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Long Cable Power Issue
4

Long Cable Power Issue

Long Cable Power Issue

(OP)
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.

RE: Long Cable Power Issue

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.

RE: Long Cable Power Issue

boreholelogger; where abouts are you located? California?

RE: Long Cable Power Issue

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
www.gke.org

RE: Long Cable Power Issue

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

RE: Long Cable Power Issue

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

RE: Long Cable Power Issue

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

RE: Long Cable Power Issue

Shucks!

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

RE: Long Cable Power Issue

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.





RE: Long Cable Power Issue

(OP)
Yeah i did thankyou.

RE: Long Cable Power Issue

(OP)
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.

RE: Long Cable Power Issue

(OP)
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.

RE: Long Cable Power Issue

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.

RE: Long Cable Power Issue

(OP)
Thats a thought!.

RE: Long Cable Power Issue

(OP)
The question is. How would you do that?.

RE: Long Cable Power Issue

"...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  

RE: Long Cable Power Issue

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

RE: Long Cable Power Issue

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
www.gke.org

RE: Long Cable Power Issue

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?

RE: Long Cable Power Issue

(OP)
Yes it was mainly to do with the bandwidth of the cable.

RE: Long Cable Power Issue

(OP)
Yeah the problem I have with the vicor DC-DC converters is that they are slightly too big for the dimensions of the pipe that they need to go into. I will keep browsing their range though thanks.

RE: Long Cable Power Issue

(OP)
Can you buy modules that put the DSL comunications over the power line?. I have never seen any, but thats not to say that there isn't. With all of our other equipment we superimpose our pulse type communications over the power wires using flip-flops etc. I would much prefer to be able to put the DSL comms over the power wire, then we can do whatever we like!.

RE: Long Cable Power Issue

skogs; the module you are proposing sounds like it needs 175V at the bottom!  Furthermore, while I think Vicor is a cool company, I can't imagine them expecting their input sources having 436 Ohms of source impedance.

Still seems to me that using a shunt or even a standard regulator down hole that gets to use all that source impedance to its advantage with the waste heat distributed over a mile as being the way to go.

So far I'm getting ugly numbers like 2W dumped on a zener. ugh!

RE: Long Cable Power Issue

The most obvious and simplest solution seems to be fitting a shunt voltage regulator and suitably large capacitor down the hole as previously suggested.

However, you mention the availability of spare wires in the cable. That raises the possibility of feeding the actual camera voltage back to the surface through a pair of sensing wires which carry no current.

It would then be possible to have a voltage regulator located at the surface that keeps the voltage at the camera constant by tracking load changes. That would eliminate any problems of size or heat dissipation of the equipment down the hole.

RE: Long Cable Power Issue

(OP)
Warpspeed,
          The cable has 4 individual copper conductors encased in a woven steel wire belted casing (Armour). The conductors have 85 ohm/km of resistance and the Armour has 23 ohm/km of resistance. The first configuration I tried was to use 2 conductors for DSL communications and the other two wires for power which made sense to me at the time. The second configuration which I am now using at the moment is two wires for DSL communications and two wires for the positive voltage (Parallel configuration to supply more current and half the resistance) and I am using the Armour as the return path. The problem with the first configuration everyone knows. The problem with the second configuration is that when the cable is spooled onto the winch then the Armour has a 0 ohm resistance (Short circuit). When the cable is fully spooled off the winch then the Armour resistance is about 59 ohms. This means that if I don't have a large voltage input range at the tool I will need to automatically change the voltage at the surface to compensate for the voltage difference.    

RE: Long Cable Power Issue

Yup, a problem in your implementation is the ground that swings along with the current peaks.  You say DSL communications; is your comm signal electrically isolated, or do you have to live with this floating gound?  I guess that your two signal lines are a diff pair.  Is there any way to isolate them with a signal transformer, to get rid of the ground fluctuations?

RE: Long Cable Power Issue

(OP)
The DSL signal communications pair is isolated via a transformer at each end.

RE: Long Cable Power Issue

I failed to make myself sufficiently clear (it happens all the time...).

!! Parallel the four wires (only 55 ohms one way) !!  Use the armour as the return path (0 - 59 ohms).  Total resistance only varies from 55 to 114 (much better, eh?).

Then - put the DSL signal onto the power line.  This is absolutely trivial since the DSL is probably about 1MHz range (just use ferrite chokes and nF capacitors).

If required - use a 10F SuperCap to smooth out the current (depends on the time frame of the variation). Or, if you need more - add a circuit to adjust the illumination LEDs current inversely to the varying camera.  The camera can probably compensate for the slight variation in brightness.

Using the above suggestions almost certainly reduces the problem to a range where there are easier solutions.  It wouldn't take much to get the voltage variation into the 9 to 48 volt range (that's a pretty wide range, more than 5:1).  Probably don't even need a voltage regulator other than the nice one built into the camera...

BTW - this is a classic case where this should have all been considered at the outset - right at the point where someone first mentions using 2.5km of cable (WHOA!!!!)

RE: Long Cable Power Issue

Smoked

If you look at their site, you will find many models with many different input voltages ranges. And I know that you know that the impedance can be brought to zero with a capacitor. If you choose an AC input model, the capacitor is there for you already. Just behind the rectifier.

