Parallel Power Sources: Use 2 12V 75W Chargers for 150W Project

[Puglife]

Hi, I am trying to make one of my project plug into the wall. It needs 150 Watts, at 12 volts. I am going to use laptop chargers to power it, but I only have 2, 75 Watt chargers.

Both of the chargers are 12 volt chargers, and I was wondering, could I use these two chargers in parallel to achieve my 150 watts?

I do not have money to buy another laptop charger, and was wondering, if i could have them in parallel, how would I go about doing that?

 

[davenn]

I am going to use laptop chargers to power it, but I only have 2, 75 Watt chargers.

that is 2.75W is it ?

so do the math, does 2.75 + 2.75 = 150?

150W @ 12V is quite substantial = 12.5A and if that is what the load needs, then you are going to require somewhat more
than that to have some headroom ie. around 175 - 200W ... now you are up to around 15 Amps

best option would be to use a PSU out of a desktop PC get one that can produce around 15Amps on the +12V railDave

 

[Puglife]

that is 2.75W is it ?

so do the math, does 2.75 + 2.75 = 150?

150W @ 12V is quite substantial = 12.5A and if that is what the load needs, then you are going to require somewhat more
than that to have some headroom ie. around 175 - 200W ... now you are up to around 15 Amps

best option would be to use a PSU out of a desktop PC get one that can produce around 15Amps on the +12V railDave

It is 2 comma 75, sorry i did not make that clear. I have two separate 75 watt laptop chargers. I do not have money to get a desktop psu, nor can I scrap one from one of my computers. I need to know if i can use the two i have in parallel to get the 150 watts

 

[davenn]

It is 2 comma 75, sorry i did not make that clear. I have two separate 75 watt laptop chargers.

ok, yes I saw they were commas, in dome areas of the world a comma is used as a decimal point so 2,75 = 2.75

for future reference, if you are describing 2 of a thing as you were trying to do either state eg ...
I have two by 75W PSU's or I have 2 x 75W PSU's ... saves much confusion

in a simply world, yes, you could just parallel the two supplies, this would give you twice the capability at the same voltage

but the world isn't simple or perfect, nor are power supplies and if the supplies are not exactly identical in their outputs
(highly unlikely that they would be) ... then one will be likely to supplying more of the current than the other
Unevenly shared load means one supply works harder and is likely to fail quickerYou also didn't comment on my comments re max power needed by the load and the need to have headroom above that amount

you unclearly hinted that you need 150W, I said if that is the case, then you need a power supply capable of more power out
so again ... what is the maximum load ? is it 150W or a lot less ?

Dave

 

[Puglife]

ok, yes I saw they were commas, in dome areas of the world a comma is used as a decimal point so 2,75 = 2.75

for future reference, if you are describing 2 of a thing as you were trying to do either state eg ...
I have two by 75W PSU's or I have 2 x 75W PSU's ... saves much confusion

in a simply world, yes, you could just parallel the two supplies, this would give you twice the capability at the same voltage

but the world isn't simple or perfect, nor are power supplies and if the supplies are not exactly identical in their outputs
(highly unlikely that they would be) ... then one will be likely to supplying more of the current than the other
Unevenly shared load means one supply works harder and is likely to fail quickerYou also didn't comment on my comments re max power needed by the load and the need to have headroom above that amount

you unclearly hinted that you need 150W, I said if that is the case, then you need a power supply capable of more power out
so again ... what is the maximum load ? is it 150W or a lot less ?

Dave

The maximium load is indeed 150 Watts, and for the future, I will be using "two" instead of two . They are the exact same laptop charger, I just so happen to have two of them. So it can work? how quickley would the first one die? would it be a faster death simply because their are two of them, or would it be because of its current draw? Thank you, your help has been really appreciated !

 

[davenn]

The maximum load is indeed 150 Watts,

OK so that doesn't give you any headroom and the PSU's would be working to the max ... this will be very bad for them
they will fail much quicker

and for the future, I will be using "two" instead of two

I don't know what that means ?

They are the exact same laptop charger, I just so happen to have two of them.

yes same make and model but not necessarily identical outputs ... see comments in previous post

So it can work?

it will work for a while till they overheat and fail, could be hours, days or a few weeks

how quickley would the first one die?

unknown ... too many unknown factorsas I stated way back earlier in the thread ... you need a single, hi power PSU at least 175W capability
to ensure reasonable reliabilityDave

 

[Puglife]

Ok, thank you, and what i meant by two, is based on the mix up, where i had written 2, 75 instead of two, 75.

What would happen, if i were to only drain 140 watts, so that none of the power sources are working on 100% output.

Also, Thank you for the help!

 

[berkeman]

Did you say what your project is? Sorry if I missed it.

You may be able to diode OR the two chargers, using two high current Schottky diodes. That might give you an overall 11V at 150W maximum load. How sensitive is your project to the input voltage?

 

[Puglife]

Did you say what your project is? Sorry if I missed it.

You may be able to diode OR the two chargers, using two high current Schottky diodes. That might give you an overall 11V at 150W maximum load. How sensitive is your project to the input voltage?

how exactly would I go about doing that?

 

[berkeman]

how exactly would I go about doing that?

Use Google Images, and search on Diode OR Power Supplies...

 

[Puglife]

Use Google Images, and search on Diode OR Power Supplies...

So this is the sort of thing you are talking about?

and this would work, and solve all issues?

 

[berkeman]

So this is the sort of thing you are talking about?

and this would work, and solve all issues?

That's what I was referring to.

