Let's see.

Quantity (2)

Current per Supply (10 amps)

Total Current Available = 2 x 10 = 20 amps.

Yes.

you copied my M Tech project!!!!.........

Thanks for pointing that out. Yes I am using the power supply for redundancy, but if no power supplies fail the total current available is still 20 Amps, until one fails then I only have 10 Amps available.

Therefore the philosophy is incorrect.

• If the second power supply has been installed as a redundant backup for the first supply, and if both are rated at 10A, then provided the load draws no more than 10A then either or both supplies will support the load. If one of them fails then the supply that has not failed will continue to support the up-to-10A load.
• If the total load is between 10A and 20A and one of the supplies fails, then the second will not be adequate to support the load and it too will fail on overcurrent, thereby turning off the load. In this case, the second supply is vital, and not a redundant backup.

While both configurations are tolerable and safe, the acceptability of each arrangement is to be determined.

Good Answer.

For redundancy of 20A load needs 3x10A supplies.

As in Redundant UPS Systems; one module is addional than the number of modules required for actual load. If one module fails, it is isolated from system and still the system can take the full load.

Regards

Hello rabat.

I have needed to do this a few times, and I have one comment, and a follow up question.

The power supplies I have used are all 120VAC 60 Hz filtered and regulated supplies. I have always cautioned the electrician energizing these to be certain the 120VAC supply for them was from the same phase.

Was this an unnecessary precaution?

The application I am using both 24VDC power supplies will receive it's 120VAC power from the same phase or source.

Let me rephrase my question, addressed to mostly other CR4 members:

The question is, is it necessary to energize the power supplies (in the intent of this thread) on the same phase?

On occasion, I have two (or more) power supplies from different parts of a building all energizing a common piece of equipment. Even though not a scheduled event, it is possible that all of my control boxes will energize this common equipment at the same time. When the power supplies are not/cannot be on the same phase, I have installed a relay isolator, to ensure the sources do not intermix.

To restate my question a little differently, is this isolation effort necessary if the power supplies are energized with different phases from the 120VAC?

For almost every appication of linear pwr supplies (transformer, 1/2 or full wave rectifier, basic filtering) where the transformer primaries are not specifically tied together in some unique manner electrically, the primaries of the transformers do not have to be in phase with each other. Since the secondaries are isolated from each other by the transformers and the rectifiers they will only effect each other in a positive way---> less ripple!

A detailed discussion is much too long for this forum but a rectified 3 phase system has much less ripple and therefore requires much less filtering than a corresponding rectified single phase system

Doorman; i would use different phase! your way would be if the breaker tripped, you would be dead. perry

If redundancy is the reason, then its not a good idea to supply both from the same source as if the source goes down, both are out!!!

As long as both have an isolation transformer as part of their design, then mormally no problems will happen. Diodes may be needed to prevent feedback from a working supply into a not working supply......depending upon design....

Each supply should be fused at 10 amps, so that if for any reason one supply takes too much load, the fuse will open the circuit.

Its a good idea to have a tiny bit of "droop" in such supplies, so that when one gets loaded first, it "droops" a tiny amount and brings the second supply "online". As setting them up to an identical voltage may prove to be difficult.....

I have built and used (and still use!) a really good double power supply (for my CNC work) from the Elektor magazine, that uses 2 x 5 amp chips to supply from 0 to 10 amps....

The circuit is very efficient and saves me money as when I am not drawing current, it only takes about 15% of the power at idle. (Most cheap Power supplies take almost the same power loaded or empty and give the excess power off as heat!!)

One chip is set up to work from 0 to 5 amps, the second "chimes in" when the load exceeds 5 amps.....eg. fills in the gap!!!

A true SMPS will be even slightly more efficient, but complex and difficult/dangerous to set up, this Elektor circuit uses a ring transformer to get the voltage into the required area first, isolating the circuit from the mains and also making the voltage levels far less dangerous. It can be set up to output from 5 to 40 volts as required.

Marking as off topic....

I forgot to mark it as off topic, now it won't let me with editting, sorry.....

Yes !

It is un-neccessary; but all the PSUs be swithed on simulteneously otherwise there is a chance of failure of the supply switche on first.

