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.

 

[itsmoked]

Well that's certainly a different question..

First thing I worry about is that if you are measuring this with a regular DMM you will not be seeing the actual current dynamics that the camera is going thru. They are happening faster than your meter is showing you. This likely means that it is using far more and less current than you have seen. As you have pointed out this means the voltage will vary at the camera(but probably more than you think).

On a bench I would run your camera thru a 1 ohm resistor with a scope across the resistor to actually see the peak current dynamic range.

Once this is know you can proceed.

This special application (in my opinion) needs something at the camera end of the cable as you are suggesting. But depending one the *TRUE* camera current dynamics that something can be several things.

1) A large MOV to limit the peak voltage to below your camera's 48V(it will work against the line resistance).

2) Possibly a small cap choosen to reduce the dynamic voltage swings caused by the current pulses being demanded thru the high impedance source lines. If the cap is too large your down hole voltage will get higher, possibly too high.

3) Alternatively, depending on the scope results, you may need a large cap and a regulator, LM237.

The math will tell you everything as soon as you know the actual current not the average.

 

[itsmoked]

boreholelogger; where abouts are you located? California?

 

[Skogsgurra]

Have a look at Vicor.

http://www.vicorpower.com

They have a very flexible range of DC/DC modules. You could probably use one of their 175-350 V input and 48 V output modules. Or something similar. It is quite small and can take +65 Centigrades ambient temperature.

Gunnar Englund

http://www.gke.org

 

[boreholelogger]

Hi itsmoked,
I have just graduated this year and I am currently located in Queensland, Australia.

 

[itsmoked]

Well then I won't be driving over to the "Central Valley" to give you a hand then, will I?

They just last week finished a multi-mile deep bore hole about 2 hours south of me in the Central Valley into one of the most seismicly active areas in the world, at the center of the famous San Andreas fault. They are going to view and instrument the hole. Then you start talking about the same thing so I was imagining a connection.

http://quake.wr.usgs.gov/recenteqs/Maps/121-37_eqs.htm

 

[boreholelogger]

Hi itsmoked,
We do that sort of work, designing geophysical instruments for that exact application but we are not involved with that operation.

 

[itsmoked]

Shucks!

BTW did you get the drift of my 1st post?

 

[felixc]

It can just be bypassing the problem, but maybe you could find a camera and lighting technology that consumes less current. A CMOS image sensor, LED lights. The fluctuations would reduce accordingly and you would not be dealing with something that has some dangerous potential. Let's hope that there is no methane in your borehole.

 

[boreholelogger]

Yeah i did thankyou.

 

[boreholelogger]

Hi felixc,
Yeah the problem that we have, is that this cable is the industry standard. We actullay use a CMOS image sensor and white LED's for the lighting. Most of the power is consumed in the DSL communications link hardware, FPGA and ARM processor. We have designed this camera specially for this cable to do real time MPEG4 video compression and send the data to the surface at high speed. The problem now is powering it.............haha.

 

[boreholelogger]

What I have done so far to reduce the problem... I have tied two conductors together for power (Parallel configuration 109 ohm instead of 218 ohm). I use two wires for communications. And I use the wire armour for the return path (23 ohm/km, although its a short circuit when the cable is spooled onto the winch). This reduces the voltage required at the surface by more than half (A significant improvement in my opinion). The power efficiency was constantly at 15% (Very bad I know). Now it is anywhere between 30 - 40% depending upon the length of cable spooled off the winch. I am still scared about surges, but I think it will be better dealing with a significantly reduced voltage.

 

[itsmoked]

Another thought might be some sort of reverse regulator, down the hole, that consumes more current the less the camera uses.

Have it be a voltage controlled current source, to keep the voltage drop constant. Have it add 0-70mA of draw.

 

[boreholelogger]

Thats a thought!.

 

[boreholelogger]

The question is. How would you do that?.

 

[VEBill]

"...two conductors together for power..." and "...two [more?] wires for communications."

It is very common practice over all industries for power and communications to share the same wires (one being DC or low frequency AC, and the other being RF or Data or whatever - a trivial issue to combine and separate).

The number of times that I've seen power and data on the same wires is innumerable. In marine work I've seen the whole submersible doen with one coaxial cable carrying power down and everything else up. Even satellite TV dishes have power and control signals going up and RF going down the same RG6 coax. Even telephone is all on one twisted pair (-48Vdc, ringing, two way audio, DSL, etc.).

Given the length of the cable, I would have made this assumption on the way in. YMMV ;-)

 

[cbarn24050]

Hi, a nice big fat zeneer diode at the camera end shoud do it.

 

[felixc]

Good idea itsmoked! Boreholelogger, this configuration is called a shunt regulator, if you want to do some search for it. A big zener, or with an additional power transistor to help channeling the heat to the casing.

http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=1155

 

[Skogsgurra]

What is wrong with the straightforward solution that I mentioned in my post? (Number 4 from top).

Vicor and many other manufacturers have standard DC/DC (or AC/DC) modules that can take a variable voltage and deliver a clean DC voltage to the load. Using series and shunt regulators to burn power seems not very logical or sound to me. It will also result in a kludge since there are no zeners that can do the work directly.


Gunnar Englund

http://www.gke.org

 

[felixc]

If it fits in the borehole instrument, it is of course worth considering. It won't heat as much as the other solutions.

Boreholelogger, what was the interest in doing the processing right at the camera? Image quality, available bandwidth on the cable?