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Old 08-22-2012, 11:42 PM   #1
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Simple way to limit DC current


I have some DC supply wires coming out from one of my servers that goes to a DIN rail, which I can patch in to other DIN rail connectors that require voltage. Basically it makes it cleaner instead of splicing each time. This is for home automation/monitoring stuff.

Now the issue is, if I accidentally short out any of that while working on it, the current limiter in the server will kick on and cause it to reboot or shut down. Is there a way I can add a limiter so it never shorts out at the server itself? I was thinking a breaker, but I don't know if it would be fast enough. This is all low amperage stuff, so chances are I could cut it off at like 5 amps and I'd be fine. There's a +5v and a +12v. I was thinking of maybe just adding a resistor, but will that decrease the voltage too?

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Old 08-23-2012, 07:05 AM   #2
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Simple way to limit DC current


There is no easy way to dynamically limit current while keeping voltage constant.

There exist circuits that will do this but they are not cheap.

It is not customary to design electrical equipment to withstand occasional shorts due to carelessness on the part of persons hooking up pieces of equipment.

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Old 08-23-2012, 07:47 AM   #3
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Simple way to limit DC current


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Originally Posted by Red Squirrel
I have some DC supply wires coming out from one of my servers that goes to a DIN rail, which I can patch in to other DIN rail connectors that require voltage. Basically it makes it cleaner instead of splicing each time. This is for home automation/monitoring stuff.

Now the issue is, if I accidentally short out any of that while working on it, the current limiter in the server will kick on and cause it to reboot or shut down. Is there a way I can add a limiter so it never shorts out at the server itself? I was thinking a breaker, but I don't know if it would be fast enough. This is all low amperage stuff, so chances are I could cut it off at like 5 amps and I'd be fine. There's a +5v and a +12v. I was thinking of maybe just adding a resistor, but will that decrease the voltage too?
What do you want to set the current limit for? 100mA or 25A? Do you care if voltage drops off up to 1.1V at the point of current limit? If these are acceptable a simple three terminal voltage regulator can be configured in a current limit mode. If you need high current and can't tolerate a voltage roll off then separate power supplies is the way to go.
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Old 08-23-2012, 11:26 AM   #4
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Simple way to limit DC current


Separate power supplies might not work out because some peripheral devices have 12V and 5V pin connections within the data feed cable. This would commingle the power feeding the DIN rail with the power supply in the server.
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Old 08-23-2012, 02:24 PM   #5
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Simple way to limit DC current


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Originally Posted by AllanJ View Post
There is no easy way to dynamically limit current while keeping voltage constant.

There exist circuits that will do this but they are not cheap.

It is not customary to design electrical equipment to withstand occasional shorts due to carelessness on the part of persons hooking up pieces of equipment.
It's actually a pretty easy thing to do using any standard adjustable voltage regulator with a low internal reference voltage. The LM317 is a typical choice. You just build a constant-current source with it using a single resistor. The problem with that in this situation is that it always results in a fixed voltage drop equal to the regulator's reference voltage plus a little bit. It's usually about 1.5V. So if he already has regulated 5V and 12V lines and is running electronic devices, he probably can't get by with this since the output would be 3.5V and 10.5V which is too low. If he had a slightly higher voltage source to work from it would be easy.
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Old 08-23-2012, 10:16 PM   #6
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Simple way to limit DC current


How about wrapping your tools in electrical tape? Just a thought.
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Old 08-24-2012, 12:06 AM   #7
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Simple way to limit DC current


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What do you want to set the current limit for? 100mA or 25A? Do you care if voltage drops off up to 1.1V at the point of current limit? If these are acceptable a simple three terminal voltage regulator can be configured in a current limit mode. If you need high current and can't tolerate a voltage roll off then separate power supplies is the way to go.
Could probably get away with like 1 amp, but really as long as I stay under the power supply's limit, so I can probably get away with like 25 amps. I can probably tolerate a 1-2v drop, I imagine the sensors and such that I'm using can handle it. In fact lot of the sensors don't really care that much about the voltage, they just need some as a reference to ground. ex: sensor off = ch is 0v, sensor on, ch is 5v. Anything above 3.3 triggers it to be 1.

Though come to think of it, I should have went with a separate power supply and simply have a relay from the server to keep it on. The idea was that I want the voltage supply off if the server is off.

Though guess for the times where I am working on stuff I can always disconnect the voltage source at the DIN rail... :P
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Old 08-24-2012, 02:59 AM   #8
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Simple way to limit DC current


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Could probably get away with like 1 amp, but really as long as I stay under the power supply's limit, so I can probably get away with like 25 amps. I can probably tolerate a 1-2v drop, I imagine the sensors and such that I'm using can handle it. In fact lot of the sensors don't really care that much about the voltage, they just need some as a reference to ground. ex: sensor off = ch is 0v, sensor on, ch is 5v. Anything above 3.3 triggers it to be 1.

