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Power transfer over cat5 (or other thin cable)

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uzzors2k

Wed Jun 15 2011, 01:58PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I tried my MKII ROV this weekend, but was less than impressed by it's ability to maneuver. Currently I run two cables down to the ROV, one cat5 cable and a two-lead 14AWG marine rated cable for power. The result is a tether that weighs too much and is too stiff for the ROV to move. The battery was purposely removed from the ROV, so putting it back is out of the question. Since I only use four out of the eight wires in the cat5 cable, I figured the others could be used for power.

Now for the problem. I did some tests using some flyback cores and a few drivers, but output power is non-existent. I can't really be bothered to create a full system like this from scratch anymore, so what are my options? Could I buy a standard 12V inverter and hack it for higher frequency/ lower voltage? Or are there other inverters types better suited for this? I'm all ears!

EDIT: I forgot to mention this: In the other four wires in the cat5 cable, control signals are sent using RS-485, and composite video is sent "bare".

ConKbot of Doom

Wed Jun 15 2011, 02:34PM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329

what sort of power are you looking for?

If youre feeling daring and can run 90-100 VDC over the cat5, you should be able to run most universal input power supplies at the ROV end, and with 4 strands, an amp wouldnt be unrealistic. However I wouldnt put 100V on an RJ45 connector. Are you using regular patch cables with RJ45 ends or just the cat5e cable itself with your own ends?

uzzors2k

Wed Jun 15 2011, 02:52PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Under 150W.

The ROV itself runs on 12V DC, so I'll need to convert back down aswell. One end of the cat5 has an RJ45 connector, the other a Bulgin underwater connector. These can be changed to suit the power supply demands however.

Sulaiman

Wed Jun 15 2011, 06:19PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

RJ-45 connectors can be rated for 1000 V 1.5 A

Using as an example the first item when sorted by price, 4-pair CAT5
Â£0.59 (+20% VAT) per meter .....
Insulation (Test) : 1 kVdc or 0.7 kVrms for 1 minute
24 awg .. 0.5 ( 1 ? ) Amp.
If you were insane, 1 kV x 1 A dc = 1 kW via RJ-45 / CAT5 (per pair !)

I expect that there are better connectors & cable than I quickly found
so two twisted-pairs for power should carry 1 A ac/dc easily,
choose voltage for power required.
(one wire is about 85 mOhm/m so about 0.85V 'lost' per 10m @ 1A etc.)

I don't like having high voltage/power in the same cable / connector as signals .. too much protection required.
(all cables fail eventually) and (makes signal tracing 'hazardous')
do-able but not advisable

P.S. try to not put high voltage or power on a common standard cable /socket
.... to most of us, if it looks like an RJ-45 I don't expect 100 V @ 100 W or more !

big5824

Wed Jun 15 2011, 06:24PM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

Have you considered some of the stranded wire with silicone insulator stuff? They are generally very flexible with a decent conductor area from what i remember.

Dr. Slack

Wed Jun 15 2011, 07:52PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

"mains" cable has to have good enough insulation for the transients of 2kV that occur daily, so with care at the ends, you could use a lightweight 3 amp cable at a hgih voltage to minimise drops. I heartily agree with Sulaiman that you don't really want to mix kv and signals on the same cable, especially if dangled in water!

uzzors2k

Wed Jun 15 2011, 08:41PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I was only thinking of stepping the voltage up to 48V or so, but if regular cat5 is good to 1kV, then I'll step it up to mains voltages. I forgot to mention that the cable is only 30 meters long, so losses become acceptable even after a 1:4 transformer ratio. The socket I use at the ROV end is a Bulgin connector like this:

So it's hard to mistake for anything else. The RJ-45 end is stuck to the tether.

What about inverters? Does anyone know how these are usually constructed, and how hard it would be change the frequency?

Wolfram

Sat Jun 18 2011, 11:54PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Uzzors wrote ...

What about inverters? Does anyone know how these are usually constructed, and how hard it would be change the frequency?

The relatively cheap 12/24V - 230V ones usually have a push pull converter controlled by an SG3525/TL494 or something similar, to step up the input voltage to around 300V DC. This voltage is fed to a MOSFET fullbridge that is switched at mains frequency by one more SG3525/TL494, with the dead time set to give a modified sine out of the bridge. Modifying the output frequency should be as simple as changing the timing resistor of the PWM controller for the bridge. There is a limit to how high the frequency can go though, as the upper FETs in the bridge are driven by some simple and slow level shifter circuitry, but a few hundred Hz is probably realistic.

mikeselectricstuff

Sun Jun 19 2011, 12:36AM

Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253

Just make sure your data lines are protected against a short to whatever supply voltage you end up using. Or at least ensure any nasties can't get past the (socketed) transceivers. Locating ground pins between data and power is also a good idea.

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