its own open point. Something like this:

N/O Relay
Contact
Battery(+) ---+---o---
| ^---o---+----> to load
k| |
[D1] [Zd] 11 volts
| |
+--------+------+
| + | |k
Charger [Relay] [D3]
| - | |
Battery(-) ---+--------+------+


When the battery V drops below 11 volts, the relay drops and
the contact opens, isolating the battery from the load and the
zener. D1 prevents the relay from "seeing" the battery voltage,
and D3 absorbs the inductive kick from the relay dropping out.
When the power is restored, the charger turns on and energizes
the relay, reconnecting the load.

Use a Schottky for D1, because it has a lower voltage drop
than a regular diode. If that still presents an unacceptable
voltage drop, use a double pole relay and replace the diode
with the second normally open contact.

You can use a differnt zener or a TL431 to set the drop out
voltage, but unless your battery spec says you can safely
discharge it below 10.5 volts, I recommend you use the
11 volt dropout point.

Ed