12V-6V stepdown circuit wanted

Too much current to be worth doing. Open it up, take one of the batteries with you to Jaycar or somewhere like that, and buy replacements. Or just turf the whole thing and buy a better one for $30 or so.

for what its worth, my mobile auto-electrician has a 7.2v makita rechargable drill that he modified to run direct from a car lighter socket (12vdc) without any form of voltage reduction (other than the resistance in the 4metre of .75mm square fig 8 that was used to connect to the lighter plug). I did ask of this setup, as I was concerned about the possible over-voltage issues and he said that the drill had been working like this for many years on a daily basis with his trade without any problems.

(He did get sick of flat batteries all the time with the original setup)

I think its worth a try using such a distance of fig 8 cable - the voltage drop in the cable is obviously enough.

Thanks Ken,

I'll give it a try . . all I can do is burn the motor out, but at >20 years, I guess it doesn't owe me much.

Try a few metres of that cord - measure the voltage at the drill - when drill motor is running, and let us know the voltage there - Im curious as to the drop. I think you will find that this will provide a pretty good voltage drop if you use a few metres.

You of course could calculate this a little .

Find the running current of the 6 volt motor . Use a multimeter , hopefully its not over 10A

Then find the resistance of the wire .Easily obtainable

Apply Ohms law V=IR

Maybe allow a 20 percent over voltage max to help with peak loads

lighter

One day FruitLoop got dressed and committed to text

OTOH is it wise to assume that the motor was indeed rated 7.2v ?? As said before many will run on a few more volts and likely have a little more power. Particularly with older ones.

-- Regards ..... Rheilly Phoull

Its a balance between reliability and power output verses duty cycle and longtivity

Hi Dave

For what it is worth: I had a situation a while back where someone wanted a similar setup, a 7.2V Makita saw sharpener. My solution was to connect a number of high current (35A) bridge rectifiers in series, the +12V from the battery going to the -ve terminal on the 1st bridge & the +ve terminal going to the -ve terminal of the next bridge in line with the final bridge +ve going to the +ve of the motor. (Battery negative going to the negative of the drill motor)

Each bridge will drop about 1.2 - 1.4V regardless of load (within reason). Therefore going from 12V to 6V, you will need 5 bridge rectifiers, which will need to be mounted on a suitable heatsink. I found in my situation, resistors did not work satisfactorily as the current varied too much. When I wanted some grunt, there was no volts & hence no torque

BTW, this arrangment is actually used in a commercial 24V -12V reducer

It is obvious to me that either everyone here is in the stone age or you are all feeling the chill of winter coming and need hot resisters and/or diode bridges to warm yourselves up on.

If, on the other hand, running off a battery (be it vehicle or small NiCds) means anything to you then you will need a more efficient solution.

If you want to drive the drill with 6V from a 12V source then its very simple - you just need a switched DC waveform at 50% duty cycle. Since the drill motor is inductive you don't need any filter capacitors because the inductance of the windings will smooth out the current to DC anyway. All you need is a 555 timer circuit set to run at as close to 50% duty cycle (on the lower side) driving an N-Channel MOSFET (I would recommend 2 * MTP3055V's from Jaycar Electronics in parralell) with a 3-AMP diode reversed from the drill connection to your +12V supply to catch any possible brush spikes. Run the 555 timer in astable mode at around 20KHz frequency. All the parts necessary to do this can be purchased at Jaycar or Dick Smith and shouldn't cost too much. If you don't want to bother figuring out the circuit yourself, then there is a kit - the High Current DC-DC converter which can be constructed to do this - you could even mod it to have a variable speed drill controller.

-Ben