12vdc Power Problems

[Michael]

I just exceeded the number of boards I can put on one run, 8 max. When I hooked up the 9th board, #8 shut down. I have also noticed when I turn ALL ON that I get flickering on many of the channels, but during normal programs, I do not see the flickering.

I checked voltage at #8 and it was 6.2vdc, but dropped to 5.8 when I connected #9.

I have a 800ma 12vdc regulated power supply - Radioshack
Between the computer and each box is 100' cat5 cable

Which way should I go to resolve this?

1 - get a bigger power supply
2 - use shorter cables
3 - run power on 4 wires instead of just 2 (requires me pulling every box, cut the ziptie mounting, and soldering on solder balls I believe)
4 - run a second com port and split the number of boxes
5 - run another 12vdc power supply in the middle of the chain (need to worry about the 2 fighting and overloading, or somehow disconnect the incomming 12vdc)
6 - pull all boxes and go back to static display, not worry about sequencing, and get some sleep

- Michael

[Mudsculpter]

split them into a second serial port,... It does wonders.

[BillAd]

I would also be worried about the amount of current being passed through the first board to the others. I don't have a PCB handy but wonder if the trace width is wide enough. Thus if you don't follow chuck's suggestion I would add a second power supply.

Bill

[xmus]

I currently run 10 on two serial ports, but they all use the same power supply (My computer PC supply). Are you using 2 wires in the Cat5 cable for power or 3? Are you using PWM FW? I'm publishing that the voltage on the 12V power rail should not fall below 7.5VDC.... Just FYI, anything below 7.5 and you will see strange behavior.

[P. Short]

Which board are you using? And PWM or non-PWM firmware?

Also, if you are daisy-chaining from board to board, what is the distance between boards?

--

Phil

[Michael]

I am using the REN16 COOP boards and the firmware burned on the chips by XMUS. It is the low power draw version that does not work well with LED's unless you add another bulb or strand of regular lights to the circuit.

I made a mistake on board count in previous post. I have 7 working and was adding the 8th board.

I only have 100' or 10' cat5 cables, all premade, but had to use 100' for everything. So total length from power supply to last board is 700'

I did not modify the cables or boards in any way so I would believe I am using only 2 of the wires in the cat5 for power. I would like to use more if it is an easy 'FIELD' fix. This is the route I would like to go if possible.

I am not currently setup to run 2 ports, all of my computers only have 1 real serial port, and I do not have a USB->RS232 converter.

- Michael

[P. Short]

The standard setup uses two wires for power and two wires for ground. It's possible to use add one wire for power and one wire for ground, but let's look at the numbers first.

If I believe what I wrote in the wiki some time back, the current draw of each Renard16 should be about 70 mA in your situation. The total then is about 560 mA, although the average current through the cable is about 280 mA (since you have the controllers evenly spaced over the distance).

The resistance of 24 AWG cable is about 30 Ohms per 1000 feet per wire. We will use that figure, because even though the wires are doubled up, the current has to travel out to the load and back again on the ground wires. Since the distance is 700 feet, the wiring resistance is about 21 ohms to the end controller and back.

Since the average current through the cable is 280 mA (some segments have more than 280 mA, som have less, but the average is 280), the voltage drop in the cable seen by the end controller is .28 * 21 Ohms = 5.9V. Using a third set of wires would reduce the resistance to about 14 Ohms, and the voltage drop at the far end would be about 4V. The voltage on the regulators on the Renard16 would then be 8V or more, which should be OK (but with no margin for adding an extra controller).

Another solution would be replacing the regulators on the Renard16 with LDO regulators, which could operate with input voltages substantially below 7.5V.

--

Phil

[holtm]

Can you put your own ends on? If you can you can shorten the cables. I think that would be the easiest field fix.
The option of putting a second power supply in the middle would work (I think, someone correct me if I'm wrong). You would have to connect the grounds on the power supplies, but separate the 12v. On the out cable of box #4 you could un-shield a 3 or 4 inch section and then find the wires with the ground and the 12v. Leave the other 4 connected as two of them carry the serial signal. Connect the 5 ground wires together (2 from each side of the cable and one from the power supply) Connect the 2 wires for 12v going to box #5 to the new 12v supply and tape or seal the 2 wires coming from box #4 so they don't short anything. Check with a meter which wires are which. Look at the RJ-45 and see which colors are on each end. One standard is Orange as 1/2 and Brown as 7/8. I think most designs use 1/2 as ground, 7/8 as power, and 4/5 as serial (blue).

[P. Short]

The concern with adding a second power supply in the middle is that you figure out where the current is flowing in the ground wires. It depends on details like where the power supplies are connected into the system (for example, is the first power supply the one in the controlling PC, or is it a separate supply). Depending on the wiring details, this could make the situation better, or it could leave it unchanged or worse.

If you are connecting a additional power supply into the middle of the setup, my inclination would be to remove the first supply altogether and power all of the controllers from the middle (you would still need the ground connection from the first controller back to the serial port on the PC). Of course, not knowing what your yard looks like, it's hard to say if this is a reasonable thing to do or not.

--

Phil

[holtm]

Phil, could he additionally connect the grounds of the two power supplies together directly with a separate wire, something a little bigger. like a 16ga wire? Would that help the situation or make is worse?

[Michael]

Thank you for those fantastic calculations, now I know some of the limits, and how to adjust for them.
It would be possible to move my power supply from inside near the computer, to out in the yard in the middle of the daisy chain. That would cut my power loss due to cable length to 1/2 of what it is currently. I will also look into getting some shorter cat5 cables. The show must go on!

- Michael

[P. Short]

I'm not sure putting a PC power supply outside in the middle of winter in Madison, Wi is the best thing to do. I would put it in a sheltered location (garage or shed or whatever) and use some 16 Ga lawn light wire to go from the p/s to the mid-point of the display.

--

Phil

[xmus]

Phil - amazing calculations, thanks for that!

Michael:
It is a very easy modification (just 4 solder bridges) but you would have to get access to the bottom of the PCB, which may not be very friendly in your enclosures.

So it appears, you have purchased CAT5 cable in pre made lenghts, and you have a lot of extra cable around. Why not invest in a crimper and ends, and shorten them up, that might really help your voltage sag issue.