Mains Power Tool Adaptor

Ugh! My battery is dead! Where's the other one? What, that's not charged either?

How many times has that happened to me? Way too often. I have a bunch of Ryobi tools but not enough batteries to power all of them. My wife also wants to use my tools while I'm at work, so she'll call me and ask "Where's the battery for the drill?"

In short, I needed something to power my tools when I've drained all my batteries or I need a tool and there aren't any batteries available.

I've thought about building a mains power adaptor for a long time. I figured I'd have to find just the right transformer and build a power supply circuit. It seemed like it would be a lot of work so I never got around to it. Like everyone else I've move to lithium battery packs and I suddenly realized that I never use my old charger which has a matched transformer and a power circuit!

Disclaimer: If you are careful, you should be able to carry out this Instructable with little or no danger. You could do this complete build without ever plugging in until everything is completed. If you attempt this, you will be dealing with with mains power at 120v which always warrants caution. Transformers and capacitors can build up a charge and be dangerous even when unplugged. Keep dry. Keep your hands away from the bare electrodes while in operation. Work with a buddy. Don't be path to ground.

I can't guarantee that this will work with any Ryobi charger. It should work with any Ryobi Nicd battery pack. I definitely can't guarantee that it will work with other brands but it does demonstrate that it should be at least possible. This was an exploration for me, I wasn't sure that it would work. If you're looking to explore and this project sounds interesting to you, I encourage you to have your own explorations.

Materials

1 Ryobi Charger for Nicd battery packs

1 Ryobi Nicd battery pack

1 outlet box (optional but recommended)

4 self tapping screws 3/4" long

cyanoacrylate glue (super glue)

electrical tape

rosin core solder

Tools

1 narrow neck phillips screwdriver

soldering iron

metal shears

Dremel tool and bits

Pocket knife (optional, useful for cleaning off burrs after dremeling)

The charger is easy, four phillips screws, none hidden, very maker friendly.

The battery is only slightly harder, the shafts that the screws are down are narrow and need a thin phillips head driver. Six screws, none hidden.

The major components in the charger are as follows. The transformer, this is the big heavy brick thing. The plug wires are soldered to it. Next, coming off the transformer is a fuse holder and fuse. This is a bonus safety feature, keep it attached. Then there's the heat sink and circuit board. Most of these parts just lift right out, the exception being the power leads that are clipped to the charging port. With a small amount of force, these slide right out.

The major components of the battery pack are as follows. The outer shell, we want this for holding onto our components and for interfacing with the tools. The clips, these come out easily once the case is taken apart. They consist of a spring and the plastic clip, one on each side. The contact clip, this is very important for interfacing with the tool. The Nicd battery pack, this is no longer needed for this project. If yours still works, you might be able to repurpose it, but mine wouldn't hold a charge.

We want everything in the charger. Remove all the components carefully, they are reasonably sturdy so you don't have to worry about breaking anything unless you're exerting a lot of force.

The circuit board from the charger does more than we want it to do. It has diagnostic circuits in it that can shut it down and we're not interested in that. But it does have a nice bridge rectifier and a power capacitor. There are two small nuts and bolts that are holding the circuit board to the heat sink. I was able to remove them with a screwdriver and my fingers.

Lets talk about what we have here and what each part does. Starting with the charger.

The power cord is decent, it's not the best and it could be easily replaced, but I decided to keep it.

The transformer is two coils of wire. The first coil generates a magnetic field. The strength of that field is dependent on how many windings it has around a steel core. The steel core is one of the reasons this thing is so heavy. The second coil picks up the magnetic field and turns it back into electricity. The ratio of the coil windings effects the voltage (and the current) of the power that comes out of it. It changes the voltage from 120v to 18v. So if the first coil has 100 windings around the core, the second coil has 15. This is called stepping down the voltage. At the same time it steps up the current so as the voltage drops the current increases and the total power is maintained.

The fuse is a great safety feature, especially since going into this, I'm not sure how much current a drill or a saw is going to draw. If they draw too much and burn out components, then I have to replace expensive parts. I can crank up the power draw now and the fuse should protect the harder to get parts.

A bridge rectifier if you're not familiar is a set of diodes that converts alternating current like what's in your wall into direct current. It's basically a maze for the electricity to travel through that always spits the positive to one side and the negative to the other. Only the power it puts out is choppy.

A power filtering capacitor takes the choppy power from the rectifier circuit and smooths it out. As the power peaks, the capacitor charges. As the power drops, the capacitor discharges and fills in the power gaps in the milliseconds where power is below where you want it to be.

Now for the battery pack, the components we want to keep are all attached to the contact clip. To remove the positive terminal from the nicd cell, rock the cell back and forth slowly until the solder joint fatigues and it comes free, or heat it with a soldering iron.

There's the negative lead and terminal, this is where the battery used to provide electrons (which have a negative charge).

