Two power supplies in parallel?

I just installed a new PCB heated bed on my printer but it seems that my main power supply (laptop type) can only run either the extruder or the bed, not both at the same time. I have a second power supply which has the same specs, so I am wondering if it would be safe to wire them in parallel so as to distribute the load among the two of them, or would this create a fireworks display?

Probably best to avoid wiring them in parallel. All it takes is a small difference in voltage and current will flow from one to the other.

That said, with most of the electronics out there you should be able to connect the Grounds together, and run the Positive on one PSU to your electronics, and the Positive on the second PSU to the Positive side of the heated bed. Leave the Negative side of the heated bed connected to your electronics, and make sure the Grounds are connected to the electronics as well. This way, you have one PSU for your electronics, and one just for the heated bed.

This will work if the electronics use N Channel FET's to switch the output of the heated bed, such as what RAMPS does (check the schematic to be sure).

If you're still not sure, tell us what sort of electronics you've got and I'll see if I can knock up a quick diagram on how to do it.

Alternativelly you can put the bed on a separate circuit with a switch like a electromagnetic relay or a SSR. This way one psu supplies for everything, the other only for the bed, and they can be kept completely separated with no contact to each other. That means not even the grounds will be in contact of one another. Again, both circuits can be completelly isolated, all v+ and gnds can be kept separate, but for that you cant use a fet. Probably a SSR would be best, tho its kinda expensive.

if you are good at electronics, you can put in a rectifier on each supply, that could prevent voltage difference problems, but load wise will be tricky, to passively induce load balancing, you can try a small value (but high power resistance, wastes some power) ... maybe 0.5ohm 20w? ... but for simplicity, no resistor is ok, some laptop PSU are current limiting, it just stops at its max supply.

JBernal Wrote:
-------------------------------------------------------
> I just installed a new PCB heated bed on my
> printer but it seems that my main power supply
> (laptop type) can only run either the extruder or
> the bed, not both at the same time. I have a
> second power supply which has the same specs, so I
> am wondering if it would be safe to wire them in
> parallel so as to distribute the load among the
> two of them, or would this create a fireworks
> display?

Edited 1 time(s). Last edit at 03/30/2013 06:37AM by redreprap.

---

______________________________________
__my mixed bag blog || aka --> [http] || ___ so 3D printing is everywhere ... dont worry, hospitals can now 3Dprint body parts, they will charge you $1million excluding surgical fees ... you will die paying your debts. thats their aim ___ if every patent expires tomorrow, everybody will surely get a 3dprinter and make EVERYTHING ! ____ there is a "DIY-DTG" t shirt printing forum, you can mod an EPSON printer to PRINT like a pro. ___ CNCzone? overly commercialized it seems ___ my country? they will be taxing you for every cm of road you use and track you to your grave using GPS and its government authorized, now they will fire all the traffic wardens instead.___ EEVBLOG? there is only 1 way to do things --> take it apart like a pro

And what about the gnd line balancing. Without that, you will be having amps from both sources sinking in only one psu gnd (whichever provides a better path). One of the gnd lines may get a current much above its rating. Perhaps most gnds have a good safety margin so this isnt too bad in practice, but it doesnt mean its correct. Probably at very least some track may heat up, and in extremis this *can* be worse.

mmm, if the PSU is a true PWM PSU, the current returns are independant. they are suppose to be floated. you can try and measure for connectivity between the +/- pin to L, N and E. there should be more than 1M ohm resistance. each will sink its own amt of return current.

also, u can try the circuit with 0.5ohm (r1 r2) ... pump them into a bare heater bed and measure the voltage across R1 and r2 to check how it balances first before operating it fully. r1 and r2 will get very hot, so this test is for short period.



NoobMan Wrote:
-------------------------------------------------------
> And what about the gnd line balancing. Without
> that, you will be having amps from both sources
> sinking in only one psu gnd (whichever provides a
> better path). One of the gnd lines may get a
> current much above its rating. Perhaps most gnds
> have a good safety margin so this isnt too bad in
> practice, but it doesnt mean its correct. Probably
> at very least some track may heat up, and in
> extremis this *can* be worse.

