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cable recommendation for dc power supply
Old 17th June 2009
  #1
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mexicola's Avatar
 

cable recommendation for dc power supply

I'm building an outboard psu for a project I'm working on. I'm having trouble finding info on what kind of cable to use to run DC power from the PSU to the unit. I'm running +/-15VDC at a max of 5A. I only need about 4 or 5 meters of cable.

Anyone know where I can source some in small quantities?
Also, what's the best brand to run DC?
Mouser has some Belden that would work, but their smallest quantity is 100 meters. I don't want to buy that much.
Old 17th June 2009
  #2
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Hrmmmmmmm...

How much do you want to spend? You can buy wank-o-phile cable for ridiculous prices or be more realistic and get commonly available options.

Personally i'd get some 10 or 12 gauge wire from your local electrical parts supplier. Here it's called Jaycar electronics.
Old 17th June 2009
  #3


Get some fine-stranded 14 AWG and some lacing tape.



-tINY

Old 17th June 2009
  #4
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Thanks guys!
Old 17th June 2009
  #5
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Hi
You haven't stated how much voltage drop you can accomodate at 5 Amps.
This will have a bearing on the thickness needed.
As it is DC there is no benefit in 'Audiophilia'.
Matt S
Old 17th June 2009
  #6
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I had this same problem a couple of years ago, extending the PS cable for my console and ended up buying a whole roll of cable. So i could sell you some if your interested. I think it was belden. I forget how many conductors it was. If you're interested shoot me an email and I can dig it out and give you the specifics.

take care,
jared

[email protected]
Old 17th June 2009
  #7
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I don't want to lose more than half a volt. Should I just go with the thickest conductor I can get?

Basically I'm building a PSU to power three 8-channel units. Each channel draws a max of 100mA for a max total of 2.4A, which is why I'm going with the 5A psu to make sure I've always got enough current. The idea is to daisy chain the three units together. The total length of power-carrying cable from the psu to the last unit (including internal wiring) would be approx 15 feet. 14 awg would be sufficient for that, right?
Old 17th June 2009
  #8
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When calculating voltage drop, do I need to take into account only the length of cable carrying the incoming voltage (in my case +/-15)? Or do I need to include the total length of the circuit including all copper traces on each pcb, component legs, jumpers, and whatnot?
Old 17th June 2009
  #9
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I would be tempted to suggest using 3 terminal regulators, a pair per channel, then sending unregulated DC from your remote supply. That way the PS will be referenced to ground at your modules, and wire losses will be no big concern.

If you don't use on board regulation consider flame proof resistors or fuses on each channel so power supply faults on one channel don't bring down the whole system.

Un-regulated will need to be higher voltage to support drop out voltage of regulators and wire losses.

JR
Old 17th June 2009
  #10
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hi
If that is your plan I would even suggest 'pre regulation' having a simple emitter follower (use a chunky power darlington) fed from a zener type regulator in the supply box and have local regulators as John suggests. This would lower the possible 'ripple' on the wiring which if the units get interconnected by audio ground and power ground might appear 'unexpectedly'. A bit of an overkill but if you select a suitable zener you can minimise the heat generated at the 'module' ends.
A handful more bits and you can put an over current limit on it too. ( a couple of diodes to tie down the base and a suitable emitter resistance to make something like a 6 Amp 'constant current' protection) which would withstand a brief short circuit, and power the thing up more gently.
Matt S
Old 17th June 2009
  #11
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I actually was planning on using on-board regulators (lm7815 and lm7915). I had intended to use the +/-15VDC 5A Power One supply HDD-15-5AG. These supposedly already have a low ripple output, and are regulated. Is it redundant to use onboard regulators with regulated DC?
Since the PSU is regulated would I still need a higher voltage using onboard regulation, or is that only for unregulated DC?
Thanks for the help so far guys!
Old 18th June 2009
  #12
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Any time local regulation is used you want the supply driving the device to have a higher output voltage than the desired output of the onboard regulators. I've been told for the LM7XXX series regulators, that this value should be at least 3 volts higher than the desired output.

