Continuous AC from 2 separate AC leads

Thread Starter

LST

Joined Jan 17, 2020
12
I have two AC mains power leads, A and B, coming from the same circuit breaker having the same neutral. Theoretically one, but only one, of the leads will be energized at any given time, although practically there might be a brief moment during switching (a few milliseconds) when both leads have power (and I can’t have one lead feed power to the other), or there may be a brief moment when neither lead has power and I need the AC to continuously provide power to a transformer/rectifier circuit which powers a 5VDC microcontroller circuit board so the microcontroller won’t reset.

What can I do on the AC side to (a) prevent lead A from ever feeding power to lead B and (b) insure continuous power? I guess (b) could have a solution on the AC or DV side.

Thanks.
 

crutschow

Joined Mar 14, 2008
25,432
What can I do on the AC side to (a) prevent lead A from ever feeding power to lead B
Do the changeover with a SPDT switch or relay (which are normally break-before-make).
To minimize any arc-over possibility between the two AC lines during the switch-over, use a DPDT device with the AC connected to the two wipers, and alternate contacts to the load.
insure continuous power?
Have a large enough capacitor on the rectifier output to power the load during the short (several ms) changeover time.
The capacitor already there, may be sufficient for that.
 

Thread Starter

LST

Joined Jan 17, 2020
12
Do the changeover with a SPDT switch or relay (which are normally break-before-make).
To minimize any arc-over possibility between the two AC lines during the switch-over, use a DPDT device with the AC connected to the two wipers, and alternate contacts to the load.

Have a large enough capacitor on the rectifier output to power the load during the short (several ms) changeover time.
The capacitor already there, may be sufficient for that.
Got it. Thanks crutschow!
 

LowQCab

Joined Nov 6, 2012
55
I have two AC mains power leads, A and B, coming from the same circuit breaker having the same neutral. Theoretically one, but only one, of the leads will be energized at any given time, although practically there might be a brief moment during switching (a few milliseconds) when both leads have power (and I can’t have one lead feed power to the other), or there may be a brief moment when neither lead has power and I need the AC to continuously provide power to a transformer/rectifier circuit which powers a 5VDC microcontroller circuit board so the microcontroller won’t reset.

What can I do on the AC side to (a) prevent lead A from ever feeding power to lead B and (b) insure continuous power? I guess (b) could have a solution on the AC or DV side.

Thanks.
Don't make it more complicated than it has to be.
No switches/relays necessary.
Use 2 power supplies, each one supplied by a different AC source,
now you have clean DC power to play with,
each power supply can be isolated from the other with 2 diodes.

You didn't specify how long you expect the power supply to remain up after
loss of ALL AC power.
If it's less than a second or so, it's probably not an issue,
as long as the power supplies you design have reasonably large storage capacitors.
If you expect longer periods with no AC,
then you can look into the controlled charging of a single Super Capacitor,
or a single Battery, (from either AC source),
but this would require slightly more complexity,
and power supplies that can power your circuit, plus
provide reasonable charging current for
charging up the Super Capacitor within an acceptable time period.
In this case you would want to have either power supply adjusted to provide a
higher voltage for charging the Super Capacitor than is required to run your circuit,
then, after the Super Capacitor, (or battery),
the voltage must be regulated back down to the requirements of your circuit.
Depending upon how much power your circuit draws,
and the size of the Super Capacitors, (or battery),
this could easily give you an hour, to several days or more, run time,
with complete loss of AC power from either source.

Of course the easy way is to just go buy a generic computer UPS,
(Uninterruptible Power Supply) from your local Computer Store,
then you have available both 120vac, and 12vdc, from the internal Battery,
even when you loose all AC power.
.
 

Thread Starter

LST

Joined Jan 17, 2020
12
Don't make it more complicated than it has to be.
No switches/relays necessary.
Use 2 power supplies, each one supplied by a different AC source,
now you have clean DC power to play with,
each power supply can be isolated from the other with 2 diodes.

You didn't specify how long you expect the power supply to remain up after
loss of ALL AC power.
If it's less than a second or so, it's probably not an issue,
as long as the power supplies you design have reasonably large storage capacitors.
...
Of course the easy way is to just go buy a generic computer UPS,
(Uninterruptible Power Supply) from your local Computer Store,
then you have available both 120vac, and 12vdc, from the internal Battery,
even when you loose all AC power.
.
Thank you for your reply.

The switch and relay solution suggested by crutschow will work with constant AC power feeding the rectifier circuit from the lower left relay lead shown below...

1581150532755.png

...but you present an interesting solution to use 2 power supplies (which I interpreted as a suggestion to use 2 transformer/rectifier circuits, each fed by one of the 2 leads) that is worth considering.

Sorry that I wasn't clear - when I said that there may be "brief moment when neither lead has power" and specified" a brief moment as "(a few milliseconds)", that was to say that power loss would be just a few milliseconds, indeed the amount of time that the contacts of a switch take to toggle, so yes, less that 1 second, and therefore I agree that given this circumstance with a large enough capacitor at the other end of the rectifier that it's not an issue.

Lastly, I'm powering a small circuit board so a relatively giant UPS isn't practical.
 
Last edited:

LowQCab

Joined Nov 6, 2012
55
It's a chronic problem in these forums that people come up with a solution as to
how to go about achieving a particular goal, and then want help with how to implement it.
When, quite often, there is a much simpler way of achieving that end result,
usually with fewer unwanted side effects,
and sometimes a cheaper, and/or, more elegant solution.

So, exactly what is it that you expect to have as an end result ??
You stated that you have 2 basically identical AC circuits with a common neutral,
are these "Hot Legs" on the same Phase in the Panel, or on opposite Phases ??
Must you have it work no matter which phase is supplying the AC ??
Why are you using 2 basically identical Phase Legs off of the same Panel ??
Are you measuring some aspect of the AC Power being supplied ??
Or are you simply powering a circuit that must never be un-powered ??
Evidently you don't need to have full isolation from the Line,
or maybe you need a direct connection to mains power, (quite dangerous),
I'm guessing because you are willing to use a clunky relay to switch it.

If possible,
you should isolate your circuit from the mains as early as possible in the circuit.
If you're not concerned with an un-isolated power supply,
you can simply use 2 "high-input-voltage" Linear Regulators,
which will take up far less circuit board real estate than transformers or relays,
and will seamlessly provide your circuits requirements.
If you are using a relay simply because you understand how to control it,
learn the advantages of using the "Enable" Pin on
the above mentioned Regulators to switch between them.

So, what is your end goal ??
How much power will it require, and at what voltage ??
.
 
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