battery voltage drop

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9 January 2017 at 18:08 #69389
chris wright 2
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@chriswright2
Hi I wish to reduce the voltage of my battery from 9.6 volt nicad to 7.2 volts to drive ancillary motor(s). I think I need to put a resistor in the circuit but what value in ohms and wattage would be required.

MT's

Chris
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9 January 2017 at 18:08 #2644
chris wright 2
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@chriswright2
9 January 2017 at 19:30 #69391
sammyk
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@sammyk
Hi Chris ,if you use a resistor it will get hot and that is a waste of battery power.i would try and use the same voltage for all the systems and if that wont work fit two nicads one 9.6 and a 7.2 nicad.but make sure only the 9.6 can only fit in the 9.6 system and the 9.6 will not fit the 7.2 system.regards sammyk
9 January 2017 at 19:37 #69392
ashley needham
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@ashleyneedham69188
Chris. Just as an idea. MFACOMODRILLS.COM do a speed regulator device that provides a variable voltage output at the turn of a pot. It is fairly small as well.

No doubt DM will have a high tech idea.

Ashley
9 January 2017 at 22:26 #69393
Tim Cooper
Participant
@timcooper90034
Component Shop sell small voltage reducers, with an Led display showing input and output voltages. Voltage is altered by adjusting a small screw. Cost under £5. I have bought several to take a lower voltage off the main batteries to power small radar motors and LED s.

Tim
9 January 2017 at 23:11 #69394
Dave Milbourn
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@davemilbourn48782
The voltage reducer has my vote provided that you make sure the thing can handle the current which the motor(s) draw.

DM
16 January 2017 at 15:19 #69457
Harvey Spreckley 2
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@harveyspreckley2
Posted by chris wright 2 on 09/01/2017 18:08:41:

Hi I wish to reduce the voltage of my battery from 9.6 volt nicad to 7.2 volts to drive ancillary motor(s). I think I need to put a resistor in the circuit but what value in ohms and wattage would be required.

MT's

Chris, There are two ways you can do this 1) You can do this using resistance and it won't get hot at all as long as the resistance used is designed to receive the current that your application demands. You will need to work out the power (Watts) required (W=Vsquared x I(amps) add 10 % and purchase a resistor that is rated at greater than that. First work out the current that your application requires, use ohms law to find the resistance that provides the necessary voltage drop. 2nd method and best is to use an in circuit adjustable voltage controller. I suggest an LM317. To find out how to make this IC., function down load the datasheet and therein you will find the electrical circuit and necessary arithmetic to make it all function perfectly. Chris S.
16 January 2017 at 20:11 #69471
sammyk
Participant
@sammyk
voltage reducer what a fantastic bit of kit ,will use one the next time i need to reduce the voltage on a model .as they say every day is a school day . what did we do before the internet.regards sammyk
17 January 2017 at 09:10 #69474
Dave Milbourn
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@davemilbourn48782
There are one or two limitations with using a resistor. First and foremost is that the resistor dissipates the "spare" voltage as heat, and so it reduces the running time of the main motor. Next there are the physical aspects of the thing to consider. At 2.4v drop for an average small motor (say 2A draw) that's 5 Watts. A 1 Ohm x 5 Watt resistor is a sizeable affair and will get HOT (not just warm), so you'll need to provide for cooling the thing. Linear voltage regulators usually have a pretty low maximum current rating; the LM317 in a T220 package (through-hole with metal tab) is usually 1.5A so that restricts the size of the ancillary motor you can use. It will also need a hefty heat-sink to carry its full current capacity.
A switch-mode voltage regulator like the High-Power one from Component shop works like a speed controller (it pulses the applied voltage) so will dissipate hardly any energy as heat. This particular one can run a 6A motor happily all day.
Having said all of that I'm a firm believer in using a second battery of the same voltage as the load requires, unless it's absolutely impossible e.g. because of restricted space/weight.
Suit yourselves.

DM
17 January 2017 at 10:55 #69475
Harvey Spreckley 2
Participant
@harveyspreckley2
Dear David, thank you for that contribution.

Perhaps I should better start with…………………….Where the hell is the prime mover………………..are you still alive Chris? We could do with more information.

David, I think that your getting rather esoteric with Mr., Wright and if that is to be the way forward then I think that you should have first asked Chris Wright "What are we talking about driving here".

