VOLTAGE DROP

Control Boards, Controllers, Tethers, Ect.
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orangeltr456
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Joined: Jul 5th, 2013, 10:44 am

VOLTAGE DROP

Post by orangeltr456 »

I am working on 2 rov projects right now 1 I am retrofitting a biology lab, basically the rov in a box model which I will refer to as 1.0 and a second one that I am building in collaboration with the local high school and university this one is refereed to as 2.0. Some details can be seen viewtopic.php?f=9&t=1140.

So the problem with 1.0 is that the kit has them running roughly 100ft of tether, which consist of 3 pairs of 14-16 gauge speeker wire for motor power and control, and a single phone wire( 4 x 24awg) for led and camera power as well as video feed. The controls consist of 3 2 way toggle switches mounted in a hobby box. ( btw not very easy to use). with a single motor running I only get about 5-6v at the motor ( 500gph bilge cartridge). which I found closely resembles Steve's testing and the calculations I have done.

I have a few options that i can think of.
1 shorten the tether ( not ideal)
2 Increase voltage to about 18 ( not that easy to fins 18v batteries or regulators that can supply the correct current total of 20 amps MAX)

3 use on board batteries and relay box for controls. only problem is this rov is small and under powered as is and do not really want to do that big of an overhaul. but a possibility if required,

I cant send 24 down the line because the motors will not handle that kind of power for extended periods of time. additionally the camera will fry, and I don't want to put regulators on board
1.0 is the #1 priority right now.

2.0 is still in the design phase but will soon have the same problems. I planned on running 1 cat 5 cable for coms between 2 arduinos and run video feed and single pair of power wires but withe the amp load ( 30A max) I would have to run close to 10-8 AWG to even get 10v to the rov. not ideal because it is heavy, expensive, and not very flexible. So I run into the same problem what would be your recommendations to fixing this problem . A 100$ 36 to 12 v regulator could work because the only 12v i need topside is logic power so a small regulator topside under 3A would power my control system and a large 30A regulator would power my rov. again on board batteries is not my favorite plan ether.
rossrov
Posts: 383
Joined: Feb 28th, 2013, 5:01 pm
Location: Australia

Re: VOLTAGE DROP

Post by rossrov »

What (watts? :D ) appears to be overlooked on recent voltage drop threads is that by upping the topside volts, say, in this case maybe 48VDC, you can decrease the tether current, and hence the wire size.
4 12 Volt motors pulling 5 Amps each at 12 Volts will be drawing 20 Amps, P=VI, a total power of 240 Watts. To get the same power, by increasing the volts from 12 to 48 we only need 5 Amps. This of course results in a thinner tether requirement, because with less current there will be less voltage drop.
Powering the motors etc from a nominal tether voltage of say 48 Volts can be done by using an efficient switchmode stepdown power supply in the ROV.
Why not go higher in volts you may ask? Basically, risk of death by electrocution. Commercial ROVs using higher voltages have properly engineered safeguard technology in place, and very expensive tethers, connections and expertise to help prevent accidents happening. Even 48 Volts could give you a shock under certain circumstances.
rossrov
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Joined: Feb 28th, 2013, 5:01 pm
Location: Australia

Re: VOLTAGE DROP

Post by rossrov »

These are intended for powering 12 volt stuff off the main batteries of electric vehicles. The $7 ones. http://www.ebay.com/bhp/car-electric-converter No idea how good or otherwise they are. 10 Amps stated, so have to use 2 or more depending on your motor current draw
jsut210
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Joined: Aug 14th, 2013, 7:53 am
Location: Maryland

Re: VOLTAGE DROP

Post by jsut210 »

You could try using three of these 6v batteries in series to get 18v http://www.ebay.com/itm/6V-12Ah-10Ah-7A ... 19e0c60a01
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orangeltr456
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Joined: Jul 5th, 2013, 10:44 am

Re: VOLTAGE DROP

Post by orangeltr456 »

Ross The idea of 48v regulated to 12 on board the rov by a switching type regulator will work great for 2.0
the problem is mainly with 1.0 see the motors are hardwired to 3 toggle switches in the control box topside( not the best way to do it i know, not my design blame rov in a box for that one) a regulator would not work on this project because there technically is no power and ground in the tether. each time the switch changes direction the polarity changes.

And in terms of watts 1.0 will be using roughly 180w max and 2.0 will be using 300- 360 w depending on motor configuration,

I have a question though. as I understand it power companies use high voltage lines say 10kv so they have to carry less current then use a transformer to drop it to say 120v. again as I understand it with ac it transfers just fine so a 100 amp heater at 120v is 12000w and 12000w at 10kv is only 1.2 amps

but dose that work with dc and voltage regulators. see regulators as far as I know ,only experimented with linear, reduce the voltage by increasing resistance as needed basically, creating heat. I was curious how current is effected through a linear regulator so I did a test .
12v reduced to 5 running a 5v motor at 500ma. current was measured at 500ma on the output side of the regulator and 575ma on the input. so instead of reducing amps by increasing volts i was actually increasing amps. Now I have no evidence to say this applys to switching regulators

If so what would happen with 2.0 is I would have say 360w 48v topside run down my 150ft tether into a 48 to 12 v switching type regulator the motors would be drawing 30A from the 12v regulator. would that mean the batteries would only be seeing a 7.5a draw ?

