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Tinkering with Electronics – Makita Drill

This has got to be one of the most boring blogs around or at least it’s getting there given that the frequency of posts have significantly declined over the last several months. And ironic it is too because playtime in my mind has been moving along non-stop. My latest rabbit-hole has been to learn all about electronics. Electrical Engineers generally start off as brilliant people and study for many, many years to become proficient, I don’t have that kind of time so I’m depending upon my high school physics to “catch-up” (teasing here). I do have some basic background knowledge but I never learned enough to actually apply of it so I’m hoping to change that this time around. If nothing else as always, I’m having fun trying.

Exploring – Makita 9.6V NiCd Drill

My Dad bought me this portable drill many years ago and it has given me good service. The drill is in great shape but I couldn’t bring myself to spend the $50 on a replacement NiCd battery pack and ultimately replaced the drill with a drill and two batteries that didn’t cost me much more than $50 combined. Since then I have purchased another new hand drill so now I was considering using the motor for my planned pick-up (coil) winder. For this session I really just wanted to confirm that the drill was still functioning normally and to do that I was going to need a 9.6 VDC power source (or similar) and I was going to have to reach the battery terminals which were located deeply inside the hand grip. I was hopeful I could use the battery charger for my power supply but to reach the terminals, I was going to have to take the drill apart.

I began by plugging in the battery charger and tested to see if I it was producing any voltage. I detected none, so I took it apart to see how it was assembled. Being a selfish and horrible blogger, I neglected to take a picture of any of findings but I can describe them. Here is a picture of an identical charger to mine, the picture is not mine. You’ll notice that there are two switch buttons, yellow for reset and red for charge. There is also a red LED to indicate the unit is charging. You can see the + and –  molded into the case indicating how the battery pack should be inserted and you’ll notice the  “key slot” on the negative side of pack prevents inserting it backwards. What you can’t see from this photo is there is a third metal contact which connects a circuit to a thermister which is wired to the first battery in the battery pack. The NiCds are shot but I salvaged the thermister.


I’m not an EE but clearly this is to monitor the charging circuit so that when the battery reaches a certain max temperature, charging is stopped. How does the thermal protection circuit work? I’m not sure, I haven’t looked it up yet but it didn’t stop me from taking the thing apart and digging deeper inside.

Four screws and the top comes off easily to reveal a pristine 120 VAC to 12 VAC transformer. Next in the circuit was a 4 pronged chip on a heat sink labeled RBV-401. Google is my friend and I was able to pull up a data sheet on it. It is a bridge rectifier which is a very common method for converting AC current to DC current using a network of 4 diodes. Hey, he DID learn something over the last month!

RBV4 Rectifier

So looking at the pin outs I can see the 12 V AC are on the center pins with the VDC on the outer two pins. Using my meter I now see a nicely stable 10 VDC are being generated. Time to wire this to my drill to see if I can make the motor run.

Dis-assembly of the drill was quite straightforward. I removed a  series of screws and carefully separated the two halves.


I wired up some leads to the outer pins of the rectifier and established that the drill motor was functioning perfectly. Although I knew the drill battery packs were rated at 9.6 VDC, I was curious what voltage  the motor was rated for. Again I Googled the part number on motor RS-750SH and by all indications it was a 12 VDC motor. I decided to hookup a spare 12 VDC power supply I had floating around and again the motor performed well. I realize that things could change when/if the drill was under load and at this point I have no idea about how many amps this thing would need to function as a drill. I don’t think it would pull very many amps however just spinning a bobbin around to wind a pickup.


Having had enough fun for one day, I decided to close the drill back up for safe keeping. I haven’t yet decided if I want to use the drill as a drill or if I want to sacrifice it for another project. It seems a bit of a waste to destroy a perfectly functional drill. Another option might be to retrofit a battery pack or even to convert it to a corded drill. I can’t decide, what do you folks think I should do?

You have been reading an excerpt from the shop journal of the Turtlecovebrewer.

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