Would it "solve all issues"? Not necessarily. As I alluded to, you lose voltage across the diodes, so you will likely be down around 11Vdc at the max 150W load. Also, that's a lot of current for a diode, so you will need to be sure to use Schottky power diodes that can handle the current (and they may need head sinking). 150W/12V = 12.5A, so you would need to find some 15A or bigger Schottky diodes and heat sink them properly. The BAT54 shown in that image is tiny, for example, and only rated at something like 100mA.

 

[Puglife]

That's what I was referring to.

Would it "solve all issues"? Not necessarily. As I alluded to, you lose voltage across the diodes, so you will likely be down around 11Vdc at the max 150W load. Also, that's a lot of current for a diode, so you will need to be sure to use Schottky power diodes that can handle the current (and they may need head sinking). 150W/12V = 12.5A, so you would need to find some 15A or bigger Schottky diodes and heat sink them properly. The BAT54 shown in that image is tiny, for example, and only rated at something like 100mA.

Thanks, a ton, You all have been extremely helpful. Thank you so much!

 

[Merlin3189]

I notice you have not said what you want to use them for. I'm still a bit sceptical about paralleling them. It would help to know how they were to be used.

 

[Puglife]

I notice you have not said what you want to use them for. I'm still a bit sceptical about paralleling them. It would help to know how they were to be used.

They are for a massive Led matrix. And to everyone, please check out my post about a separate project , I have not been getting much help on this thread, Thanks!
https://www.physicsforums.com/threads/lithium-ion-charging-voltage.859590/#post-5394019

 

[berkeman]

And to everyone, please check out my post about a separate project , I have not been getting much help on this thread, Thanks!
https://www.physicsforums.com/threads/lithium-ion-charging-voltage.859590/#post-5394019

I've closed that thread due to safety concerns. We do not think you should be trying to design a Lithium Ion battery charger. There is too high a risk of fire if not done very well. Save that for a project that you do in a year, when you have much more experience. Just buy a charger for that now.

 

[davenn]

They are for a massive Led matrix. And to everyone, please check out my post about a separate project , I have not been getting much help on this thread, Thanks!
https://www.physicsforums.com/threads/lithium-ion-charging-voltage.859590/#post-5394019

That thread was closed for a reason
That being, you wouldn't listen to good advice on how LiPO's need to be charged safely

It doesn't need to be discussed further in this threadDave

 

[Merlin3189]

So would it be possible to power this as two half matrices?
Open collecter drivers should not care where the power comes from.

 

[Puglife]

That thread was closed for a reason
That being, you wouldn't listen to good advice on how LiPO's need to be charged safely

It doesn't need to be discussed further in this threadDave

sorry, i didnt realize at the time that it was closed

 

[Jeff Rosenbury]

Why can I still post here? (#$%^& hackers)

In general it's not possible to diode power supplies like that. This is because some semiconductors have a positive temperature coefficient and some have a negative. With the positive (I think) one draws slightly more power, gets hotter, then draws more still until it dies. The negative gets hotter, draws less power, the other picks up the slack and both work fine.

So it's at best a 50-50 shot if it works (BJT vs. CMOS, BTW). So there is some risk to equipment and conceivably a risk of fire.

Also you shouldn't push your power supplies beyond about 50% of nominal for projects like this. Power supplies are rated for nominal maximum with specific equipment. Guessing is a good way to blow things up.

 

[Merlin3189]

My worry is more to do with the interaction of the regulation feedback loops.
If PSU A notices a slight rise in voltage, it will try to reduce its output. PSU B will then see a fall in voltage and try to increase its output, which makes it worse for PSU A, which further reduces output, etc. Until PSU B is supplying its maximum output. One PSU ends up trying to carry the whole load.
Paralleling regulated supplies seems to be an inherently unstable situation until one of them current limits.
(This is my thought experiment: I haven't done the experiment.)

 

[Jeff Rosenbury]

My worry is more to do with the interaction of the regulation feedback loops.
If PSU A notices a slight rise in voltage, it will try to reduce its output. PSU B will then see a fall in voltage and try to increase its output, which makes it worse for PSU A, which further reduces output, etc. Until PSU B is supplying its maximum output. One PSU ends up trying to carry the whole load.
Paralleling regulated supplies seems to be an inherently unstable situation until one of them current limits.
(This is my thought experiment: I haven't done the experiment.)

That is certainly possible if the voltages are poorly matched. But remember the extra diode drop which should cover the effect.

 

[berkeman]

In general it's not possible to diode power supplies like that. This is because some semiconductors have a positive temperature coefficient and some have a negative. With the positive (I think) one draws slightly more power, gets hotter, then draws more still until it dies. The negative gets hotter, draws less power, the other picks up the slack and both work fine.

In this case, the 12V power bricks will have current limiting, so they will balance out the power draw near max current output.

Also you shouldn't push your power supplies beyond about 50% of nominal for projects like this. Power supplies are rated for nominal maximum with specific equipment. Guessing is a good way to blow things up.

Nah, the maximum power supply ratings are just that. It should be fine to run them near max power for extended periods of time. Unless they are of poor quality or something.

 

[berkeman]

So would it be possible to power this as two half matrices?
Open collecter drivers should not care where the power comes from.

This is the best idea so far. Divide the project up into two power domains...

 

[Jeff Rosenbury]

Nah, the maximum power supply ratings are just that. It should be fine to run them near max power for extended periods of time. Unless they are of poor quality or something.

And what manufacturing company ever used cheaper substitutes when they could use the engineer specified part? [sarcasm]

Relying on current limiters and unknown safety circuits is poor practice, IMO.

"Should work ..." Edmund Duke, Starcraft.