Why is connecting 2 PSU's in series so dangerous?

[Chris_M]

I am looking for a higher volt PSU as the 12V 40A one I am currently using is really limiting my charging capabilities. After some reading I am wondering why daisy chaining 2 psu's in series is so dangerous? And is there anyway to make it more safe? After doing some research on RH site they have a 15V PSU that they do this simply with on video and both units are even touching each other. Any info on the subject would be appreciated....thanks

[suomynon]

Given that some years ago I received an electrical engineering degree, I should be able to answer this confidently. But I guess I didn't study hard enough to know the answer for sure...So let me wing it:

I think it is because, unless you implement a more sophisticated setup, you're planning to wire it up like this:


Black Output Wire -- PS1gnd -- PS1+ -- PS2gnd -- PS2+ -- Red Output Wire


And the problem is (and this is the important bit) that each supply has their ground attached to your house's ground, so the REAL wiring (whether you like it or not) will include a short that looks like this:


Black Output Wire -- PS1gnd -- PS1+ -- PS2gnd -- PS2+ -- Red Output Wire
....................................|............................. |
....................................|............................. |
...................................-- (house's ground) --


So then you're gonna be tempted to correct for that by cutting off the ground wire on the plug of PS2, defeating the grounding protection included with PS2. So if anything fails, you have no protection (with this setup).

See here for more:
http://sites.google.com/site/tjinguy...ts/diy-24v-47a

[Chris_M]

That makes a lot of sense...thanks for the description. With that being said watch this video.
http://www.youtube.com/watch?v=Uo7Cf...layer_embedded
How can they safely tell the public that this can be done? I would assume that a server PSU and this one contain pretty much the same components. For $140 its pretty cheap to just pick up 2 of these Voltz PSU's and call it a day if it will work without a hitch.

[Hekicopter]

The problem lies with the DC negative of the server power supplies being connected to the case of the supply. If you don't isolate the DC ground from the case of one of the server supplies, if they touch it'll create a short circuit. By isolating the DC ground you can still leave the AC ground lead attached which still provides the ground protection in the event of a short within the supply.

For Voltz to be able to put 2 PS's in series the way they show in the video then they have to have the DC ground isolated from the case of the power supplies.

HTH

[RocketSled]

None of these answers is quite correct. There is no one "right" answer.

The problem is that a modern-day computer switching-regulated power supply contains "active" power management features that control their voltage and current outputs. They're not just step-down transformers or big resistors, and while their output circuits are typically isolated/floating with respect to the AC input, they still can't be treated as if they're just a battery. They're closed loop "servo controllers" designed to adjust dynamically and operate within a very specific voltage and current range. Since they sense load and adjust, they can be "fooled" by another externally supplied voltage being applied to their outputs, and this can lead to faulty operation, damage or outright failure.

When you gang them together, depending on how they're designed and wired, you may get exactly what you want, or you may get one supply overdriving the other, or you may get the two supplies fighting each other for "dominance" (with a highly variable and potentially unregulated output as a result).

It's not that you *can't* gang P/Ss, it's that unless you know the P/S is designed to be ganged you're shooting craps. Might work, might work sub-optimally, might not work at all, or it might burst in to flames the moment you apply power or sometime later when you're not paying attention to it...

[Chris_M]

Well it looks like the Voltz are going to be the safest and least expensive alternative to getting a high rated PSU.

[garyji]

I'm using the VOLTZ setup with great results like the video.

G.

[write2dgray]

Posts 2, 4, and 5 are all correct. Part design for series loads and part design for ground protection.

There is also CE certification to consider. From the manufacturer of the P350 power supply, “We have consulted our engineer and got that it's Ok to use P350 in series, but we don't suggest you do it in this way, which maybe beyond the standard for protection range of CE.”

CE certification ensures testing has taken place to verify compliance with safety and health standards. Some power supplies are designed to accommodate series and/or parallel configurations and have appropriate protection built in. The P350 power supplies, although they may work initially, were not designed for this operation and may create a hazardous situation for the user in the event of a failure.

The Iota and PowerMax power supplies are designed for series operation and the Mean Well RSP units are designed for parallel operation.

Cheers,
David

[newhelliguy]

I am using the 2 voltz setup as the video but I noticed one will get warm for a while and the fan will run then the other will get warm, like was said they seem to be fighting for dominance.

I know they are both putting in voltage because the voltage is 30 something but they alternately heat and cool.

I thought that was weird but this explains it.

They do hold voltage for the full amp ratings and get the job done.

[TJinGuy]

Originally Posted by newhelliguy

I am using the 2 voltz setup as the video but I noticed one will get warm for a while and the fan will run then the other will get warm, like was said they seem to be fighting for dominance.

They are in series, there is no fighting for dominance. They each get loaded equally.

