small automatic transfer switches

Does anyone have any recommendations for a small, cheap, reliable ATS?
(I know, pick two, you can't have all three) I'm looking for something
to power one or two 120V out-of-band network device(s) in each
location with a single power supply each, much less than 10 amps
total, with two 120v input cords. The primary input cord will go to
the UPS and the other directly to a wall outlet to be able to access
the UPS when if fails to turn on after the power returns :slight_smile:

I found the usual suspects, APC, TrippLite, ServerTech, etc. but they
are mostly 8 or more outlets and upwards of $300-$900 each.

I also found this neat one, Zonit uATS, which is a small box that
piggybacks onto the powered device's C14 input and has two power cords
coming out of it. But it seems to cost just as much as the bigger


better yet, $134

I have had good luck with BayTech in the past.

Doesnt the packetflux sitemonitor generator controller do that?

That unit is 220V. I bought it once by mistake. Josh' first link is the
15A/120V version.

If all you need is a single port (still 15A limit), and can handle a 70ms
switching time, I've had success with this marine transfer switch:

You'll have to add your own ends/outlets, as it is intended to be hardwired
in place.

+1 on Baytech

The APC SU042 series sell for dirt on ebay.


If you are not looking for "monitoring" of it.
A DPDT 120v 10amp Relay with three power cords cut and attached will make
an ATS for under $30.

Velocity Online

Or the SU041 if you have some patience to wait
for a motivated seller and only need/want NEMA 5-15.

Although as all of these used devices are getting up
there in age, the reliability number is likely going
downwards (so, which two are the priority again?)

If you’re willing to risk that solution, and want monitoring, a $10 Microcontroller
and ~$1.00 worth of ancillary resistors and diodes will get you monitoring. If you
want to get really fancy, you could mount it all to a custom designed PCB for
around $10 ($5/ for 3 copies of the PCB) from <>.

If you want to get really fancy, you can change out the power cords for real
PCB mount IEC outlets and put the whole thing in sheet metal for ~$45 more,
yielding a total cost of <$100 + whatever you value your time at.

The software for the MC would be dirt simple and probably take less than an
hour to write and fully integrate into your monitoring system.

The time to design the PCB for the fully loaded version is probably a couple
of hours with Eagle (if you use an MC, relays, and outlets that have Eagle
Libraries for their parts). Takes 10 days+shipping from Oregon to get the
PCBs. For an extra $5, they’ll ship USPS Priority. One really nice thing about
Eagle and OSH Park is that you can do small stuff in the free version of Eagle
and you can submit the Eagle .BRD file directly to OSH without having to turn
it into gerber files. OSH gives you a very accurate preview of your boards
which is a nice final check before submitting the job for fabrication.

Note: I DO NOT RECOMMEND using this solution. It has a number f shortcomings.

1. It depends on some external force to make the decsion about starting
  or stopping the generator.

2. It can lead to a really rough phase transition when switching.
  If you’re just feeding a UPS and you can make sure that one side
  is down well before you switch to the other side, this is probably OK.
  If you’re feeding some sort of motor and there’s potential for a live
  switch, this can be very hard on said motor and can lead to graphic
  and spectacular failures of said motor as it attempts to change
  it’s armature position instantaneously to match the phase of the
  new power source.

3. It would almost certainly never pass UL, CSA, or any other certification.

4. It’s the kind of thing NEBS was developed to prevent.

5. The potential for combustion if the world ever violates your expectations
  is not insignificant.

Now, with the same hardware, if you have the MC do some additional detecting and
control the switchover process, you’ll add a few failure modes into the system,
but you can make the whole thing a lot safer and actually more reliable. You can
at least build something that will not damage your equipment or catch fire.

The cost would be about the same (same hardware, after all), but you’d need to wire up
a few more pins (more traces on the PCB) to the MCU and you might need a couple more resistors
and diodes.


I have packetflux deployed and find it buggy and of little actual value, im sorry I spent the money.

have you reached out to support? I wish all vendors stood behind their products as much as Forest does.

I love mine, i have them deployed at all my sites.


There's also WTI, which we use:



There's also WTI, which we use:

And for the small deployments their RSM series is great as well: automatic transfer switch, remote power switching and remote serial console all in one box. Those boxes are more expensive, but if you need all of that functionality in 1U they can be really useful.