mini-ups

We need an uninterruptible power supply, functionally similar to the sort we
use on our computers, but with 24vac input and output, and of the smallest
possible size. Output at 24vac will be less than 5 watts, for no more than
10 minutes. Can we buy one? And if not, then can we make one?

cheers
brock

 

Hi Brock,
The answer is a definite maybe.
What is the application? Control circuitry, security/fire detection?
Regards,
Tom

 

The application is control circuitry. We are HVAC service technicians
in south florida, and we have problem with systems using solid state
controllers that default either to 'off' or to an undesirable setting
after a power interruption.

cheers
Brock

 

Why don't you install a standard UPS before the 120 to 24 VAC
transformer? That way it is an off the shelf part that is easy to find,
and easy to replace after it dies of old age or a too close lightning
strike.

Instead of hard wiring the transformer, see if the building inspector
will let you install a single outlet for the UPS, then use a high
temperature cord to plug into the UPS. Add a clamp to hold the AC plug
into the outlet, then post a warning that it is not to be disconnected,
except to service the system.

I see low end UPS new for under $60 quite often, and have seen a few
250 watt for $40. harbor freight had some not too long ago, and Staples,
Office Depot and Circuit City have them on sale quite often.

The only problem I see is that you ay need a sine wave supply, or at
least a stepped waveform to prevent heating the transformer.