24v - 12v Regulation

[Pcolaboy]

I'm working on a submersible ROV and could use some information on voltage regulation. Please read through the following overview first.

The ROV has three 12V DC motors that will draw a maximum of 10A when engaged. In addition, I have two 20watt 12V lamps that will constantly draw 4.2A. Therefore my maximum total draw will be around 15A.

I have a series of DPDT relays (2 for each motor) onboard the ROV that receive 5v inputs for their coils from a control box on the surface via CAT-5.

The 12V power for the motors will be supplied from the surface with two 12V batteries run in series to produce 24V and will come down to the ROV on 160feet of 14AWG wire and distributed to the relays for power to the motors.

Using an online voltage drop calculator, I determined that the available volts at the end of the wire will be 12.38 volts based on the assumed amp draw, gauge and length of cable, etc. The calculator is located at the bottom of this page: http://www.powerstream.com/Wire_Size.htm.

Realizing that the actual AMP draw will often be much less than 15A, I'm now starting to worry about "over-volting" the system. The Relay Contacts are rated up to 8A @ 30VDC but I'm not sure about how well the motors will handle extra voltage. Relay Spec Sheet (model RTE24005)

QUESTION:
Is there some sort of simplistic voltage regulation circuit that I could build to limit voltage (coming into the relays) to 12V no matter what the amp load is?

I've looked into DC-DC converters online but the ones in my price range can only provide 100watts of 12V. According to my calculations, I'll be using up to 180 watts.

Thanks in advance,

Pcolaboy

 

[positronicle]

Why are your batteries not on-board the ROV?

 

[ericgibbs]

hi Pcolaboy

The tables for 14AWG state 2.525R/1000ft.

So for a total length of 160ft gives, [160/1000] * 2.525 = 0.404R

For a double length of 160ft gives 0.808R

So the voltage drop for a single length of 160ft at 15Amps, 15 * 0.404 = 6.06Volts drop.

A double length of 160ft, gives a 12.12Volt drop

Question is the cable length overall 160 * 2 feet.? Is it a 160ft length of double core.?

I am doubtful about the calculator, try the same numbers but using aluminium wire...

EDIT: the 14AWG for power transmission is rated at 5.9Amps!

As the system is designed to work on a 12Vbty, why dont you have the 12V battery in the submersible.?

 

[jpanhalt]

Here are some other options to consider:
1) Get rid of the relays and use PWM to control the motors in an H-bridge to get forward and reverse.
2) The amp capacity of the wire is temp-limited in air. In water, it is probably higher.
3) I you do use a voltage regulator for the heavy currents, use a switching power supply/regulator rather than a liner regulator for efficiency. There are several simple ones in the 6A range (e.g., MC33167 and TI -- PTN78020). The TI is relatively expensive ($20) but is a complete unit. I don't have any datasheets on hand for something in the 8 to 10 amp range.

John

 

[mneary]

I've looked into DC-DC converters online but the ones in my price range can only provide 100watts of 12V. According to my calculations, I'll be using up to 180 watts.

You should have a small battery (7Ah, 12V) in the sub for the motors, and use a DC-DC converter to keep it topped off. Use a second DC-DC converter for the lamps only (no need for a second battery). Then you just need to keep the average motor consumption below 100W.

 

[crutschow]

. but I'm not sure about how well the motors will handle extra voltage. Relay Spec Sheet (model RTE24005)

Plus it would likely zap your lights when the motors are off.

QUESTION:
Is there some sort of simplistic voltage regulation circuit that I could build to limit voltage (coming into the relays) to 12V no matter what the amp load is?

I don't know if there's any "off the shelf" simplistic solution that you can afford. The high current and low voltage drop requirements may require a custom circuit. A simple design could consist of a power MOSFET regulating the voltage in a simple two transistor regulator circuit. The maximum power dissipated by the MOSFET would be about 45W so it would have to be mounted on a good heatsink.