[mcottingham]

Hello All,

I am going to re-ignite this thread. I recently purchased an iCharger 4010DUO and am planning on connecting 4 670W DELL 1950 PSUs together in series to get 48v DC. I know that this is WAY overkill, but those that know me will know that that is what I do .

I will make a new thread with my build and pictures, however, I had a comment on this link https://sites.google.com/site/tjingu...ts/diy-24v-47a that is referenced all over the place. They mark the one chassis as "Ungrounded Case", however, the case is touching the other case that IS grounded, grounding it . I guess maybe they marked this case because they cases are not physically connected to eachother??

In my build I will be isolating the chassis ground, on 3 of the 4 supplies, but I would like to place a zenner diode from the board to the chassis so that there is still a bit of a fail-safe in place.

[Gregor99]

Originally Posted by mcottingham

I will make a new thread with my build and pictures, however, I had a comment on this link https://sites.google.com/site/tjingu...ts/diy-24v-47a that is referenced all over the place. They mark the one chassis as "Ungrounded Case", however, the case is touching the other case that IS grounded, grounding it . I guess maybe they marked this case because they cases are not physically connected to eachother??

In my build I will be isolating the chassis ground, on 3 of the 4 supplies, but I would like to place a zenner diode from the board to the chassis so that there is still a bit of a fail-safe in place.

When that was written, it was common to float the DC ground by disconnecting the ground from the AC side. The cases cannot touch. That was the dark ages of serial supplies.

Now we do it but disconnecting the ground on the output side. The case remains grounded to the AC side and the cases can touch. How its done varies by supply. Usually its a matter of electrically isolating one or two case stand offs from the pc board. In some cases, traces on and wires on the board must be cut.

Also be aware the 4010 does not have supply stability protection and can be damaged if one of the four supplies triggers one of the safeties while charging. The manufacture warns against such conditions. Serialize at your own risk.

[RedOctobyr]

I suspect that the "Ungrounded case" one has the internal ground removed, between the circuit board/negative terminal, and the metal case.

If you leave that internal ground connected, the negative of supply #2 is at +12V, as it comes in series, off of the +12V output from supply #1. However, the case of the supply is usually grounded to the negative terminal of the supply. Typically, that negative terminal is at 0V, not +12V.

So now the case of supply #2 is also at +12V, not 0V. The case of supply #1 is at 0V, like normal.

So when you let the two cases touch, you short them, with +12V going to 0V.

This is why you isolate the ground of supply #2. That disconnects the metal case from the negative terminal of supply #2. Meaning that the case of supply #2 is no longer at +12V.

In that picture (and in my setup, with the case of supply #2 isolated from ground), both metal cases can touch, and it's not a problem. If you touch the cases together, the case of #2 is at the same ground as the case of #1. So it's grounded in that sense, I suppose, but only as long as you keep them touching.

Sorry if I misunderstood your question.

[S.Millar]

Originally Posted by mcottingham

Hello All,

I am going to re-ignite this thread. I recently purchased an iCharger 4010DUO and am planning on connecting 4 670W DELL 1950 PSUs together in series to get 48v DC. I know that this is WAY overkill, but those that know me will know that that is what I do .

I will make a new thread with my build and pictures, however, I had a comment on this link https://sites.google.com/site/tjingu...ts/diy-24v-47a that is referenced all over the place. They mark the one chassis as "Ungrounded Case", however, the case is touching the other case that IS grounded, grounding it . I guess maybe they marked this case because they cases are not physically connected to eachother??

In my build I will be isolating the chassis ground, on 3 of the 4 supplies, but I would like to place a zenner diode from the board to the chassis so that there is still a bit of a fail-safe in place.

Why not just use 2 or 3 of these? http://www.rchelination.com/setting-...-power-supply/

2 will give you 1800watts which is more than enough to max out your house outlet and a 2000W generator. 2 of these will be much lighter than the 4 supplys you are planning to use.

[mcottingham]

Originally Posted by S.Millar

Why not just use 2 or 3 of these? http://www.rchelination.com/setting-...-power-supply/

2 will give you 1800watts which is more than enough to max out your house outlet and a 2000W generator. 2 of these will be much lighter than the 4 supplys you are planning to use.

I'm mostly using these PSUs because I had 4 sitting around in some old servers that I have. They do not have any fans in them either so that may become an issue when I start parallel charging 450 packs. So far everything is working OK and I have been able to deliver 10A to my charger without any issues. I am going to build a case to mount this stuff into that has proper ventilation fans. I guess we'll see if I end up running into any heat issues.

My build can be seen here: https://www.helifreak.com/showthread.php?t=597253

[S.Millar]

I think its great that you are planning ahead with your charging set-up. I think you will find that it will last you for the foreseeable future.
Too often new guys under buy on things like chargers and radios only to end up replacing it shortly after.

I will be looking at your build thread. You can get those HP 1200fb power supplies very cheap on ebay if you decide to change your mind.