---

______________________________________
__my mixed bag blog || aka --> [http] || ___ so 3D printing is everywhere ... dont worry, hospitals can now 3Dprint body parts, they will charge you $1million excluding surgical fees ... you will die paying your debts. thats their aim ___ if every patent expires tomorrow, everybody will surely get a 3dprinter and make EVERYTHING ! ____ there is a "DIY-DTG" t shirt printing forum, you can mod an EPSON printer to PRINT like a pro. ___ CNCzone? overly commercialized it seems ___ my country? they will be taxing you for every cm of road you use and track you to your grave using GPS and its government authorized, now they will fire all the traffic wardens instead.___ EEVBLOG? there is only 1 way to do things --> take it apart like a pro

It's a Sanguino board, rev 1.3a
I also thought about running the ground through the board and the + to the bed, actually tried just connecting the bed directly to one power supply, but something about the way the load is handled makes either power supply auto-shutoff when attached directly, but not when run through the board. It's weird.
I suppose it's time I go out and get a heavy-duty power supply. Just wanted to know if I could save some cash by being cheap and using what I've already got sitting around.


Cefiar Wrote:
-------------------------------------------------------
>
> This will work if the electronics use N Channel
> FET's to switch the output of the heated bed, such
> as what RAMPS does (check the schematic to be
> sure).
>
> If you're still not sure, tell us what sort of
> electronics you've got and I'll see if I can knock
> up a quick diagram on how to do it.

the R of the heater bed is low, u measured that? over current situations, some PWM shutsdown. there has to be some extra resistance in the PCB to justify it working though.

cheapest PSU? any china computer PSU 300w, new or old.

[www.instructables.com]

---

______________________________________
__my mixed bag blog || aka --> [http] || ___ so 3D printing is everywhere ... dont worry, hospitals can now 3Dprint body parts, they will charge you $1million excluding surgical fees ... you will die paying your debts. thats their aim ___ if every patent expires tomorrow, everybody will surely get a 3dprinter and make EVERYTHING ! ____ there is a "DIY-DTG" t shirt printing forum, you can mod an EPSON printer to PRINT like a pro. ___ CNCzone? overly commercialized it seems ___ my country? they will be taxing you for every cm of road you use and track you to your grave using GPS and its government authorized, now they will fire all the traffic wardens instead.___ EEVBLOG? there is only 1 way to do things --> take it apart like a pro

The power supplies are 120 watts. They have no problem running the extruder and the old 4-resistor heated bed (could get it to 70 degrees but no higher) but they can't run the PCB.
I have my eye on a 12v 20-amp project power supply, has 22-amp surge. Hopefully it will do the trick.

I'm currently using a 203w 17 amp Xbox 360 power supply and it works well for my printrbot setup. I'm not sure if your setup requires more though. Mine runs both and has really good protection. It has a 5v and multiple 12v lines. Just a thought you can pick em up cheep. It will say on the back so make sure you get the 203w version if you go that route. I got it for $5.

---

--------------| For Everything |--------------------------
Check it out here:
[reprapsquad.wordpress.com].
---------| For Everything Prototype Related |------
Now featuring comp case mods:
[RepRapLab.wordpress.com]

--------------| Find us at Twitter|------------------------
@REPRAPSQUAD (RS Main)
[mobile.twitter.com]
@REPRAPSQUADHQ (ProtoLab)
[mobile.twitter.com]

if you have a voltmeter of some kind, you should probe the resistance of the heater bed and work out that power consumption and see if it is the bottleneck. if the power numbers are approx correct then, you should lower the resistance of the heater to get more power in and so raise the bed temp.

---

______________________________________
__my mixed bag blog || aka --> [http] || ___ so 3D printing is everywhere ... dont worry, hospitals can now 3Dprint body parts, they will charge you $1million excluding surgical fees ... you will die paying your debts. thats their aim ___ if every patent expires tomorrow, everybody will surely get a 3dprinter and make EVERYTHING ! ____ there is a "DIY-DTG" t shirt printing forum, you can mod an EPSON printer to PRINT like a pro. ___ CNCzone? overly commercialized it seems ___ my country? they will be taxing you for every cm of road you use and track you to your grave using GPS and its government authorized, now they will fire all the traffic wardens instead.___ EEVBLOG? there is only 1 way to do things --> take it apart like a pro

You have to have two power supplies which having same parameters other wise parallel operation may caused damages