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
On Thu, 20 Nov 2008 00:05:19 -0800 (PST), kol...@gmail.com wrote: >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
> 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.
> 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
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.