N-channel MOSFETs are more readily available and have lower on resistance. But that would require a separate wire to bring in the 24V from you batteries so the transistor can be fully turned on when needed for the maximum load. Do you have an extra wire in your CAT-5 for this purpose? Otherwise you could use a P-MOSFET.

If you're interested in this approach I can post a circuit for either the N-chan or P-chan design.

 

[ronsimpson]

You have 24 volts and want 12 volts at 4 to 14 amps. A switching power supply built for 12 to 24V in and 12 volts out should work.

Do you care about power loss? You could use 48 volts. A 48 volts to 12 volts switching supply will draw less current.
12V 11A in 12V 10A out
24V 5.5A in 12V 10A out
48V 2.7A in 12V 10A out
At 48 volts you wire loss will be much less. OR You could use smaller wire.

 

[Pcolaboy]

Why are your batteries not on-board the ROV?

Believe me I've been trying to figure out a fairly inexpensive approach to building a pressure hull for a battery.

On the one hand I could buy an inexpensive ($20)12v lawn tractor battery, but would have to design my own casing - available plans and personal experience are lacking. On the other hand I could buy a very nice sealed-lead-acid battery at $70+shipping, but I would then want to buy an Otterbox at $50 to insure protection of the more expensive battery. Am I making any sense? The SLA batteries are much more compact per amp-hour and many will fit nicely in the largest Otterbox. Ultimately I guess this is the way I'm gonna have to go - just have to tell...or rather, beg, my wife for the extra buget money .

I will certainly update the post with whatever solution I go with. Any suggestions on a watertight case for a battery that can handle 100ft of depth would certainly be appreciated. Battery dimensions being roughly 8" Long, 6" Wide, and 7 3/4" High from what I've been finding.

Thanks for all the great replies!

Pcola

 

[Pcolaboy]

hi Pcolaboy


Question is the cable length overall 160 * 2 feet.? Is it a 160ft length of double core.?

EDIT: the 14AWG for power transmission is rated at 5.9Amps!

As the system is designed to work on a 12Vbty, why dont you have the 12V battery in the submersible.?

According to the online calculator, it has you input the one-way length but says that the round-trip (double) length is used in the calculation. The 14AWG or even 12 AWG I intended to use was essentially Monster-type stranded speaker wire (oxygen-free). I found a 200' spool of the 14AWG selling locally for $29.

There are a few peeps I know that have built ROVs with virtually an identical setup sending down 12v on a simple 75ft outdoor 14gauge extension cord. Obviously they aren't getting nearly 12V to the motors. I wanted full capability out of the motors and a greater range of travel that I was hoping a longer cable and more input voltage would afford .

Thanks for the reply!

Pcola

 

[ericgibbs]

According to the online calculator, it has you input the one-way length but says that the round-trip (double) length is used in the calculation. The 14AWG or even 12 AWG I intended to use was essentially Monster-type stranded speaker wire (oxygen-free). I found a 200' spool of the 14AWG selling locally for $29.

There are a few peeps I know that have built ROVs with virtually an identical setup sending down 12v on a simple 75ft outdoor 14gauge extension cord. Obviously they aren't getting nearly 12V to the motors. I wanted full capability out of the motors and a greater range of travel that I was hoping a longer cable and more input voltage would afford .

Thanks for the reply!

Pcola

hi,
So far you have only given us the 'steady state' current requirement for the motors,
I expect you are aware the starting/stalling current will be much higher, you could 'current/voltage starve' the motors
when they are attempting to start, especially as they are starting with the 'prop' load applied.

 

[Pcolaboy]

hi,
So far you have only given us the 'steady state' current requirement for the motors,
I expect you are aware the starting/stalling current will be much higher, you could 'current/voltage starve' the motors
when they are attempting to start, especially as they are starting with the 'prop' load applied.