Though come to think of it, I should have went with a separate power supply and simply have a relay from the server to keep it on. The idea was that I want the voltage supply off if the server is off.

Though guess for the times where I am working on stuff I can always disconnect the voltage source at the DIN rail... :P
If you can tolerate a 1.5V drop, then use an LM317 constant current source with a resistor selected for like 5A. That will give you about a 0.8-1.2V drop in normal use and will handle up to an amp.
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Old 08-24-2012, 10:49 AM   #9
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Simple way to limit DC current


So that I understand this....your using your computer server power supply to power some of your home automation stuff?

If so....bad idea.....

As mpoulton said....an LM317 will do the job (I still have some laying around...next to my 555 timers)...but that really is a bandaid on the real problem....your using the wrong power supply.

There are plenty of sources for power supplies....if your real cheap...Automation Direct....the better ones would be Power One or Sola....Allen Bradley if you have lots of money....

One other advantage to an independant PS....you can shut if off without shutting down the server....

Your sever PS is a lot more expensive than any DC brick you use....why take the chance of toasting it.
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Old 08-24-2012, 04:18 PM   #10
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Simple way to limit DC current


The reason I use the server power supply is that if something happens where the OS gets hosed up, locks up, etc and a relay is stuck open (ex: the furnace) I can remote into the IPMI interface and shut down the server. Though, I could probably also use a relay to keep an external power supply open and accomplish the same thing. The server PSU is maybe around 100 bucks, so not really any more expensive than another one. Another reason to use the same PSU is that it's common with the USB voltage. The +5 volts matches with the output of the PSU. The grounds are common, etc. Though I guess if I was to use an external PSU I can just hookup the negative to ground and should be common as well right?
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Old 08-24-2012, 11:55 PM   #11
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Simple way to limit DC current


In the kind of work I do, I always use independant power supplies for different systems.

In your case, one advantage is that you can shut down one without killing the other one.

On your setup....not sure why you need to tie the commons together. I'm assuming you have a communications board in your server that is communicating with your automation controller. Ethernet or USB? If Ethernet....isolation is built in....if USB...then the USB is will ground it.

Just make sure you have your AC power right. Most power supplies don't care if neut is hot or hot is neut.....it's an AC/AC/DC switcher....(switching power supply).

One other thing....you run the risk of damaging your computer motherboard by playing with the power on your server PS.....yea, that PS may only be $100....what about the rest of the machine??????????
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Old 08-25-2012, 04:25 AM   #12
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Simple way to limit DC current


Yeah there are various communications boards, and everything actually uses a common ground for the negative (the actual earth ground) so it's important that all my voltages are in reference to that ground. If I start adding separate power supplies into the system they may not all match up. Or can I just hook up the negative to the same ground to "force" it to match?
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Old 08-25-2012, 07:40 AM   #13
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Simple way to limit DC current


On the DC side of the power supplies (the output side) all the common* or ground lines (usually black) may be connected together and to the equipment chassis. These conductors in turn are connected to ground (not neutral) of the home electrical system, typically via the equipment grounding conductor in the power cord and in the household wiring, and the ground pin of plugs and receptacles. It is also a good idea to interconnect the chassis of all of the pieces of equipment using a separate ground wire (12 gauge to 16 gauge suggested).

* The "minus five" and "minus twelve" leads are treated as hot leads and their presence means we really should not call the common or (on the DC side) the black leads as "negative".
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Last edited by AllanJ; 08-25-2012 at 07:43 AM.
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Old 08-25-2012, 10:30 AM   #14
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Simple way to limit DC current


Follow what Allen said. You can take all the commons on those DC power supplies and tie them to your earth ground.

In the case of a -5Vdc power....it's can be a +5Vdc PS, but the + lead is tied to ground.

All those power supplies are floating....i.e., the output referenced to ground will be what ever ground is....up to a point. It's been awhile...but I believe the UL requirement for isolation between line and load is something like 1200 V or more (HiPot Test). In other words, you 'could' have the common on your power supply output tied to a reference that is 1000v above the reference of the line....I wouldn't suggest it....but it should handle it....

So...yea....you can tie allo those power supply load side commons together.

BTW....in the analog world...this is where a nice heavy ground buss comes in handy. I deal with a lot of analog....grounding is real important....especially when I'm running T/C's a couple hundred feet.

I would install a nice copper ground bar (you can get them with 10-32 screws already tapped in) and use that as your central grounding point. You will reduce the chances of ground loops.
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Old 08-25-2012, 11:18 PM   #15
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Simple way to limit DC current


Ahh ok, so I can just tie it all together, good to know. Think I'll do that then, just look for another power supply, and I'll hook it up to a relay that's connected to the server's power supply, and I'll get what I want: that power is off if the server is off.


Last edited by Red Squirrel; 08-25-2012 at 11:21 PM.
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