It all depends on the actual output impedance and in the case of a diode coupled redundant power supply configuration you should not expect any added current capability for two reasons. First, by definition for redundancy each supply should be capable of driving the load alone. Second, the voltage drop across a diode is inversely related to the temperature of the junction. So as one supply starts to provide a little more current through it's power diode (A) than the other diode (B) a positive feedback happens. This happens because the forward bias equation for a PN junction is actually :

i=I_S(e^(qv/nkT) -1)

The variables and constants are the following:

• i Forward diode current
• I_S Reverses diode saturation current
• v Potential voltage across the diode
• q Charge of an electron: 1.6E-19 joules/volt
• k Boltzmann's constant: 1.38E-23 joules/°K
• T Absolute temperature of junction in Kelvin
• n Empirical scaling constant, typically between 0.5 and 2 for silicon. (Doping and topology effects this value.)
• e Euler's constant: ≈2.71828

So with a constant current going through a diode, as the diode temperature increases from self heating the diode voltage drop decreases.

Some power supplies can be used thus (e.g. Power One Type W) - some can't (without additional circuitry). If in doubt, consult the datasheet for your power supplies, or contact the manufacturers.

Generally speaking, the answer is usually no unless the supplies are specifically designed to be paralleled. what happens is that one supply will be a fraction of a volt higher than the other and it will supply virtually all of the current and the other supply will essentially just be barely ticking over. It also depends on the type of supply it is. linear supplies do not parallel worth a flip. Switch mode supplies with MOSFET outputs often can because MOSFETS have a negative temperature coefficient. As the current increases, the on resistance increases and the current goes back down. so MOSFETs in parallel share loads fairly well. If you are using switch-mode supplies with MOSFET outputs, you should probably put a very low resistance high wattage resistor in series with each supply.

Friends,

Last two answers are best. Let me add--go to the manufacturer's literature and check their application instructions. If they say you can parallel the outputs, then you can. If they say that the inputs of paralleled supplies must all be on the same phase or same circuit, then that is a mandatory requirement and you violate it at your own peril/liability. Depending on the degree of reliability or redundancy needed in your system, particularly if life safety or security is involved, you may want to include methods to monitor the health and operation of each of the supplies in addition to monitoring the total current draw, etc. Remember that most fire codes will require that any electrically-locked exit doors be unlocked in the event of a fire alarm. This is true even in a mental institution with locked wards.

--JMM

Regards.

Today most of the Dual Supplies are manufactured to be connected in parallel to double the current & in series to double the Voltage ratings.

These supplies are with a sngle power cord so no question of connecting to different phases.

The output of all supplies is DC, if connected to different phases of the same grid there seems to be no problem as the N & E are common.

We ar not connecting AC in parralel where In-Phase connection is required.

Yes....but bear in mind how you achieve this connection I would use 2 diodes and at least one condenser.

Why the condenser?

To make sure that .....main supply ripples are minmum

have always a ceramic 0.001µF capacitor to remove hi freq noice/ripples

Regards

Yes !

Well known Mic having low background noise.

LOL!!

Like it!!!

So by preferring the use of more common terms [as agreed by all the parties] so as not to confuse, I am to be likened to a perpetrator of the spanish inquisition? & Wouldn't a luddite prefer an obsolete term?

You have spewed a verbose rant, do you feel better? Why don't you and your clan of egomaniacs try for the accuracy you claim to relish .!.. next time?

Hmm

So is the IQ of 3 brains in parallel additive like Capacitors? _{only my refrigerator has has a condensor}

By the looks of this thread, I think the resistor formula is more likely to be correct

Talking to yourself?

How many guests does it take to screw in a light bulb?

I hope you now feel better........

Actually, I do feel better. Thank You. I'm glad you liked it.

I wasn't actually addressing you at that point in time (check back!!), but I am still VERY glad you do feel better my friend!!!

Maybe we should start a thread on names for electronic names/devices that have either been forgotten or are not generally used anymore!

Wasn't the original Condenser/Capacitor a "Leyden Jar"? That also showed if there was a charge stored on it visually!!

Rheostat? Nowadays a variable potentiometer......Though I personally tend to think of Rheostats as handling amps rather than milliamps.....

Cat's whisker, an early forerunner of the diode, but needed careful handling. My Father as a kid woould make them from a short length of aluminium rod and a piece of coal....for his "Crystal set".......

Valves are still in use, but not quite so much by the amateur anymore.....

Anymore Guys?

A variable resistor was once called a Rheostat.......

Look here:-

Rheostat

Yes !

But normally big [Hi-Power used to control current thru or Voltage accross] a power device like Fan speed, light dimming, etc etc.

Exactly as I already mentioned some days ago in post #37 (this is nowpost #54 or so!!!) AND it was re-mentioned in the link I posted.

Now you have mentioned it a third time......

Was that REALLY needed ?