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Black Forest
09-29-2014, 05:25 AM
Is it possible to use two 24 power supplies to supply one row of terminal blocks in order to increase the amps available? Would the term be to connect them in parallel???

For instance I have a 24v 10amp and a 24v 3amp if combined would that give me 13amps available?

Stuart Br
09-29-2014, 07:25 AM
Parallel connection is the correct term.
Generally a very bad idea unless the PSUs are designed for the task.
Magic smoke to be expected.
There are circuits out on the web that show diodes connected on the outputs of each supply to prevent reverse currents flowing due to different output voltages. Not particularly efficient.

radkins
09-29-2014, 08:01 AM
I tried this once with two 5 AMP 12 Volt supplies using the diodes and it did work, well sort of anyway. At the higher loads one of them, for whatever the reason, seemed to be carrying most of the load and with both sides protected by a 5 AMP fuse (each) that one that seemed to be carrying the heavier load would sometimes pop the fuse at about a 7-8 AMP load when they were combined. Now bear in mind I am in no way an expert and this set-up was only an amateur effort, actually I find the post very interesting due to my somewhat failed effort and I am eager to see what the experts, that I am sure will chime in, have to say

browne92
09-29-2014, 09:33 AM
+1 with Stuart.

ironmonger
09-29-2014, 10:23 AM
+ the bad idea of parallel connections.

If you have multiple 24V loads just find one that has a 3 amp requirement and dedicate one supply to that. I would sooner find the current limiting pot and dial it up rather than connect them in parallel.

paul

RichR
09-29-2014, 10:57 AM
Hi Black Forest
Either get a bigger power supply or separate the load into two appropriately sized loads. Trying to parallel the two supplies using isolation diodes is likely
to fail, especially since their current ratings are so mismatched. The problem with this technique is one supply will always have a slightly larger output
voltage than the other, and it will dominate by trying to supply most of the current. If the diode in series with that supply warms up due to the current, its
voltage drop goes down and that supply tries to provide even more current.

macona
09-29-2014, 11:56 AM
It really depends on the supply. Some are designed to be paralled, some are not. Look in the manual and see what it says.

Mike Burch
09-29-2014, 05:18 PM
A very bad idea.
And using diodes is not clever either, especially if the voltage is critical. A silicon diode has a forward voltage drop of around 0.7v.

J Tiers
09-29-2014, 08:50 PM
It really depends on the supply. Some are designed to be paralled, some are not. Look in the manual and see what it says.

Ding Ding Ding!

This is the correct answer.

If you want the simple answer, it is do not try it...... because that assures not damaging either one from paralleling.....

PStechPaul
09-29-2014, 09:02 PM
Paralleling unregulated supplies is usually OK, especially with output diodes. You may be able to check the voltages of the two regulated supplies and perhaps adjust one or both so they match closely. Also, you could "deregulate" the outputs by adding low value resistors so that each supply drops about the same voltage at their rated capacity, which would help with current sharing.

So the 24V 10A supply might use R=0.5/10=0.05 ohm (5W) and the smaller would use 0.5/3=0.17 ohm (1.5W).

dp
09-29-2014, 09:18 PM
Depending on where the regulation feedback is pulled from each supply can confuse the other, even with diode isolation. It is a pretty bad way to power something. Redundant supplies that I work with coast until needed and since they're always on, they're available as fast as the fail over circuitry can handle the transfer. That is for reliability though, not capacity. I agree with others that splitting the load across isolated supplies is the best practice.

J Tiers
09-29-2014, 10:54 PM
Supplies that can be paralleled are in some cases set up to use ONE feedback as the master, and feed the control signal to all of them. That's one good way, but obviously must be designed-in.

dp
09-29-2014, 11:33 PM
Exactly - the loop gains and phase responses of the two systems have to be identical in order for both loops to respond in unison, amplitude, and in sign else they will battle each other to the finish.

darryl
09-29-2014, 11:56 PM
A bad idea if the power supplies are regulated. The 'old fashioned' brute force power supply, with rectifier and filter, can work well this way. If both have the same output voltage under no load, then neither will try to power the other and both will supply current to the load. If one has a higher output than the other, it will charge the filter in the lower voltage one to the higher voltage and there will be no ill effect to either. When a light load is on, the one with the higher output voltage will supply all the current. When the load is heavier and the voltage is drooping, the lower voltage one will start to contribute current. However the first one will be trying to supply most of the current and could become overloaded before the second one contributes its share of current. If the voltages are widely different, you could have the higher voltage one damaging the lower voltage one- usually the problem here would be the capacitor voltage rating, but you could also be coming near the breakdown voltage of the rectifiers. And if you did couple a pair of supplies where the output voltages are quite different, the higher voltage one would be stuck trying to supply all the needed current, while the lower voltage one does nothing except suck idle current from the line input.

If both power supplies have the same output voltage, as in maybe they are the same make and model, then they will try to share the load equally and your current rating will be double that of a single supply. You do though have the possibility of one failing- then the other will become overloaded if your current demand exceeds it's rated output.

As JT suggested, one regulator can control multiple output devices. In a high current power supply there may be multiple devices, but they normally are fed from a single 'brute force' power supply capable of enough current and voltage. You can have separate input voltages from separate transformers, but this approach means more parts and probably a larger package than you'd have with a single, larger power transformer. It works if you're forced to fit it all into a narrow package where certain dimensions are restricted and you can't fit a larger transformer.

