2 quick questions April 28, 2015 11:51PM |
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Re: 2 quick questions April 29, 2015 12:35AM |
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Re: 2 quick questions April 30, 2015 12:01PM |
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Re: 2 quick questions May 02, 2015 01:49PM |
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Re: 2 quick questions May 02, 2015 03:02PM |
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Re: 2 quick questions May 02, 2015 03:35PM |
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Re: 2 quick questions May 03, 2015 09:21AM |
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Re: 2 quick questions May 03, 2015 04:28PM |
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Re: 2 quick questions May 03, 2015 05:17PM |
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Re: 2 quick questions May 06, 2015 08:51AM |
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Re: 2 quick questions May 06, 2015 10:52AM |
Registered: 9 years ago Posts: 30 |
Quote
alan richard
Well that's all very interesting but it still doesn't answer the question as to why anyone should connect their reprap power DC negative to ground.
I'm not saying you shouldn't. I'm still only asking why.
-a
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alan richard
Actually. Back to safety. There is a significant issue with some printers. . .
My printer has a metal frame with the power supply inside it and 230v going into it.
Quite correctly, the metal parts are earthed to prevent mains voltage shock risk.
As I mentioned - reprap controllers switch the heaters with a negative feed (permanent positive on the heatbed and hotend.)
Which means; If that negative feed wire gets chaffed or pinched or somehow contacts the metal work, then the heater is switched on. (if DCneg is connected to ground)
Permanently and without control and you wouldn't even know about it (apart from the heat, obviously)
That's not safe.
On any kind of electrical machine or appliance, if a wire touches the chassis it should either do nothing or blow a fuse. It should not energize a component without you knowing it.
Especially not a heater (have you seen thejollygrimreapers video of a runaway hotend, dripping aluminium?!)
So for that reason alone I won't be connecting my -ve side to earth. I'll just have to live with the . . . the . . . what was it again?
If your printer has no earthed chassis or the PSU is external, that doesn't apply.
-a
Re: 2 quick questions May 06, 2015 01:28PM |
Registered: 9 years ago Posts: 158 |
Re: 2 quick questions May 06, 2015 03:26PM |
Registered: 10 years ago Posts: 14,672 |
Quote
csambrook
Let's imagine we have two bits of electronics powered from separate 12VDC power sources and that the power sources do not connect the DC negative to ground. Each is therefore free to float its rails some way off true ground provided its the DC negative and DC positive both float together and stay 12V apart. Each bit of electronics is happy in itself as it only sees the difference between its two rails, 12VDC. Relative to true ground however one might be (say) 0V & 12V and the other 20V & 32V (a bit extreme maybe but it's an example)
Now lets connect a comms cable between the two, it could be RS232 or LAN or anything else. The comms cable -ve will form a connection between the two DC negatives so we'll have 0V connected to 20V and current will flow. That current will be limited by the capability of the power supplies but it could be quite large as one power supply pulls the other's negative to match it's own. In practical situations the differences in potential are not normally enough to give a safety concern but we now have a comms cable which is unbalanced, it has more current flowing in its negative wire than in its signal wire, and this can lead to all sorts of very hard to track down comms failures.
Re: 2 quick questions May 06, 2015 05:16PM |
Registered: 9 years ago Posts: 30 |
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dc42
Quote
csambrook
Let's imagine we have two bits of electronics powered from separate 12VDC power sources and that the power sources do not connect the DC negative to ground. Each is therefore free to float its rails some way off true ground provided its the DC negative and DC positive both float together and stay 12V apart. Each bit of electronics is happy in itself as it only sees the difference between its two rails, 12VDC. Relative to true ground however one might be (say) 0V & 12V and the other 20V & 32V (a bit extreme maybe but it's an example)
Now lets connect a comms cable between the two, it could be RS232 or LAN or anything else. The comms cable -ve will form a connection between the two DC negatives so we'll have 0V connected to 20V and current will flow. That current will be limited by the capability of the power supplies but it could be quite large as one power supply pulls the other's negative to match it's own. In practical situations the differences in potential are not normally enough to give a safety concern but we now have a comms cable which is unbalanced, it has more current flowing in its negative wire than in its signal wire, and this can lead to all sorts of very hard to track down comms failures.
I'm sorry, that explanation is misleading.
If both pieces of electronics had 12VDC supplies that were truly floating, then there would be no problem with connecting a comms cable between them. Both 12V supplies would still be floating, but tied to float at the same level.
If one were floating and one were grounded, then again there would be no problem. The 12V supply that was previously floating would be tied to ground when the comms cable is connected.
If both were grounded, and the mains grounds they are connected to were truly at the same potential (i.e. ground), there would again be no problem.
Problems arise when one of the following is true:
1. Both supply outputs are grounded, but the grounds not always at the same potential. This can occur if then two devices are plugged into different power sockets, and other devices on the same mains circuit generate ground noise. In this case, the communications cable (e.g. USB cable) connecting the two devices will carry ground current due to the difference in ground potential. This is the common situation when a 3D printer is connected to a PC using a USB connection.
Possible solutions include:
(a) Use an alternative form of connection that doesn't connect the grounds, e.g. Ethernet. This is how I run my printers, except when debugging new firmware. An Ethernet connection does not join the two grounds.
(b) Power the printer and the PC from the same mains distribution block, not from different mains sockets. Don't power anything else that might create ground noise from the same distribution block.
(c) Put a ferrite bead around the USB cable. This will reduce any ground current caused by short-lived transient differences between the ground potentials. This is best used in conjunction with (b).
2. At most one supply output is grounded, and there is significant capacitive coupling in the PSU(s) providing the non-grounded output(s) between the mains input and the output. In this case, transients on the mains may get coupled through to the PSU outputs, so that the outputs are not truly floating at high frequencies. When a USB cable is connected between the devices, high frequency ground currents may flow.
Possible solutions include:
(a) Use an alternative form of connection that doesn't connect the grounds, e.g. Ethernet.
(b) Power at least the device with the non-grounded output via a surge suppressing mains distribution block. This assumes that the device does not itself generate mains transients.
(c) Put a ferrite bead around the USB cable. This will reduce any ground current caused by short-lived transient differences between the ground potentials.
Re: 2 quick questions May 07, 2015 05:22AM |
Registered: 9 years ago Posts: 158 |