Surely not too many model boats run motors of or greater than 1.5 amps, this is a hig'ish current for any models electric motor [Monoperms apart] in my experience and you know the LM317 can't be burned out; like most modern Voltage controllers the 317 is guarded by over current and over heat resetting cut outs. In short it's dead safe for 95% of all modelers models, I would have thought. One thing more for anybody reading these remarks by Dave Milbourn and myself should be aware that the 317 is not an easy beast to wire up for a novice electrician and if you have a less obtuse voltage than 7.2V's to acquire you might well look to a range of Voltage controllers such as the 78xx series and in particular for your own purpose look at the two xx figures which will for most peoples purposes will read 03;05;06;08;09;10;12 this final figure of course is the output voltage of the particular IC., which is fixed unlike the 317 which you have to organise by arithmetic and resistance to output your desired voltage, but with a maximum current output of 1A. UA78xx = 1.5A max. So for the industry standard electronics voltage requirement we would look for LM7805 to provide a fixed voltage of 5V – Imax=1A. All else is readily available on the 78xx data sheet. One more thought the LM/UA7806/08's may well satisfy your needs for 7.2V's and of course they are very easy to implement, even for a novice.

I am not a buy it now electronics user for anything so Mr., Milbourn's dissertation on that method of control may well be the better way for anybody who does not feel confident enough to carry out the construction of his own electronic device and David's pwm., method just has to be the soundest method of driving any motor under any circumstance, but certainly not the cheapest especially if bought in the high street.

Chris S

Edited By Harvey Spreckley 2 on 17/01/2017 11:10:19
17 January 2017 at 11:57 #69476
Dave Milbourn
Participant
@davemilbourn48782
Surely not too many model boats run motors of or greater than 1.5 amps

1.5A??? Oh you would be very surprised, especially if you regard a Monoperm as a high-ish current motor. An easily-imagined scenario might be to run the main motor/motors on 9.6v and wish to drop that down to 7.2v for a bow-thruster. These are generally powered with 3-pole 380 motors whose nominal rating is 7.2v and will cheerfully take up to 10A, although a more usual continuous current would be of the order of 3-5A. I wouldn't recommend any of our customers to use anything less than a 10A speed controller for one of these – and "buy it now" electronic speed controllers for brushed motors routinely go up to 30A continuous rating.

Granted if the ancillary motor is for something like a radar sweep arm then a simple linear regulator would do the job, but I added my caveat because Chris Wright hadn't specified the actual use and may not be aware of the limitations of using a resistor in certain circumstances.

I don't think I've ever been described as "esoteric" before. Indeed I would have left the matter alone after my original brief reply – it was you who introduced Ohm's Law et seq.

DM
17 January 2017 at 20:08 #69484
Harvey Spreckley 2
Participant
@harveyspreckley2
Dear David,

OK. I bow out with good grace I hope and some amazement , as explaine in another thread I am a newbie to MB's. My model engineering experience is gained from 00 gauge rail where 4-6 engines of the finest can be run on a 1.5 amp feed current all given the reinterpretation provided in cab., (dcc} and for that matter straightforward dc., models given the right kind of circuit.

I bow out, but it would, I believe, be remiss of me to leave any follower of this thread thinking that electronics is an idiots way of working. Somewhere, somehow there is always an answer to be found in electronics for work that cannot be accomplished any other way. For instance transistor or relay switching to a higher mode of power [pressure(V's)] or current [[I amps] from the original source. When you get electronically stuck as they say in Lancashire, 'for bobbins' just keep reading the data sheets and pursue that manufacturers advice.

Chris
17 January 2017 at 23:18 #69489
Dave Milbourn
Participant
@davemilbourn48782
Dear Christopher

I'm not at all sure what point your second paragraph is trying to make. Believe me when I say that probably less than 1% of the folk who read this Forum would be prepared to research manufacturers' data sheets and draw up their own solutions to a problem which, with a click of the Send key, can be solved by obtaining an off-the-shelf solution – often for relatively pennies. There are folk out there who actually think it's somehow "cool" not to understand anything to do with volts or amps. It's for those folk that I was asked to write this article **LINK**

Dave M
18 January 2017 at 18:10 #69499
Paul T
Participant
@pault84577
Dave

Thank you for tipping me off about this thread, I needed a good laugh and I found it right here in ample quantities.

I wonder where I am going wrong as non of my boats draw less than 1.5a but I think my grandson has got a plastic toy that might just qualify.

Paul

'esoteric allusions' doesn't this describe what you used to write whilst running ACtion Electronics?
18 January 2017 at 19:38 #69500
Dave Milbourn
Participant
@davemilbourn48782
'esoteric allusions' doesn't this describe what you used to write whilst running ACtion Electronics?

No, you berk! They were called Instructions – otherwise known as the coloured part of the packaging that was thrown away first.

Keep the faith.

DM
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