These days, switching regulators are very common, and they would present a nightmare trying to run them in parallel.

atty
09-30-2014, 12:04 AM
If you must......you can cheat a little. Grab up a couple of 12V car-type batteries, rig 'em in series, then drop the paralleled supplies across the pair. Take your load from the batteries. It will act as a bit of a buffer to the load presented to the power supplies, and possibly prevent all the ills that were previously described. I say "possibly". It may only delay the inevitable, but it'd be damned fun to try.

Mike Burch
09-30-2014, 12:37 AM
If you must......you can cheat a little. Grab up a couple of 12V car-type batteries, rig 'em in series, then drop the paralleled supplies across the pair. Take your load from the batteries. It will act as a bit of a buffer to the load presented to the power supplies, and possibly prevent all the ills that were previously described. I say "possibly". It may only delay the inevitable, but it'd be damned fun to try.

Unless the supplies are designed to be used as battery chargers, do not do this. Most switch-mode power supplies (i.e., not designated chargers) will let their magic smoke out if you do.

Black Forest
09-30-2014, 01:14 AM
OK I will not hook the two together. I am having some issues with the control cabinet on my sheep shearing machine. I will try today with a tractor 24v battery and see if the problem goes away. If so then I will buy a bigger power supply. The one I have is only 10 amps and should be enough but maybe I calculated wrong!

J Tiers
09-30-2014, 08:29 AM
These days, switching regulators are very common, and they would present a nightmare trying to run them in parallel.

If they are "current mode" supplies, they are extremely easy to design as capable of being paralleled. Other types, aer less so, but it is possible to do. Same setup, one master, and everyone else has to use the same control signal.

if not so designed, they may not fail (not if well designed) but also will not work well together, won't share load, and won't do what you want..

Paul Alciatore
10-01-2014, 11:38 PM
I have frequently seen this done with identical, UNREGULATED supplies to provide redundancy, not more current. Each of the supplies was capable of supplying the full load current by itself. AND the regulation was done further down the line, at the individual loads. They were combined with steering diodes that prevented current from flowing back into either of the supplies.

I suppose it could be made to work for increasing the current, again with identical, unregulated supplies, but care would have to be taken. I would not try to do it with regulated supplies for the reasons given by many others above. Bad idea.

PStechPaul
10-02-2014, 12:38 AM
The resistor decoupling I proposed should work for unmatched, regulated supplies, unless either supply fails with a short circuit output or unusually high voltage or other unlikely event. And in that case, various overcurrent or overvoltage (crowbar) mechanisms would probably kick in and prevent catastrophic damage to either supply or the load. But it is likely not practical or efficient for a 30% increase in capacity when such supplies are so inexpensive.

Overvoltage detection and protection, along with a fuse or circuit breaker, are important considerations if the load is sensitive and expensive.

darryl
10-02-2014, 01:33 AM
Perhaps what we could be talking about is why some switching power supplies have problems with some loads that are well within their ratings. This might be what's happening with BF- a supply that should be capable of powering the system somehow can't do it-

I have some like this. One is a 1.5 amp, 12v switchmode ps that won't run a 12v led light that takes about 200ma. But it's fine with a 10 ohm resister for a load-

Black Forest
10-02-2014, 05:52 AM
Here is a link to the power supply that I have.

http://www.automation24.co.uk/power-supplies/power-supply-unit-phoenix-2866323-trio-ps/-1ac/24dc/10-i28-520-0.htm

MrSleepy
10-02-2014, 06:27 AM
If your total load is too much for one supply , why not divide the circuit into zones each having its own supply. In your case split it into hydraulic power and control power, or further ..hydraulic1 hydraulic2 and control. It should also ease fault finding in the future and giving the control a nice clean supply of its own is always a benefit.

Rob

Stuart Br
10-02-2014, 06:52 AM
Here is a link to the power supply that I have.

http://www.automation24.co.uk/power-supplies/power-supply-unit-phoenix-2866323-trio-ps/-1ac/24dc/10-i28-520-0.htm

The datasheet does say that it can be used for parallel operation providing the PSUs are of the same type

"Parallel operation
Devices of the same type can be connected in parallel to increase both redundancy and power. By default upon
delivery, no further adjustments are required. If the output voltage is adjusted, a uniform distribution of
power is guaranteed by setting all parallel operated power supply units to exactly the same output voltage.
To ensure symmetrical current distribution we recommend that all cable connections from the power supply unit to the
busbar are the same length and have the same cross section.
Depending on the system, for parallel connection of more than two power supplies a protective circuit should be
installed at each individual device output (e.g., decoupling diode, DC fuse or circuit breaker). This prevents high return
currents in the event of a secondary device fault"

Black Forest
10-02-2014, 06:58 AM
Maybe my logic is wrong in how I have the input switches and limit switches connected. On the Siemens Logo's that I am using the inputs are only one terminal. I feed the switches a positive electric connection. When the switch is in the make position that feeds a positive electric current to the input on the Logo. The negative is through the negative connected to the Logo. It worked for me in the past. Is that a normal type of circuit?

Paul Alciatore
10-02-2014, 11:52 AM
This can be a workable scheme, but some caution should be taken to prevent problems if one supply does fail. Be sure that the un-powered circuit's inputs and outputs will not draw too much current in that condition
.


If your total load is too much for one supply , why not divide the circuit into zones each having its own supply. In your case split it into hydraulic power and control power, or further ..hydraulic1 hydraulic2 and control. It should also ease fault finding in the future and giving the control a nice clean supply of its own is always a benefit.

Rob