Long Cable Power Issue 4

[boreholelogger]

I have a 2.565km piece of wire-line 4 conductor cable wound onto a winch that is used in the mineral exploration industry. The cable has an 85 ohm/km resistance which means that there is 218 ohms resistance from one end of the cable to the other, (Where the camera will be connected) and 218 ohms return line back to the power supply. I require 30VDC @ 400mA across the camera system which will be connected on to the end of this cable. I have ran a few tests and the camera system's current changes from 320mA to 400mA constantly which means that the voltage across the camera system fluctuates alot to provide constant power. The issue I have is that the system requires 200VDC at one end of the cable to provide 30VDC @ approx 400mA across a 75 ohm resistor (This resistor sort of simulates the camera system). Also the initial voltage supplied to the camera is 200VDC for a short moment which would blow the camera's regulator to smitherines. The camera's regulator can withstand 9 - 48VDC. Does anyone know how to help me OR anyone that can help me with this issue?. Or does anyone know of a 30 - 300VDC regulator that can be mounted on a PCB?. Also maybe someone might have a circuit that has used a transorb or something to minimise the impact upon the camera system on the initial startup.

 

[boreholelogger]

Its that simple huh!

 

[VEBill]

"...then how do you do it?."

DSL signals are typically around 1MHz (RF).

Your power is obviously DC.

All that you would need to 'diplex' these two onto one wire (sharing a common return) would be something like a small value capacitor (probably nF range) to block the DC and a small ferrite core to block the 1MHz DSL RF signal. Use the same set-up at each end.

In the worst case, it might be slightly more complicated than I describe, but not much more.

After all, DSL was actually designed to travel down telephone wires at the very same time as -48Vdc, ringing AC, two-way audio, caller-ID signals, and anything else. So obviously this proposal to combine power and DSL is hard-up against the 'been there, done that, got the T-shirt' end of the difficulty spectrum.

 

[Warpspeed]

What you say is quite true, but a balanced push pull system using reasonably wideband centre tapped pulse transformers would offer more natural initial power supply noise rejection, (and wider potential signal bandwidth) than a simple choke/cap at each end.

A reasonable attempt at impedance matching will improve the signal to noise ratio and signal integrity. It is not just the dc that gets lost in cable attenuation, there will be high signal attenuation as well.

The highest possible signal to noise ratio should be an important consideration when combining signal with raw power.

 

[VEBill]

DSL is capable of going through plain old telephone lines for many, many miles. The DSL RF carrier frequencies are sufficiently high (~1MHz) that it is naturally immune to most power supply noise (typicall not as high as 1MHz range).

In summary - the problem here is the DC power. One apparent solution is to use the muticonductor wire paralleled as a single conductor with the armour as the return. I've already shown that this is a possble solution, perhaps in conjunction with other suggestions as well.

Using the four conductors as one precludes using balanced DSL, but I don't think that the DSL is an issue over 2.5km. DSL could probably go 2.5km over a damp string (a slight exaggeration ;-) ).

In general - if someone mentions the phrase '...kilometers of cable...', then the assigned engineers should stop right there and start to be very very very careful.

 

[itsmoked]

As VE1BLL is saying, I'll add that DSL is generally considered an "upto 10,000ft" method on POTS lines.

 

[BigWallyB]

ok. I'm new but my thot is along the lines of Itsmoked first post. Leaving everything original (4 lines), cable impedance is constant. If u supply 200Vdc at the input, worst case (at lowest camera z)u will get 29.13V at the camera. Best case 200Vdc in, 320mA draw, camera will see 35V. What is the problem? Thats not even close to ur limits. U say camera will handle 9-48 volts. A simple cap across the camera will shunt any transients to ground (as in power up).

Life is what happens while we're making other plans.

Wally

 

[itsmoked]

Hi BigWallyB.

Maybe you didn't see Bore's requirement "change" up a few posts from this one that sez, "I really must have 30V minimum".

The 436 Ohms is going to create a down-hole fluctuation of 34V.... If you stack that on top of the "30V" you get 64V and a lot of exotic minerals spattered around the level of the bore hole that the camera is at. Hence the need for some sort of regulation, since he really needs his power to stay between 30V-48V.

 

[boreholelogger]

Thats right itsmoked!.

 

[boreholelogger]

Ok well for now DSl over power is out of the question as the management wants it working ASAP!. Thus no time to implement it for this prototype.

So far we have concluded that there is going to be only two options.

1. Use one wire as the power wire and another as the return (Large loss but voltage is stable). Use the other two wires for the DSL communications link. The Armour is just chassis ground. We find a 200VDC power supply and a transorb to surpress the intial 200V transient spike at the camera. I would think that this is the easiest to implement but is very lossy and I am not very comfortable about the voltage.

What are the rules for wiring up 200V, do you need a licensed electrician etc, I have never had to worry about this sort of voltage before?.

2. Use two wires in parallel for power and the Armour as the return path (Much less loss, safer voltage, voltage will need to vary with cable spooled off the winch).

 

[itsmoked]

Hmmmm well for starters... if you are suddenly talking "code and licensed electricians", you better stop talking about using the armor for anything but a safety ground.

Next, I have been assuming you are a bore hole guy who will be wandering from site to site all over the planet with your Zero aluminum case full of "instruments" and a VCR.

But now you are talking about licensed electricians... I don't see where they come in with respect to your instrument. If you took a video camera out somewhere would you talk about an electrician? No, you say it only runs on 8VDC. Then I ask if you took a TV out to the field would you talk about an electrician? Televisions have 25,000VDC in them. Anyway the point I'm trying to make is.. um .. well .. it.. er..
I guess we need more functional description of your project as compared to your camera/dsl.

 

[boreholelogger]

Yes I see your point itsmoked. It just crossed my mind and I thought it may be an issue. But there are not alot of options with this type of cable and application.

 

[Warpspeed]

At least here on oZ, a licensed electrician (electrical contractor) is only required to install fixed wiring that is directly connected to the power supply grid. Beyond the fixed power point you can wire it up yourself. As you will probably be out in the middle of nowhere most of the time, as long as it is done in a safe professional manner with due care there should be no problem.

Just make sure there can never be any exposed live parts that can be easily contacted without using a tool of some sort. Female connectors supply power, bare exposed male pins receive power. Choose your connectors very carefully, think about wet conditions and possible mechanical abuse/damage. Include some sort of lamp or indicator to show when high voltage power is on, and include a prominent "high voltage" warning label. That sort of logical good design should keep everyone out of trouble.

If someone does get fried, provided you have taken every reasonable precaution the Coroner will not get mad at you.