Electrical help please

[Guest]

Have a Makita 7.2 volt cordless drill with 'clapped' battery. As I can now buy two cordless drills for the price of a new Makita battery I would like to 'wire' the drill to use on board. Question: How do I reduce 12 volts to 7.2 (8 volts !) or thereabouts.

 

[burgundyben]

Ohms Law

You need to know how many amps the drill will draw under load, then use the equation resistance = volts/amps to calculate the ohm rating of the resistor you need to put into a circuit with a twelve volt battery, resistor can be bought from RS or Farnell or similar.

Circuit would have the resistor and the drill in series (circuit then being a potential divider)

If you dont understand this either you should not try it or I have got it wrong.

Thats all the help I can be, please someone turn the the date confusion thing.





add you sarcastic remark about someone elses tag line here.....

 

[jfm]

Suggested bodge solution

The resistor idea will work but needs a high power resistor, I mean a big wore-wound job with a heat sink, not a tinsy winsy tic-tac sized thing that you solder in a circuit board. You may find it hard to get such a component.

Alternative is to bodge it as follows. If the makita has a variable speed trigger, you could just connect straight to 12v, then put a self tapper screw or summink into the drill trigger assembly to make sure you only squeeze the trigger 2/3 of the way down, so the motor never "sees" more than about 8v. Guess it by the sound of the rpm. Doesn't have to be precise.

Whatever you do you will need heavy wiring though. I thort these motors were 150 watts ish, so at 8v you will need a "flex" that will handle 20 amps. That's heavy stuff, so take care to use stuff that wont burn out. Also, if you are connecting to a 12v socket already on yor boat, make sure the 12v feed wires to the socket (behind the panel) are also similarly butch. (Check the wattage in case my 150watt estimate is a bit high)

 

[BarryH]

Re: Ohms Law

If you have got it wrong, it all goes...KERR..BANG.....fizzle, fizzle, fizzle

Wha'dya mean "I'm always playing with this engine" its the only way to get it to run!

 

[Geoffs]

I don't think it is really practical to do what you ask, as the current drawn by the drill is large. You'll need large voltage regulator to do the job, which will probably cost as much as a new drill, these days.

The only other possible answer, is if your battery has external links between cells, when you can tap off 8 volts from 4 cells.

Old Chinese proverb 'Man who sail boat into rice field, soon get into paddy'

 

[BarryH]

I have in the past changed the cells in a drill battery. Carefully take the casing apart and you will find that inside that the battery is made up from normal Ni Cads. Being that its quiet and old drill you could pick up the cells from maplins or someone. If the batterys dead take it to bits, what you got to lose!

Wha'dya mean "I'm always playing with this engine" its the only way to get it to run!

 

[chippie]

I have done exactly what you propose also with a makita 7.2 volt drill. I split the casing and soldered some flex to the terminals, dont even worry about polarity as the reverse switch takes care of that. In practice the drill will be fine with 12v as most holes dont take long to drill and the motor copes ok. the model drill that I used has a thermal cutout which will operate if overheating is a problem. (It hasnt been so far).I used heavy alligator clips direct to the battery because I was in the process of re wiring .
This setup works well.
I find it good for screw driving as well (square drive screws are best.)

Cheers