So, if you're feeding 7815/7915s this value should be +18/-18 volts or more. I'd go for a PSU design that uses an LM317/LM337 with a power transistor for added current output (check out the PDF on the national semiconductor website). That way you'd have the ability to increase the output voltage of the supply if need be for more modules.
Old 18th June 2009
  #13
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Hi
Bluesprocket beat me to replying but I would observe that the LM317 /337 is only 1.5 amp max and adding an extra transistor can take it to 2 or 3 times this value but it is getting a bit 'messy' by this stage.
You should be able to mod the power one module to give 18 Volts, it requires a resistor change.
Load the power one module so that it is taking 5 Amps, then measure the voltage on the biggest capacitor. To get 18 Volts out this will need to be 18 + about 4 Volts DC MINIMUM when the mains is at the lowest expected level. I would expect them to be OK as I seem to remember the higher rated units being around 24 Volts when fully loaded.
Matt S
Old 19th June 2009
  #14
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Thanks for the responses! You guys have been very informative and helpful!
Awesome, I'm glad to know the Power One supply can have it's voltage raised since I've already ordered it!
I've never used a Power One unit before, so I don't quite know the layout. I'm guessing the resistor you're talking about would be just after the main filter caps?

Quick question...if I got rid of the onboard regulators, I wouldn't need to raise the voltage, correct? I could then just get away with using fusing resistors on each channel, right?
Old 19th June 2009
  #15
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Quote:
Originally Posted by mexicola View Post
I've never used a Power One unit before, so I don't quite know the layout. I'm guessing the resistor you're talking about would be just after the main filter caps?

Quick question...if I got rid of the onboard regulators, I wouldn't need to raise the voltage, correct? I could then just get away with using fusing resistors on each channel, right?
Having never used their supplies I'm not sure which resistors it is either. It shouldn't be too hard to figure out though.

As for removing the onboard regulation, it all depends on how they are regulating the power off the filter caps. As long as the pre-regulation voltage isn't too high for the 7XXX regulators to work with (which I can't imagine it will be) you should be ok. Just make sure to use flame proof resisitors before the onboard regulators for current protection.

Though personally I can't imagine that too much regulation is ever a bad thing. If the regulated output of the supply can be raised to the proper level and keep the proper current for the system, I'd just use the supply's regulation in concert with that in my system. You'd just be insuring that the available DC is even better regulated. Unless of course someone can see an issue I'm missing.
Old 19th June 2009
  #16
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Hi
If you can take a couple of pictures of the power one and Email them I should be able to direct you to the resistors that need changing. You could simply use the supply 'as is' and wire it direct to the modules. Star the supply from the power one module to each of the chassis.
Matt S
Old 19th June 2009
  #17
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Thanks again for the help! The Power One should be arriving any day now. When it does, I'll snap a pic and email it.

Matt, so you're saying if I dump the onboard regulators, I won't need to bother raising the supply's voltage? If that's the case, I might go that route and just use fusible resistors on each module for current protection, like Blue Sprocket suggested. Power One claims their supplies are "highly regulated" anyway.
Any recommendations for fusing resistors? I'm guessing I'd want 1R so as not to reduce the voltage. Mouser has these:
NFR25H0001008JA500
Old 19th June 2009
  #18
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Hi
Without knowing what your modules are it is difficult to say but possibly have a 4.7 Ohm resistor for EACH module which will have the benefit of some isolation between modules.
You will need to wind the 15 Volt supply up to 15.5 to compensate, no problem as the power ones have trim of a bit over a volt each way.
Matt S
Old 19th June 2009
  #19
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Awesome! Thanks, Matt. That's exactly the info I needed to know.
Old 19th June 2009
  #20
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What John Roberts suggested is the best option: 3-leg regulators are very cheap today, lot of surplus is available for funny prices.
Old 19th June 2009
  #21
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Quote:
Originally Posted by Wavebourn View Post
What John Roberts suggested is the best option: 3-leg regulators are very cheap today, lot of surplus is available for funny prices.
Just curious, but what makes having onboard regulators the better option if the Power One supply is already highly regulated? Is it because due to the large number of channel modules, it would provide smoother local power for each channel? I'm not second guessing you, I'd just like to know because there's still time for me to do either but I'm going to have to make a decision soon.
Old 19th June 2009
  #22
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Hi
Having regulation 'local' is best so if you can swing a couple of extra volts from the Power One then use the regulators as well.
Matt S
Old 19th June 2009
  #23
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Cool, thanks!
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