I apologies if this is a obvious question

thanks jsut for the link
it is an option to run 3 6v batteries for 1.0 and just use the voltage drop for the motors they can handle initial 18v spikes.. but I would have to have a small regulator for the camera and light. not that hard to do.
martinw
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Joined: Sep 20th, 2011, 11:02 am
Location: Aberdeenshire

Re: VOLTAGE DROP

Post by martinw »

It's all to do with efficiency of the voltage regulator in question, linear voltage regulators are typically less efficient than switching regulators and therefore generate more heat (especially when there's a large voltage difference across them, for example when dropping from 24VDC to 5VDC), but linear voltage regulators aren't as electrically "noisy" as their switched mode counterparts. That's engineering for you, pros and cons wherever you look :)

You can work out how much current can be drawn from a particular linear voltage regulator at a particular voltage drop using the junction temperature.

http://forums.parallax.com/showthread.p ... alculation

Basically on a 1 amp 7805 with a decent heatsink dropping from 24VDC to 5VDC you can get about 200mA output at 5VDC without running into any thermal issues. The input voltage must be above the output voltage by about 2VDC, i.e. 6.5VDC to 7VDC to get the maximum 1 amp stated.

Hope this helps.

Martin
rossrov
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Location: Australia

Re: VOLTAGE DROP

Post by rossrov »

456, your mention of transformers is well placed, the switchmode supply can in this case be considered the DC version of a transformer. If we had 100% efficiency then there would be no losses, power in = power out. You would really need to do some further testing, as efficiency is often stated without mention of other conditions, such as input voltage and output load. If the ebay supply in the plastic box does what it says it does, then the fact that there is no big heatsink on the outside says to me that it will be pretty efficient. Loss of efficiency is going to result in power going somewhere we don't want it to go, in our case in the form of heat.
Linear regulators such as 7805, 7812 etc etc should really only be used where only a few milliamps are required, such as a microcontroller, sensor etc as they rely on heat loss or inefficiency to do their very job.
With regards to the toggle switch ROV, personally I would be using two 12 batteries to give 24 volts and using a longer or thinner tether. Easier to charge (cheap off-the-shelf charges abound) than an odd voltage and you can use them for other things later on.
Just for arguments sake, you could actually use the step-down regulators, provided their start-up time was sufficiently short, with the toggle switch system by adding a few diodes and a relay
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orangeltr456
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Re: VOLTAGE DROP

Post by orangeltr456 »

Ross
To make sure I am on the same page a switching regulator such as this onehttp://www.alliedelec.com/search/produc ... #tab=specs would have the same effect as a transformer, as in 48v topside sent down to regulator sent to motors. if all motors are running at the same time I can expect to draw 30 amps.

30 amps at 12 v = 360 w
regulator is 86% efficient
so would be drawing 410w from my 48v battery bank
410w/48v = 8.5A
is that safe to say or am mistaken.
if this is true this would work great for 2.0 the micro controlled rov.


Now back to 1.0 toggle switch controlled.

so I do not mind the idea of sending say 24v through the control box and down to the rov with 2 motors running at the same time v would drop to about 10. not that big of a deal. that would require the least amount of modifications. Or even changing the wire size and running 36v.

1 last option would be to change it to relays and cat 5 + a single pair of power wires for the tether and a small regulator supplying power to a relay box. I am not really sure what I want to do . Any advice would be appreciated.

Thanks
Under_The_Surface
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Joined: May 3rd, 2013, 12:10 am

Re: VOLTAGE DROP

Post by Under_The_Surface »

another option maybe to make a something like a garage with the batteries in that the rov can move out of at a certain depth.
think link is sort of what im trying to say https://www.youtube.com/watch?v=SgFonmHL90E
although you you have to make or buy a winch to reel the rov back in
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orangeltr456
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Re: VOLTAGE DROP

Post by orangeltr456 »

I had thought about this idea but with 1.0 a toggle switch designe with power being sent down each time the toggle switch is activated a garage type system wouldn't work plus this being such a small craft and not really good for working at depth it would be over kill to do something like that.

The thing is I may use a cage/ weight on my 2.0 project because it will be used at depth in a river with current. To minimize the effect of the drag on the rov the majority of the cable will be weighed down so it hangs off the bottom and then a shorter length of cable with have to be drug around by the rov itself
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