Gunnar Englund
www.gke.org

RE: Long Cable Power Issue

geezz skogs that site is hard to wade through.. :(

I couldn't find one that would do the job.  Doesn't mean there isn't one. I just couldn't find one.  So I went to other DC-DC makers but always had problems with the desired 30V output.

If he uses a LM137 it would need to dump 12.5W when the camera/lights were doing their 320mA thing and 2W when doing the 400mA thang. Probably means something like 6W average which a pretty moderate heat sink would handle easily.  What kind of temps hang out down a mile?  I would guess ummmmm 50F?

VE1BLL's idea is a good one. Perhaps mixed with warpspeed's so the two wires running the sense voltage to the surface double as the DSL lines.  

BTW: Why are we calling this DSL?  Is this really a set of modems?

RE: Long Cable Power Issue

Re: warpspeed's suggestion - There might be an issue with having 2km+ long sense wires that are also in close parallel with the power wires.  By the time that you filter the induced noise out of them, they would probably be too slow to maintain timely control of the distant voltage.


Four Wire Parallel solution - the math...

Using the four wires in parallel with the armour as the return, the total resistance varies from 55 (cable spooled) to 114 ohms (fully out).

Thus if you were to feed in 60 volts at the top end, the voltage limits at the bottom end would be within 14.4 to 42.4 (which is within the camera's limits of 9-48VDC).

Here is the math:

Vin    mA    R    IRdrop    Vout
60    320    55    17.6    42.4 (max)
60    400    55    22.0    38.0
60    320    114    36.5    23.5
60    400    114    45.6    14.4 (min)

Note that no voltage regulator is required at the bottom end.

Furthermore, if you add a 10F supercap (or something similar) at the bottom end, you can probably filter out some of the current variations to stabilize the voltage towards the middle of the range.

I'd add a voltage crowbar (perhaps an SCR crowbar using the cable resistance as the voltage dropping load) at the bottom end to protect the circuits. If you need to reset it, then you simply cycle power at the top end.

As already mentioned, it is trivial to insert and extract the ~1MHz DSL signal.

RE: Long Cable Power Issue

Smoked,

You are a stubborn young boy. The camera can take anything between 9 and 48 volts. What is the problem? Stuck with your darling?

Gunnar Englund
www.gke.org

RE: Long Cable Power Issue

Take a look at how the power industry does things: long distance transmission uses high voltage.

If the cable will withstand it, take a supply of 300V or so. The load is 12W, equating to 40mA @ 300V. Assuming that you use a single core each for the positive and negative supply, that is a loop resistance of 436Ω and a volt-drop at the load of about 17V, leaving about 283V available to supply the load. A DC/DC converter like the ones Skogs is describing would do the job very well.

Rather than say something is 'too big' (e.g. the Vicor converters), tell us the dimensions you have to work within. It helps because we might be able to suggest something useful if we have all the information.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

RE: Long Cable Power Issue

Higher voltages are 'Good Squared'. The IR drop is reduced because the current is less, AND any remaining IR drops are less of an issue with higher voltages.  They're 'Good Squared'.

Size souldn't matter because, although the instrument diameter may be limited, the remaiming third dimension can be as much as about a km or so...

RE: Long Cable Power Issue

(OP)
Basically yes this system is an off the shelf modem at the surface and a custom built design at the tool.

RE: Long Cable Power Issue

(OP)
Yeah the camera board can take 9-48 Volts but the LED power supply board can take 30 - 48V. By using any less than 30VDC at the tool, the voltage fluctuates to much at the tool end due to the increased current draw for constant power. That is why 30VDC is my aim.

RE: Long Cable Power Issue

(OP)
Yes using high voltages are fine in theory, but when you have people that are not always qualified in the use of electronics, like people using this stuff in the indonesian jungles running off generators etc. I would prefer to keep the voltage as low as possible and still be able to run the equipment successfully (Especially if I am using the armour as a return path). I think the best idea is to put all the 4 wires in parallel and use the armour as a return path. That way everything is safe and we could even use a longer cable.

VE1BLL,
       You say putting DSL communcations over the power wire is trivial, then how do you do it?.

RE: Long Cable Power Issue

(OP)
I appreciate all the feedback I have received it has all be very helpful!. Thankyou to everyone that has contributed and that I am sure will continue to contribute to this matter.

RE: Long Cable Power Issue

Ah ! signal transformers at each end, lovely.

One technique is to have a centre tap in each signal transformer winding. You can then feed dc into one centre tap at one end, and take it out at the centre tap at the other end.

The dc current splits two ways and travels down both sides of the signal circuit in parallel. That will effectively halve the dc cable resistance. The dc current will not effect either transformer because it will be balanced at the centre tap.

The two other available conductors could be paralleled and used as a dc return path.

A suitably large filter cap, and a shunt voltage regulator down the hole should then be all you need.

RE: Long Cable Power Issue

(OP)
Its that simple huh!

RE: Long Cable Power Issue

"...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.

RE: Long Cable Power Issue

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.

RE: Long Cable Power Issue

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.

RE: Long Cable Power Issue

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

RE: Long Cable Power Issue

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

RE: Long Cable Power Issue

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.

RE: Long Cable Power Issue

(OP)
Thats right itsmoked!.

RE: Long Cable Power Issue

(OP)
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).

RE: Long Cable Power Issue

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.

RE: Long Cable Power Issue

(OP)
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.

RE: Long Cable Power Issue

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.

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