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eHam Forums => Misc => Topic started by: K8AXW on October 31, 2012, 08:24:29 PM



Title: Voltage drop-out alarm
Post by: K8AXW on October 31, 2012, 08:24:29 PM
Occasionally I'll read a "Help Needed" item here that falls in the electronic field but not ham related.  I now have such a need.

Many years ago I designed and built a plug-in voltage dropout alarm for an old ham who was tethered to an oxygen concentrator.  He was afraid of going to sleep and not waking up because of a power failure and not knowing it.  This alarm allowed him to rest peacefully.

My wife has sleep apnea and is forced to use a C-pap machine/mask to keep her breathing during the night.  Whenever there is a power failure she doesn't know it until she breathes in and there isn't anything there.  She wakes up in a semi-panic ripping the mask from her face.

I could easily build another alarm that uses a latching relay that drops out along with a 9V battery to power the Sonalert as I did for my old friend. Although the small relay never failed or got hot I was always uneasy using it. 

I would like to have a solid state circuit that senses a line voltage dropout and triggers a Sonalert or similar alerting device that can be powered from a 9V battery. 

If such a circuit is designed with a time delay of say 5 seconds to avoid false triggering from the common 2 or 3 second power failures, it would be icing on the cake. The only "control" needed would be a RESET button.

Thanks for any consideration.

73

Al - K8AXW


Title: RE: Voltage drop-out alarm
Post by: WN2C on October 31, 2012, 08:38:09 PM
I may be reading in to this but... I too sleep with a C-pap.  I don't see the need for an alarm to wake me when the power goes out and or see the need to wake to an alarm sounding and having my c-pap not working.  Just sounds...unnecessary.  If she has not been using it for long she will get used to it and get used to waking with it not working, making noise (you too), hose slipping off ...etc, etc.

73 de wn2c  Rick


Title: RE: Voltage drop-out alarm
Post by: K8AXW on October 31, 2012, 09:32:09 PM
Rick, I somewhat agree with you but since I've never had to use a C-pap I really can't comment.  My wife has been using one for several years and I know that when the power goes off she wakes up in a semi-panic. 

Maybe a Sonalert will cause her to wake up in a full panic.  I have no idea.  All I know is she gets upset when it happens.  What I'm looking for is grease for a squeaky wheel.  Know what I mean Verne?

Al - K8AXW


Title: RE: Voltage drop-out alarm
Post by: KA1MDA on November 01, 2012, 01:21:33 AM
If you wire up a relay so it's pulled in when there is power and drops out during a power failure, it's a failsafe design, since a relay coil failure would be the same as a power outage and trip the alarm.

The weakest link here is the battery. Without monitoring the battery condition, how can you be sure the battery has enough juice to activate the alarm? If power outages are infrequent, monitoring battery condition becomes even more important, as the unit may go untested for months or even years.

Personally, I would go with the KISS design and use a relay. Much less to go wrong with a simple relay than solid state devices. Unlike transistors, triacs, and scrs, relays are relatively imune to voltage spikes.


Title: RE: Voltage drop-out alarm
Post by: AC4RD on November 01, 2012, 04:15:20 AM
How about a small UPS (uninterruptable power supply)?  The sort of thing that you plug a computer into, and it runs the computer for a few minutes if the power goes out--gives you time to shut down.  Maybe plug the cpap machine into one of those?


Title: RE: Voltage drop-out alarm
Post by: AA4PB on November 01, 2012, 05:32:31 AM
I'd probably stay with your original KISS relay design. Be sure to have a fuse in the AC line to the relay coil and it should be safe enough. As soon as you start connecting solid state to the AC lines then you have to be concerned with possible voltage spikes. Relays are pretty forgiving.

Of course, you could get real fancy and use a large capacitor (or a rechargable battery) to power the alarm and use the AC power to keep the capacitor (or battery) charged. Using a large capacitor would mean that when power goes out the alarm would sound for several seconds and then stop by itself when it discharges the capacitor. No "off" button required - everything would be fully automatic.


Title: RE: Voltage drop-out alarm
Post by: KCJ9091 on November 01, 2012, 06:42:08 AM
I was also going to suggest the UPS.  It will keep the machine running and will give a power loss alarm.  You can find inexpensive used ones that at worst need a need battery replacement.


Title: RE: Voltage drop-out alarm
Post by: KE3WD on November 01, 2012, 10:57:46 AM
You make the assumption that a solid state solution will be more robust than the simple electromagnetic relay solution that has already proven itself in at least one empirical situation that you yourself implemented? 

Go with the tried and true relay dropout circuit that wires the alarm via the NC contacts.

Adding a UPS is a very good idea and is cost effective.

The UPS will likely have its own power off alarm in it as well.


73


Title: RE: Voltage drop-out alarm
Post by: KE4DRN on November 01, 2012, 05:30:14 PM
hi

if you decide to use a UPS, be sure to get one that
has full sine wave output, not a stepped or modified sine wave.

the APC SmartUps series is full sine wave and can also
be started up if no AC power is available and will run
on battery for a short time.

73 james


Title: RE: Voltage drop-out alarm
Post by: K8AXW on November 01, 2012, 10:36:01 PM
Thank you all for your replies.  The UPS is one thing I never thought of even though I presently use one on my PC! The really big drawback is the cost.  But, what's money?

I just might go to the one I previously designed with the relay.  Even though I spent 40 years in a power plant where thousands of relays were in use with very few failures, I still have this reluctance of having an energized relay on 24/7. 

As for the battery, there isn't any current draw and the battery life is the "shelf life."  No different that the 4 smoke alarms scatter throughout the house, shack and shop.

Again, thanks all.

73

Al - K8AXW


Title: RE: Voltage drop-out alarm
Post by: KCJ9091 on November 02, 2012, 06:36:26 AM
Good working UPS units can be found in thrift stores for $20 up depending on the size.   The relay and sonalert will cost that much.


Title: RE: Voltage drop-out alarm
Post by: W9GB on November 02, 2012, 07:49:51 AM
Quote from: K8AXW
The UPS is one thing I never thought of even though I presently use one on my PC! The really big drawback is the cost.
The newest APC units are now using a SMPS charger and inverter.  This significantly reduces weight and size.  
APC has also added new monitoring and control electronics.

I exchanged/upgraded to the APC BR1300G model last month.  
APC paid for UPS shipping back (recycled) of 3 old APC models that I had, for discount.

APC TRADE-UPS Program
http://buy.apc.com/commerce/storefronts/tradeups/tradein.aspx


Title: RE: Voltage drop-out alarm
Post by: AA4PB on November 02, 2012, 08:06:05 AM
My wife has a large type c-pap machine that has a sonalert and a 9V battery built into it. If the power goes out while the unit is turned on then the alarm will sound. She got it used and the alarm test didn't work. I took it apart and found no 9V battery installed.

The nice thing is that the alarm doesn't sound if you loose power while the machine is not being used.