Power supply smoking -- HELP PLEASE!

[firstyear]

Hi Guys n Gals, hope someone can help me....

Silly question but how does everyone hook up their power supplies in parallel and have a single V+ and V- coming out?


I have 3 x (48v 7a) power supplies set up in parallel (48v 21a); and my breakout board needs single V+ and V- input wires @ 48v 21a.

I connected the 3 power supplies to the mains; but when I tried to bridge the 3 individual V+ exiting from the power supplies to form a single V+ lead to the BOB; one of the power supplies starting smoking!

I removed the bridge connector from the 3 individual V+ and it seems fine?

Called in the electrician and he was a little stumped, suggesting it was backfeeding and to get a single power supply instead..

not good..
Any help is super dooper greatly appreciated

[epineh]

Hi, there is a little bit of info missing here but I am going to take a stab in the dark...

I assume you are paralleling 3 switchmode power supplies together ?

If so, even though they are rated at 48 Vdc output there will most likely be a small voltage difference between them, in fact most switchmode's I have seen have a voltage adjustment on them implying they may not be EXACTLY the rated output. So what may be happening is that one or two are fighting each other as far as the output voltage is concerned, so most likely the one that is set at a slightly lower voltage is trying to push the voltage level up and smoking up as a result.

So what to do to fix the problem...well I don't know what you are powering from this setup, but my guess is a 3 axis cnc machine of some sort (this IS CncZone after all )

If this is the case then you could power each individual axis with its own power supply and not join them together in any way, so you don't have to change power supplies and waste any more money.

Now before you go ahead and do this you should maybe let us know what exactly you need from this setup so we can make sure we are giving you the right information, as you could also possibly introduce more problems by splitting up the supplies. Generally grounding will be the "introduced" problem, but with some careful wiring this can be avoided.

Hope this helps.

Cheers.

Russell.

[firstyear]

Hi Russel

You Legend! Thanks so much for getting back to me; yup i missed a few important details..

I have a dmm-tech 3 axis cnc kit (400w servos/drivers and BOB and 3 power supplies); I'm trying to hook them up but have hit a smokey snag..

The BOB requires a single V+ and a single V- input from the 3 power supplies

The power supplies from the Dmm-Tech kit are AC/DC switching MeanWell S-350-48
350W, 48Vdc, 7.3A

Im trying to avoid having to buy a replacment if possible, but I may have too if im out of my depth/knowledge?

Let me know what you think?

[mactec54]

firstyear

You should be Just running 3 Plus wires to a 3 in 1 connecter & 3 minus wires to a 3 in 1 connector,there are many ways to do this, a drawing below will give you a rough idea of what you have to do, if you do it like the drawing, solder & tape the joint very well, you will need 2 sets of wires one for the plus & one for the Negative

[mactec54]

firstyear

The power supplies have to be wired in parallel,

Post a photo I will make sure you have it right,

A heads up, if you are using the USB cable to the breakout board, you Don't want to have the 5v power supply as well, this will cause problems with your computer, The USB is supplying all the power the Bob needs

[mike_Kilroy]

epineh is correct: u have proved empirically that the power supplies cannot be wired in parallel of they will work against each other and cause ur smoke. accept that you cannot do this with those ps units.

put them in series as suggest if you want to buy new drives and probably bob: unless your stuff is designed for 96vdc input, you will likely blow all up by putting 2 in series: put all 3 in series and ur new 48v*3= 134v for sure will blow stuff up.

since it appears u cannot separate out the 3 axes to 3 separate power supplies, you must use one. that said, think of what this means........... 48v 7amp...... power supply running 3pc 400w servos'.... 400w/48v= 8 amps..... since u saw a spec that suggested u have 21amps available, what does that mean? prob they figure 7 amps each for that total.... but.... each of those servo drives is a "power converter:" that means it take in that solid 48v and turns it into a variable voltage proportional to the motor speed - 0 v for 0 rpm, 48v for max speed..... so unless you run all motors at max speed, you dont need 48v into them. how 'bout this for an idear: what goes up must come down, so why not figure average output speed is 1/2 motor max, or 24v not 48. and then too, bet you do WORK cutting or whatever at way less than 1/2 speed - prob 1/4 speed or less. so figure 1/4 speed for doing work (HP like).... so you only will output 1/4 of 48v or 12v to the motor with lots of current (the stuff that makes the torque). so if you run the motor at full torque load, and do work upto 1/4 motor speed, YOU ONLY NEED 1/4 THE INPUT POWER! Hence you need not 8 amps per motor but 8/4=2 amps..... hmmmm.... 2*3=6 amps total max continuous load from your power supply input..... sure sounds like ONE power supply should be PLENTY for you so dont bother paralleling those other 2 - sell them on ebay and go to dinner out instead

[epineh]

OK I see what you are up to now...

To be honest I don't use switchmode supplies all that much as I prefer the good old transformer/rect/cap supply, each to their own

I had a quick look through the DMM site and they don't really go into much information regarding paralleling their power supplies, but it may be as simple as putting a power diode on each supply before you join them together, this way they cannot feed into each other, causing problems.

I am sure you are not the first person to parallel switchmode supplies together, I will do a bit of googling later as I have to run out the door right now.

Cheers.

Russell.

[Al_The_Man]

I am not a fan of paralleling P.S. either, especially switching.
The only draw back to the Diode is, if any one voltage output is slightly lower than another, the diode will be reverse biased and the PS will effectively be out of circuit.
Al.

[mactec54]

mike_Kilroy

The Dmm system is designed to take 20amp, The Breakout board has a 20 amp fuse to protect it, here is how it should be wired

There is ajustment on each power supply so there is no problem to balance each at 48v

[mike_Kilroy]

designed for upto 20 amps, AND 48v, not 96 or 134v. If it & the drives were capable of 96v or 134v then they would be rated for that. 48v is a typical max limit for FETs used in low cost drives; next step is typically 80-ish volt with higher voltage FETs, then for the 130v and up IGBTs. so to series the ps and put in higher than 48v will likely break stuff.

rating of 20a is a MAX; any smaller amount can be used also. My point on using 1pc of his 48v 7a rated supply is that for his machine the chances are probably 95% that 7 amps will be plenty and will allow all 3 motors to output 8 amps each at the same time. We seldom use a 20 amp ps for 3pc 400watt 48v drives as this is usually WAY overkill for the reason I explained.

[mactec54]

mike_Kilroy

The Drives are rated up to 70v dc max & if you see the drawing I did of the wiring it is for Parallel wiring

They may be low cost, but are very sohisticated, & equal to most high cost drives

[mike_Kilroy]

mactech, I was not trying to suggest these drives are less that great; just that a single 7 amp supply will likely be sufficient. again, due to the drives acting like dc-dc converters, that 7 amps in from the single power supply can supply 8 amps to each drive motor or 24amps output at 1/4 speed on them. I had thought i read one of the posts saying to wire the supplies in SERIES and so wanted to nix this idea to prevent damage. but again, 1 ps should be sufficient for a 3 axis supply that is not going to do 7amps worth of work above 1/4 motor rated speed on all 3 axes at the same time.