I'll try to locate that information from the manufacturer. The motors are essentially 12v bilge pumps without the housing using a small hobby propeller instead of the factory impeller. The use of these types motors for small submersible ROVs is quite common since they are already watertight to about 40-50 feet in depth. They must be pretty resilient since I know of a guy that's been putting less than 10v to them for nearly two years with almost the same exact power/control setup that I'm intending. I'll post any details from the manufacturer that I find.

Thanks for the feedback!

Pcola

 

[mneary]

Isn't the interior of your ROV dry? Why would you need to 'protect' a small-ish 12V battery?

12V DC motors don't usually have a problem with low voltage unless you let them stall.

 

[Pcolaboy]

Isn't the interior of your ROV dry? Why would you need to 'protect' a small-ish 12V battery?

12V DC motors don't usually have a problem with low voltage unless you let them stall.

I'm not sure what you're referring to. Most amateur ROV's (mine being no exception) are basically frames made of PVC pipe with a video camera, lights, and motors. The only 'interior' space would be the housing for the electronics and camera.

At depth, I strongly suspect that sea water will easily enter a standard car battery's cells through the maintenance caps on top. A sealed-lead-acid battery or even a gel-cell would be less of a worry but they are too expensive for this project.

 

[ericgibbs]

Realizing that the actual AMP draw will often be much less than 15A, I'm now starting to worry about "over-volting" the system. The Relay Contacts are rated up to 8A @ 30VDC but I'm not sure about how well the motors will handle extra voltage

hi,
I would think the 12V lamps would suffer the worst if the motors were not driving.
The voltage drop in the cable would be much less in the power cable, you would overvolt the lamps.

With regard to the motors, any excess voltage would immediately drop as soon as the relay closed to power the motors.!
So in effect the motors would only see a small over voltage when one was running compared to both running.

I would suggest you have the 160ft cable coiled on the bench and give it try, dont blow the 12V lamps.

 

[crutschow]

At depth, I strongly suspect that sea water will easily enter a standard car battery's cells through the maintenance caps on top.

Are you actually using this in sea (salt) water? That would make corrosion and electrolysis a significant concern.

 

[ericgibbs]

hi Pcolaboy

Do you have a spare wire in the control cable going down to the ROV.?
[not for a power cable]

It just might be possible to have a voltage regulator located on the surface, with a low power 'ROV voltage sensing' cable feeding back to the surface.

 

[jpanhalt]

An odd thought occurred to me while thinking about the pressure problem. What if you filled all of the air spaces in the batteries with "mineral oil"? In fact, you could put the battery in a tub full of oil, put a top on it, and attach it to your ROV. Pure mineral oil will not react with sulfuric acid. What you need is an oil that doesn't have any unsaturated hydrocarbons in it. Cetane (kerosene) might be a good choice. If you can't find anything, you could make it by stirring the "oil" with sulfuric acid for several hours, then decant the oil (top layer), wash with water (i.e., stir with water then recover the top--oil--layer), and you would have something to use.

Of course, at today's price of oil, that may not be the cheapest solution.
John

Edit: Corrected typo on top and bottom layers.

 

[Pcolaboy]

Are you actually using this in sea (salt) water? That would make corrosion and electrolysis a significant concern.

You're exactly right. That's why I'm considering building a sealed battery container filled with mineral oil. The container would have watertight bulkhead fittings to pass wiring through - there are actually several of these fittings on the market: Bulgin Components

Pcola

 

[mneary]

As i mentioned before, you only need a smallish battery (2Ah might be enough). Coupled with a switching regulator to keep it charged (and prevent over voltage), you should have enough.

Here's where it gets brilliant. The switching regulator only needs two parts! Just put a relay winding across the battery terminals; with a variable resistor in series. Adjust the resistance until the relay just pulls in at 13.8 volts (full charge for the SLA battery). The contacts apply your 24V (through the ~1 ohm cable) to the battery when the relay drops out, and they open before the battery is overcharged. Relay: 12V nominal. Variable resistor: 0 to 2X the relay resistance.