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Two Stupid Questions

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Hi,
I need answers to these two stupid questions:

#1-How can I use a CMOS-level output (logic HI) to trigger a 555 timer for 1 hour (delay), which in turn triggers another 555 timer for 1 minute (alarm)?

#2-Does anyone have a battery-backup circuit for 12-volts? Back-up MUST occur within 40 nS of AC Mains failure. Injection time is based on 74AHCT00 series CMOS requirements.

Thanks.
 

Klaus

New Member
Q2:#2-Does anyone have a battery-backup circuit for 12-volts? Back-up MUST occur within 40 nS of AC Mains failure. Injection time is based on 74AHCT00 series CMOS requirements.

Is the 12V critical? If you can run your circuit on sligthly less voltage you could use a 12V battery (one which can handle constant charging) and regulate its voltage for your circuit requirement, say 10V or so to allow for regulator drop.
The battery is constantly charged from the mains to maintain its capacity. For a wet cell set the constant voltage charger to 13.8 - 13.9V to avoid over charging.
At any dropout of mains, the battery still supplies the power - no interruption at all.
Klaus
 
Yes, the 12-volts supplied by the battery is critical. Also, I won't be able to use a rechargeable battery due to potential explosion hazard. I might be able to use a gel-cell, but I don't know much about them. Any other thoughts?
 

Klaus

New Member
Well, 40 nano seconds is pretty fast switching for back up power, you *really* do not want any interruptions at all :)
You do not state what currents are involved, if they are small you might use a large capacitor to bridge the dropout gap until your standby power supply comes on line. Not much chance of a detailed answer from sparse information given, is there?
Good luck,
Klaus
 
Klaus said:
Well, 40 nano seconds is pretty fast switching for back up power, you *really* do not want any interruptions at all :)
You do not state what currents are involved, if they are small you might use a large capacitor to bridge the dropout gap until your standby power supply comes on line. Not much chance of a detailed answer from sparse information given, is there?
Good luck,
Klaus
Hi Klaus,
I did say they were stupid questions! Actually, if I follow the data sheet correctly for a 74AHCT139 decoder, I should have divided by 2 to get a power-interrupt time. It has a rise/fall time of 20nS. I should be trying for a 10nS injection time. However...by redesigning my power supply per your advice, I think I may not need such a fast backup. Max current will be around 300mA, so... would a 4700uF @35V in parallel with a 0.1uF @50V tantalum be a good choice?
Keep trying...I'm kinda hardheaded! Thanks.
 
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