multiple source different current, same voltage in parallel

Thread Starter

fifi18

Joined May 25, 2018
2
Hello,eletric circuit.png
Is been a long time when I learn electric circuits in my school but i have a question.

Is possible to connect multiple suppliers at the same voltage but different currents?
I did a circuit to better understand.

And last question what kind of device or component (?) I can connect to have the output the same voltage as input, but the current is the summation from all input current.
Do I need this (?) device/component, or I can connect the cables and the current will flow

thank you!
 

LowQCab

Joined Nov 6, 2012
3,939
This is not a good plan, especially if You are planning to use Batteries.

Your plan above looks like it might be for Solar-Panels,
if so, don't try to re-invent the wheel,
there are plenty of effective ways to interface multiple Panels,
none of them are achieved with no added-on Components.
.
.
.
 

Thread Starter

fifi18

Joined May 25, 2018
2
Maybe is missing information, the suppliers 15V with different A just go in one direction, and is not batteries.
I just want to know if have a component (?) that works like this, or I simple remove the component (?) and connect the wires for the next device
 

Tonyr1084

Joined Sep 24, 2015
7,782
Having multiple voltage sources in parallel does not change the voltage.
Having multiple different amp hour sources in parallel will add up their total amps.
Having a device connected to these multiples means the device will draw as much current as it may require.

I'll ignore the 0V, 0A portion as it has no effect. The smallest supply is capable of only 3 amps while the largest is capable of 10 amps. Suppose your device draws 5 amps. The 3 amp and 4 amp sources are insufficient on their own but combined they have sufficient power to drive the device - hence forth called the circuit. Each source will do its best to supply what is needed while at the same time sharing the load. However do to the unavoidable differences in output voltages as minor as they may be, one supply will be more heavily taxed than the others. It will be the first to fail. I haven't forgotten the 10 amp supply. In total and in theory you have 17 amps available for your circuit. Suppose your circuit draws 12 amps. Likely - and this is a guess - the 3 amp source will fail first leaving the remaining 14 amps of supply to carry the load. 12 amps is going to be more harmful to the 4 amp supply and it may likely fail. Again this is guessing due to the imperfections of each final voltage. Once the second supply fails the third will fall like a domino.

Ohms law tells us that "the circuit" will draw so much power (watts). That's volts times amps. The amount of power the circuit draws can be considered as resistance. So if the circuit draws 180W and the source voltage is 15V then 180W ÷ 15V = 12A. That means the circuit has an equivalent resistance of 1.25Ω. Since the circuit is unlikely to change its resistance then the next thing that will be affected is voltage. As you approach the max current any supply can give then its voltage will drop off. Exceed the capabilities of the supply and the voltage will plummet and likely overheat the source. As the voltage drops the available amps drops. Remember, the circuit isn't going to change its resistance based on the supply. It will remain basically a constant. Having unbalanced supplies means the weaker supply will be more prone to voltage drop-off than the others. As it fails you begin a cascading failure.

The best approach, and I suppose your question is more an exercise in theory and understanding than in a practical circuit since nobody makes a zero volt zero amp supply. Basically that's just an open wire. So the best approach is to have a supply sufficient to carry the whole load with headroom. What's headroom? Suppose your circuit requires 10 amps. Headroom is a supply that is not just capable of powering that given amperage but having more than is needed. Some people here say twice the available amps is recommended. I'm more of a minimalist. While others recommend 200% I go with a more standard approach of 133% or 150%. Why those numbers? Because in the industry it's been my experience that good grade commercial equipment favor the 133% approach. That means a smaller supply and lower cost to manufacture. The 150% comes from aerospace, automotive and medical equipment. Anything that is "Life" critical or "Mission" critical generally gets the 150% build. It's more expensive but it's more reliable. Whether it's a supply or a component, having that extra headroom is the difference between something that lasts a lifetime or something that lasts years down to something that may work on the bench for a few minutes but then leak the magic smoke.

The short answer to your question: 15V at 10A plus 4A plus 3A equals (in theory) 17A. But you've subjected yourself to three unknowns. Which supply will experience a drop-off before the others. Once drop-off begins - failure is sure to follow. Of course, that depends on how much the circuit is drawing.
 

MisterBill2

Joined Jan 23, 2018
17,827
I have used multiple non-regulated power supplies of the same voltage in parallel to supply higher current loads. It can work it can be a handy solution to not enough supply current rating.
 

Tonyr1084

Joined Sep 24, 2015
7,782
I have used multiple non-regulated power supplies of the same voltage in parallel to supply higher current loads. It can work it can be a handy solution to not enough supply current rating.
Yeah, it can be done. Doing so is not the best advice though. At least that's my opinion.
The balancing act to make that system function would be monumental at best.
This is where I stand.

A few decades ago I took some circuit breakers from a scrapped aircraft panel. A 2 amp breaker, 3A, 5A, 7A and 10A breakers and put them in parallel. I thought I had a pretty nifty rig. I could select any of those values or combine them to get different values of capacity. 5A and 3A would give me 8A. 7A and 2A would give me 9A. 12A was possible, so was 13A, 15A, 17A, 18A and so on. But it didn't work that way. When one tripped the remaining would cascade.
 

