wire limits

Zap-o

So I have tried to look up (Google) a formula to calculate the correct gage wire for my application. I keep coming up empty. Either I get charts or good guesses. Here is the data sheet http://www.assmann.us/specs/AK322-2-R.pdf

I want to use a VGA cable to run 5V at up to 8Amps on my device. How many strands of wire could I use and be safe. The total distance is 6 feet. I am thinking 4 conductors will do but want to be sure. Any ideas?

kwinn

You either need to know the gauge of the wires in the cable or do a little empirical data gathering. To see if this is feasible you could measure the voltage drop across 4 wires with various currents through it. Start with 1 amp or less initially to avoid problems. A 5 ohm 5W resistor in series with your cable should give you your answer.

Pliers

Using 4 conductors would be the minimum. I would use at least 5. You also need to use 5 conductors for your return line (common).
That leaves you with 5 extra lines.

ElectricAye

I think your Assmann data sheet says the wire size is 28 AWG. Bear in mind that connectors often have contact resistance, too, so wire resistance might not be telling the whole story. It might be best to check with Assmann to see what that cable is rated for. I don't think VGA cables are normally used for your kind of load.

RDL2004

8 amps through a VGA cable? I think you need something like 14-12 gauge stranded for that. Your cable's data sheet didn't mention anything about the actual size of the wires. Since it's supposed to be used as a signal cable and not a power cable I would think they are pretty small wires, like 26-30 gauge or there about.

http://en.wikipedia.org/wiki/American_wire_gauge

http://www.belden.com/pdfs/03Belden_Master_Catalog/22Technical%20Information_Glossary/22Technical_Information.pdf


edit: Okay, now I see it, the cable's wires are 28 gauge.

davejames

...ok - so, 28AWG lists as 65 Ohms per 1000ft. That's 0.065 Ohms per foot.

A 6 foot length would result in 0.39 Ohms (6 * 0.065). Multiply that by 8A, and you encounter an approximate 3V drop across the wire.

Putting 6 28AWG wires in parallel would result in a combined 0.065 Ohms (back to single wire resistance).

Multiply that by 8A and you'll have about a 0.5V drop.

And yes, you'll need the same number of "return wires".


Moral of the story? Run as many 28 gauge wires as you can if you're determined to use that size. Using a larger gauge would be reduce the number of wires involved for source and return.

Peter KG6LSE

For VGA use the GND cover shiled as a conductor too . its got a some meat to it ..


Peter

HollyMinkowski

Peter, that's a good idea. to use the shield to carry power.
It is a lot larger than the wires. :-)

I have wondered why it is possible for a wire to handle more
power at a high voltage than at a low voltage? I say this because
it seems like an awful lot of power is carried by relatively small
wires on those big high voltage transmission towers. The wires
look small and yet can power an entire town. I was speculating
on another thread the other day about sending power and data
up a very long twisted pair of wires to a quad copter and use it
as a sort of radio tower. The wires would need to be very small and
lightweight but would still need to handle several amps. So I assumed
that sending up high voltage ac and converting it on the copter to
low voltage DC would probably work. Was my reasoning sound or was the idea
just silly?

ElectricAye

HollyMinkowski wrote: »

...
I have wondered why it is possible for a wire to handle more
power at a high voltage than at a low voltage?...

Power Transmitted = Voltage * Current. This is the familiar power equation P = VI.
You want to maximize power transmitted, so in theory you would like Voltage and Current to be as high as you can make them.
But another thing you must think about, Power Dissipated = (Current)²*(Line Resistance), so in order to reduce wasted energy, you need to minimize Current.
Problem is, you have no control over Line Resistance. And because Power DIssipated goes up with the square of the Current, it's best to jack up the Voltage.

Of course, once the wires get too small, then you run up against simple problems of Voltage Drop = Current*(Line Resistance), so running power over very small wires does have a practical limit.

bill190

HollyMinkowski wrote: »

I have wondered why it is possible for a wire to handle more
power at a high voltage than at a low voltage?

"Voltage drop". The voltage will be less at the end of a long wire. Plug in a long 100 ft. extension cord to an outlet, measure the voltage at the plug, then at the end of the cord. Place a load on the end of the cord, measure again.

Using a larger gauge wire will reduce voltage drop.

"Stepping up" a voltage to very high voltage will allow it to be carried for very long distances. (Like 500,000 volts.)

AC (Alternating Current) has the advantage that is can easily be stepped up in voltage, then stepped down using transformers -> Thus allowing for distribution over very long distances.

The decision to use AC for electric companies...
http://en.wikipedia.org/wiki/War_of_Currents

Voltage Drop Calculator...
http://www.csgnetwork.com/voltagedropcalc.html

Wire gauge sizes for 12 volt DC automotive (LARGE!)...
http://www.offroaders.com/tech/12-volt-wire-gauge-amps.htm

AC Wire size calculator...
http://www.csgnetwork.com/wiresizecalc.html

bill190

Oops... I forgot the most important part!

That is the current or amperage a wire carries determines the size of wire required. And when you step up a voltage with a transformer, the voltage on the wire increases whereas the current carried decreases.

So lower current/amperage and a smaller wire can be used.

The general idea is that a wire is the correct size if it is not warm or hot when in use.

Peter KG6LSE

Ill warn you that IF you Do use the GND you can put trash on the wire and possibly cause a data issue on the Sig wires Via capsatice coupling or induction ...... so no running shop vacs witha VGA cable with data on it .


At least VGA is analoug so the worst is you will have a noisy Picture ..

Got to love transients ..


Peter

Peter KG6LSE

Holly with todays SMPS converters your idea is not that far out there .


250 V is just high enough to run some serious power But low enough to use COTS gear

Use 250VDC and a off the shelf SMPS to get 5V or 12 or what not .. .

to get 250 V DC use a inverter and a cascade to up it .

Peter

ElectricAye

HollyMinkowski wrote: »

.... The wires would need to be very small and
lightweight but would still need to handle several amps. So I assumed
that sending up high voltage ac and converting it on the copter to
low voltage DC would probably work....

It wouldn't be too hard to figure out the size of the wire and the weight of the wire for a given height. But bear in mind that there will be wind resistance on the wire, too. And that ~sideways force will affect what sort of loads your copter will experience. Any sideways vector will detract from your vertical lifting capability and will therefore limit your altitude. Also, if you plan on using very high voltages, bear in mind your wire might then need better insulation, and that will add weight, too. The trick, I suppose, would be to find a "sweet spot" amid all of these competing requirements.

localroger

Holly -- for tethered flighted radio tower creation, you can use a balloon. It doesn't require any power at all. I have read of people doing this with natural gas as the lighter than air gas, since many homes have a virtually unlimited supply of it available and even though it's not nearly as bouyant as helium it's a lot cheaper and antenna wires don't weigh much.

As for more power at high voltages, yeah it's voltage drop -- three volts is insignificant at 220V but it's a real potential problem at 12V and kit absolutely kills you at 3V3.

ElectricAye

localroger wrote: »

... I have read of people doing this with natural gas as the lighter than air gas, since many homes have a virtually unlimited supply of it....

Just don't smoke - or spark. Also, just to avoid the confusion: natural gas is NOT propane. Propane is heavier than air. And it is dangerous, too.

Zap-o

Thanks for all the help.

I decided to use a heaver gauge wire selected from the beldin link RDL2004 posted up there.