2 AC inputs, 1 AC output

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dnszero
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2 AC inputs, 1 AC output

Post by dnszero » Fri Apr 04, 2003 2:16 pm

I want to be able to connect 1 AC receptacle to 2 seperate AC outlets (preferably with out destroying the connected equipment or killing myself).<p>Why? Well, I've got 2 seperate sources of AC and I need to be able to keep a computer running if one or the other goes down. So, I need something that would be able to switch from one to the other quickly, or to just use both at once.<p>I'm not sure how difficult this is, but it seems like it shouldn't be too hard.<p>Thanks in advance for any thoughts or advice.
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Crowbar
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Re: 2 AC inputs, 1 AC output

Post by Crowbar » Fri Apr 04, 2003 3:14 pm

What you need is a UPS. The smaller ones are not that expensive anymore, the biggest factor being how much uptime do you need.
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Chris Smith
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Re: 2 AC inputs, 1 AC output

Post by Chris Smith » Fri Apr 04, 2003 3:27 pm

Trying to synchronize Two AC sources is the hard part, and if you don’t, they fight each other on their wave fronts. <p>A UPS simply switches on fast enough to “not miss a beat” but remains off when its not needed. <p>You could use the switching method used in a UPS to do what you want. But power source must be off, or not connected, and it switches on fast enough to catch the circuit before it fails.

dnszero
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Re: 2 AC inputs, 1 AC output

Post by dnszero » Fri Apr 04, 2003 6:08 pm

Well, actually I am using a UPS and that's the problem.
The UPS's I have occasionally fail in such a manner that they no longer supply power to the connected gear. What I'm wanting to achieve, essentially, is a device that will allow me to bypass the UPS when it has failed (but when the wall outlet is still working).<p>Maybe some sort of relay setup? And actually, it's not even essential that switch from one power source to the other be transparent. My gear tends to be at rather remote locations (avg drive of 2-5 hours away). So, I'd actually be happy with just about any solution even if it switched so slowly that gear rebooted. Hell, even if it takes 2 hours to switch over, that'd be an improvement. :)
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Re: 2 AC inputs, 1 AC output

Post by rshayes » Sat Apr 05, 2003 4:51 am

Relays are a fairly simple solution to this problem since you don't need fast switching. Remember that they do not operate or drop out instantaneously. These times may be several tenths of a second for relays capable of switching AC power. Be careful that these delays do not result in even instantaneous shorts across the AC line or between different AC sources. Fault conditions such as that can make substantial metal contacts either dissapear or weld solid.<p>Get relays with rugged construction and conservative ratings. Inrush currents can weld undersized contacts closed. Some companies that used to make these relays were Potter & Brumfield, Sigma, and Guardian, among others. Try looking in the catalogs of some of the older distributors, such as Newark Electronics and Allied (they may be part of Hallmark now). If they give ratings for motor starting or incandescent lamp loads you are probably looking in the right section. These loads characteristcally have much higher starting currents than operating currents, and the relays are built more ruggedly to withstand this. They also tend to have good insulation and screw terminals with barriers between the terminals. Use insulated crimp on ring lugs for wiring, don't just twist wire around the screw and tighten it down. After wiring, a little bit of RTV over the connections will reduce the chance of contact or shorts. Shorts can be caused by water, insects, and possibly small animals, as well as dropped screwdrivers. Enclose everything in a box, preferably metal. Fuses in the power leads are a good idea. If you can, ask an electrician for advice. Remember that the peak voltage on a 110 VAC line can be close to 200 volts with high peak current capacities (100's of amps).<p>An alternate method would be to operate separate DC supplies from each AC source and combine their outputs with diodes. Each supply needs to be capable of supplying the full load. This has the advantage that you do not have to do any AC wiring; all of the switching is done at low voltage DC. The power supplies will already be designed to meet the various regulatory requirements and will already have protective features such as fusing. For 5 volt loads, you would probably want to use large shottky barrier rectifier diodes for low voltage drop. The power supplies will have to be adjusted slightly higher to compensate for this. Don't operate the diodes at anything close to their maximum rated current. This rating is usually based on optimum conditions and high power dissipation in the diode. Look at the voltage vs current plots in the data sheets.<p>Basically, the highest power supply supplies the load. If this one fails (due to loss of input voltage), the second highest takes over, and et cetera. As long as any supply is working, the load will be powered, with no problems with transfer time.

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frhrwa
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Re: 2 AC inputs, 1 AC output

Post by frhrwa » Sat Apr 05, 2003 11:23 am

geesh, all this time I thought the UPS (uninteruptable power supply)meant you ran off the inverter? The AC power converted to DC (charger) to keep the batteries charged, the inverter kept the outside AC totally isolated from the output going to your computer or whatever, therefor, no switching time.. this protected it from spikes also... And, the BBU (battery back up) was nothing more than a very fast relay or switching setup, running off the AC power until brown out, loss of power, spike, or what have you, then it switched to battery invertor? so its basically a charger for the batteries, batteries for the invertor, and some sort of fast switcher for the output..?<p>[ April 05, 2003: Message edited by: wayne ]</p>
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