ElectricSpidey

Joined Dec 2, 2017
2,756
If all three supplies had current limiting, there would be no cascade failure. (in theory)

In theory and only in theory...

You set the current limits as 10 amps, 4 amps and 3 amps.

You set the voltage of the 10 amp supply at 15 volts.

Then you set the 4 amp supply voltage a little below the 10 amp supply and the 3 amp supply a little below the 4 amp supply.

As the 10 amp supply reaches its set point the voltage begins to drop and the 4 amp supply begins to supply current and so on.

And that is only my personal fantasy and I have absolutely no clue if it would actually work. :)
 

MisterBill2

Joined Jan 23, 2018
17,827
NOTE!!!! The supplies that I have run in parallel were all UNREGULATED SUPPLIES of the same rated output voltages and similar current ratings. (but not identical current ratings). It is more complex to operate regulated supplies in parallel, AND it is important that none of them have over-voltage protection. One additional detail is not running at the maximum current that one would guess, based on the sum of the current ratings. So two six amp supplies would be OK for ten amps not 12.
 

Pyrex

Joined Feb 16, 2022
239
fifi18,
sure you can.
In the real world, those PSU have a little bit unequal voltages. But this is not a problem for PSU with current limiting circuitry. You can take 17A from the whole circuit
 

Tonyr1084

Joined Sep 24, 2015
7,782
You can take 17A from the whole circuit
In theory.

I vaguely remember needing 24VAC and using two 12V transformers in series. Different scenario - I know - but the results were less than hoped for.

Many power supplies have dual secondaries that can be set up in parallel or in series (voltage wise in series and current wise in parallel) IT WILL WORK. But when such is engineered there's an allowance for headroom. In other words it's over engineered.
 

Tonyr1084

Joined Sep 24, 2015
7,782
Seen this on many occasions: Single primary, dual secondaries. Even seen multiple secondaries. Also have seen dual primaries which allow operation on 110VAC or 220VAC.
1700578838303.png
Granted, in this scenario both secondaries are constructed to deliver 12 volts at 1 amp. But in the real world unless the transformer is built with EXTREMELY precise construction techniques and exacting number of turns of wire gauge that is exactly the same, both coils will have what's considered to be 12 volts but one might be at 12.01 V while the other is at 11.99V. Or likely worse. Same applies to amperage. They will combine. But using individual power sources, not one constructed as pictured above, means you have even less control of how precise each output will be. ESR (Equivalent Series Resistance) (not previously mentioned) will also have some minor play in individual power supplies.

And yes, this is all theory. I didn't get the impression the TS has built such a machine. Nor did I get the impression TS want's to build such a machine.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
17,827
Seen this on many occasions: Single primary, dual secondaries. Even seen multiple secondaries. Also have seen dual primaries which allow operation on 110VAC or 220VAC.
View attachment 308085
Granted, in this scenario both secondaries are constructed to deliver 12 volts at 1 amp. But in the real world unless the transformer is built with EXTREMELY precise construction techniques and exacting number of turns of wire gauge that is exactly the same, both coils will have what's considered to be 12 volts but one might be at 12.01 V while the other is at 11.99V. Or likely worse. Same applies to amperage. They will combine. But using individual power sources, not one constructed as pictured above, means you have even less control of how precise each output will be. ESR (Equivalent Series Resistance) (not previously mentioned) will also have some minor play in individual power supplies.

And yes, this is all theory. I didn't get the impression the TS has built such a machine. Nor did I get the impression TS want's to build such a machine.
There is a very good brand of transformers that are intended to be used that way. two identical primaries and two identical secondaries. And very good quality in al of their aspects.
 

Pyrex

Joined Feb 16, 2022
239
I do agree, that use secondaries in parallel of the transformer is not so easy as windings are not absolutely equal. And, much worse, usage of different transformers in parallel.
On the other hand, a lot of transformers are so designed to use the secondaries in series and in parallel. And an extreme precison is not a mandatory.

But if we talking about various DC PSU's, which have current limiting circuit, I do not see any problems to operate in parallel
 

ThePanMan

Joined Mar 13, 2020
757
Late to this conversation. Yes, you can parallel differing supplies and combine their total amp output. But as others have said, be ware of problems that can arise. It's ill advised. Yes, you can do it. The question should be "SHOULD I do it?" My answer to THAT question is "Well, not really. The world won't come to an end but it's not good practice for any hobbyist." Learn to do it right or stunt your learning growth.
 

ThePanMan

Joined Mar 13, 2020
757
I have a transformer in my shop that has two primaries. Can be powered from 120VAC or 240VAC. The outputs are three fold with multiple center and off-center taps. Without grabbing it I recall the MAIN secondary is 65V with center tap and 28V taps as well. There's another secondary that is 12V-0V-12V and finally there's a 5V secondary. Taking THAT transformer and putting it in series with another transformer of different origins take away some of the certainty one can expect from a transformer that is built that way and one that is bodged that way. Bodgification is not a good practice.
 
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