dc output switchers
since most switchers isolate the inputs from the outputs, can two separate board outputs be paralleled together or should i use a diode summ point to prevent sinking current back into one? say i get two 12v 2.5 amp switchers, can i make an equivalent 5 amp from them?
Similar to you, I wanted to take a handful of switching PSs scavenged from about half a metric buttload of old computers that had outputs rated for 12 volts @ 7 amps each, and combine them to get a 12 volt @ 50 (+/-) amp supply for use in powering a stepper motor that wanted to be driven with 12 volts @ 40 amps.
I pretty much got a chorus of "No way to do that - they'll eat each other" responses.
Ended up powering the rig I was trying to build from a car battery - It actually ended up being cheaper and easier to go that route and add a battery charger, than buying or building a 12 @ 50 power supply. Go figure!
-- Don Bruder - dakidd@sonic.net - If your "From:" address isn't on my whitelist, or the subject of the message doesn't contain the exact text "PopperAndShadow" somewhere, any message sent to this address will go in the garbage without my ever knowing it arrived. Sorry... for more info
That depends on their design. Some are made for N+1 redundant applications, where they will all put out the exact same voltage to the load(s), and share the load(s) evenly. If not, all bets are off.
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switchers,
Generally the answer is NO (or at least not advisable) for several reasons...
1) Source impedance: Two ideal voltage sources should never be connected in parallel. How much current would flow from one to the other if one was 1% higher voltage than the other? 2) Miss matched loading. How can you be sure they will both supply half the current needed by the load? You might expect the one with the slightly higher out put voltage to supply all the current. The regulator in the low one is crying out.."help help I've turned down the output stage but the output voltage is still too high..it's 5.1V and it should be 5V". 3) Stability. A power supply is like a power amplifier. Think of it like this... Take a low power voltage reference like a 5V zenner diode and connect it to the input of a power amplifier with unity gain. The output is also 5V but you can take many 10's of amps out of it unlike the zenner. Now imagine that the load current is AC. The power supply/amplifier has to maintain a constant voltage despite the output current changing over a wide frequency range. Now do you see why a power supply is like a low gain DC amplifier. The problem is that amplifiers can become unstable. Particularly when large amounts of feed back are employed to reduce the gain to unity. Putting two power amplifiers in parrallel might work but they might also start oscilating. Even if they don't oscilate you can have a problem with stability....for example they might not respond well when the load current changes. The output voltage might "ring" and go out of spec when you hit the power supply with a current spike.Diodes stop 1) but not 2) or 3). In addition there will be a voltage drop accross the diode that varies from 0 to 1.5V depending on the diode and current - hardly does wonders for the regulation.
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