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multiple DC devices in parallel over different distances of wire

multiple DC devices in parallel over different distances of wire


  #1  
Old 11-14-14, 03:04 PM
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multiple DC devices in parallel over different distances of wire

I have 10 x 0.1A 9V DC devices that I'm feeding in parallel over different distances of wire. I'm using a total of ~250' of Cat6 (24awg stranded) wire (2 conductors in each to provide 9V to each device). The longest cable made up is around 30', the shortest is probably 12'. The other conductors in each wire are used for low voltage audio signals. (the devices are headphone amps - for in ear monitoring on stage).

Question 1) how much of a power supply do I need to power all of these 9V .1A devices?

I found a table specifying my 24awg stranded wire is probably around 6ohms impedance (or I guess 12 ohms round trip...?)

Initially, I bought an inexpensive 9V (supposedly) 3A power supply. After wiring about half of them up, I showed around 3.2V, and my devices showed low battery lights. (2 of them didn't light up at all - as they are supplied by a single wire for both devices - paralleled off at the end of the cable).

Question 2) supplying two devices from a single cable (and running different distances of cable to each device) should be okay, right? Once my power supply is big enough, everything should be supplied a constant voltage from my power supply, and pull as much amps as each needs, right?

Thanks in advance!!
 
  #2  
Old 11-14-14, 06:06 PM
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Ten 0.1 amp devices wired in parallel need one amp total. You should have some extra "reserve" power since for continuous operation the total load should be 80% or less of the power supply capability. Also the devices, although advertised as drawing 0.1 amp, might in reality draw a little more.

Resistance of the wires for DC (and also very low frequency like 60 Hz AC) uses a different formula compared with impedance for audio or video. Look up a table of copper wire resistance to figure out the total (round trip) DC resistance of the wires.

I don't have the resistance values for 24 gauge in front of me but, assuming each light or device has its own wire from the power supply (not tapped or spliced or sharing part of the wire run to another device) you can figure the voltage drop in the wires using the formula voltage drop V equals current (0.1 amp) times the resistance of the out and back length of the wire, 60 feet worst case. More than a third of a volt lost in the wires is undesirable and the only way to get around that is to use fatter wires.

Your cat-6 cables may have six to twelve conductors inside each and if so you could double up as needed, namely use two or three conductors for the positive and two or three conductors for the negative to get the equivalent of fatter wires. A redundant run of 2 conductors has half the resistance of one wire of the same kind; 3 conductors in a redundant run have a third of the resistance.
 

Last edited by AllanJ; 11-14-14 at 06:23 PM.
  #3  
Old 11-15-14, 07:10 AM
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Thanks for the reply! So you are saying I need to account for a voltage drop of the 60ft of wire? So my power supply will end up needing to be higher than the 9v I need at the device? Also, thicker conductors will have less voltage drop, which makes sense.... And that points out that I probably will have to deal with different lengths of wire, and balance out what voltage will reach each device due to different wire lengths and number of conductors used. So, V=IR .. I probably can't use resistors to even out impedance of each leg, right?
 
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Old 11-15-14, 08:15 AM
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Hi,
The numbers don't seem to be correct. 24 gauge wire shows a copper resistance of 0.026 ohms per foot. For a 60 foot round trip that would be 1.56 ohms. With a single device running over that wire drawing 0.1 amp you would drop 0.156 volts.
American wire gauge - Wikipedia, the free encyclopedia

I'm not sure where/how you got the 3.2 volt reading, but something doesn't add up. A single device on a 30 foot cable would see about 8.85 volts from a 9 volt supply.

Are you running them all over one loop? Are you connecting several in series?

Bud
 
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Old 11-15-14, 09:33 AM
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No, they should be in parallel. At each end, the device is obviously plugged in, but at the other end, all positive leads are tied to the positive lead of the PS, and all the negative leads are tied together with the PS. I assumed the voltage drop was due to not providing enough amps due to higher resistance of the wire... But I guesstimated in my head that 3A would be enough... Which is why I'm on this forum... Lol. I thought maybe I'd over simplified in my head planning this out. Maybe the PS doesn't provide as many amps as it claims? It had a low whistling sound trying to drive everything.

There are currently about 4 x 28ft cables feeding one box each, and one cable feeding 2boxes (paralleled out at the end of the cable - only using two 24awg conductors in each cable (for 1+ & 1-). I had two other 12ish foot cables that made, I hadn't wired in yet, and I have some more to make yet.

I'd have thought that the 9v 3A PS should have done it...
 
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Old 11-15-14, 09:53 AM
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Maybe you mentioned it, but what is the low voltage cut-off?

Do you have any resistors? A 91 ohm resistor would draw about 0.1 amps and allow you to measure the resulting voltage at the end of a run. If the voltage is still 9v at the ps and way less at the load resistor then something is wrong/different about the cable. If the end voltage remains close to the 9 volts then it implies your device is drawing far more than 0.1 amps

Bud
 
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Old 11-15-14, 10:24 AM
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A simple way of sorting this out would be to connect 10 of these lamps on a short wire connected directly to the power supply.
Measuring the voltage drop at the power supply should tell you if it is the problem or not.
 
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Old 11-15-14, 07:46 PM
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Thanks guys! I think I have a couple ideas to troubleshoot moving forward. In answer to the minimum voltage question, they can run off of a 9v battery, so they have a green light above like 7.5v, red battery light comes on with the green somewhere between 3.5 or 4 and 7.5, and then less than that, you get only the red battery light. I had the red battery light on all devices except the two that were fed by the single cable. The 3.2v reading was at the power supply.
 
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Old 11-16-14, 01:39 AM
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With the 3.2 volt reading at the power supply I would be concerned that those devices are drawing far more than 0.1 amp (100 ma). Either that or the power supply is no where near capable of 3 amps.

Let us know what you find.

Bud
 
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Old 11-16-14, 06:14 AM
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If these are off-the shelf headphone amps it would help if you could provide the make & model number.
 
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Old 11-16-14, 03:21 PM
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The model names of the amps are Behringer powerplay P1. They have pretty decent reviews for battery life, so I kind of doubt they are drawing a ton... But I do have to keep that open as an option. Is it possible that a kinked/damaged cable would draw excessive amps and cause my voltage to drop like that?

I wasn't able to do anything today, but will go back in tomorrow to start troubleshooting. Any hunches as to whether or not a 12V supply would be safe to use? I probably have a couple laying around... I've not seen a max voltage rating published for these guys.
 
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Old 11-19-14, 08:59 AM
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resolved

I got in last night to troubleshoot, and this is all working now! Thanks for the feedback.

I added one wire at a time to the power supply until I found the wire that was causing my problem. It was the cable that supplied the two different devices. One of the DC pigtail ends was shorted together (+ and -) inside of the shrink wrap. Isolating those fixed my problems completely. I noticed during the troubleshooting procedure that those conductors were actually heating up - which is obviously where all my amps were going!

The voltage drop over the entire circuit now is minimal. I'm right above 9V on the entire circuit with 8 devices wired in. I have two more cables to make, but don't expect any further issues. Thanks again, guys!
 
  #13  
Old 11-19-14, 11:12 AM
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Good job. Nice when it all works out.

Bud
 
 

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