There's the positive terminal, which doesn't have a lead attached to it.

The thermocouple and it's terminal. This protected the battery from getting too hot. When the thermocouple opened, (turned off) the tool would shut down.

The majority of what the circuit board does is not needed for what we want for this project. However we do need to get rid of any circuitry that would shut down or limit the power as is done when a battery is charging. Just looking at the circuit, it looked simple enough but I wanted to proceed carefully.

The picture above shows how power is routed through the board. At the very end of the negative loop, there's a power transistor that serves as a switch to turn off power. We don't want that so it should be disabled. I took out the transistor and put a jumper across it. This is an unnecessary step because there's an easier way around the transistor. I include what I did here because it might help with figuring out a different model charger or brand. After disabling the transistor with a jumper, I could run the tool off the charger circuit.

In the end. I soldered the negative lead from the contact terminal to the negative side of the bridge rectifier. This was much simpler.

It started to become apparent that the parts were not all fitting in the battery case. The transformer barely fit but wouldn't allow anything else in the case. It was also too tall. The circuit board was also too big. It wouldn't fit at all.

Since most of the board served no purpose now, a portion of it could be removed. I grabbed my metal shears (tin snips) and cut off the top and side of the board. Tested again and I was still able to power a drill off the setup. It's not pretty, but it worked and saved me a lot of work having to rebuild the circuit with jumpers.

After looking at the transformer and the case, it looked like it would fit if I removed one of the screw posts, so I cut it out with a Dremel tool. This allowed the transformer to sit in the case and have enough space for the cut down circuit board. Unfortunately the transformer was still too tall. The case would not close.

I've seen people drastically cut down transformers. I didn't want to go that route.

I wanted to avoid this, but in the end it worked out fine. I cut a hole in the bottom of the case, and now the transformer fit and the case could be closed.

I now had enough room that I could re-include the heat sink if it were cut down. So that's what I did. I probably could have re-applied some thermal paste if I was really worried about it.

Now all there is to do, is to fit everything in.

I glued the contact terminals to the inside of the case post with the cyanoacrylate glue. There's no Nicd cell to hold it in anymore, so if it's not glued or otherwise fastened in place, it will slide out of position. I thought of globbing Silicone into the void, but that would take longer to cure and I'm impatient.

I also put the fuse and the thermocouple in the stem of the case. They fit well. I put piece of electrical tape over the back of the fuse holder to prevent shorts against the metal thermocouple.

The power cable side of the transformer should face down. String the power cable through the hole in the case. The fit is still a tight one.

Put a piece of electrical tape over the back of the circuit board to prevent shorts against the transformer. The circuit board and heat sink can now be fitted in place behind the transformer and under the post. This fit is also tight.

Now reinsert the springs and the clips into the case and then close the whole thing up.

The adaptor is now functional but the bottom terminals of the power cord are unprotected. Looking around, I found an electrical outlet box and cut it down with the Dremel tool. I also gouged out a spot for the electrical cord to clip into. I then attached it to the bottom of the case with four 3/4 inch self tapping screws.

So far the rig tests out. I haven't blown the fuse and I haven't tripped the thermocouple yet. It's sturdy but heavy. I could paint the bottom cover black, but I'm not overly concerned about it. This is a tool, not fashion.

I would have loved to fit it all in the original case but I couldn't find a way to do that.

As an alternate, I could have left all the components in the charger, and ran an extension cord from the charger to a hollowed out battery case. That might have been easier but this was more interesting. This solution is more compact and portable.

I went and demonstrated the adaptor to a coworker the other day. Within a few minutes of showing him how "well" it worked, the fuse popped.

Actually I think I figured out why. I'd tested the adaptor on my old drills that were meant for Nicd packs. I demonstrated it to my coworker on my Lithium pack drill (you know, the lime green kind). I wonder if the newer drill draws more current than the older drills. In any event, there are quite a few tools that have a high power draw, like the reciprocating saws.

I decided that I would add a circuit breaker to the build instead of a fuse. Instead of buying a new fuse, cracking the case and replacing it each time, all I'd have to do is hit the breaker. I also thought about adding a current limiting circuit, but I found a four amp breaker on ebay for less than $3 so that cinched the deal.

What do you need?

Your newly built Mains Adaptor

Most of the tools in the first part of this build (except the super glue).

A 4 amp circuit breaker. It seems they use 4 amp breakers on boats, for what I'm not sure but I'm not arguing. It also seems they use 4 amp breakers in aircraft.

I decided to mount my breaker to the back bottom of the case. I thought about poking it out the side, but I wasn't sure that would weaken the casing, since I was going to have to remove another screw post. The back seemed to be a safer bet. The hole doesn't have to be perfect because the circuit breaker has it's own escutcheon plate that covers things nicely.

The power supply circuit fits, barely. I didn't include a picture of the circuit soldered on because it was so cramped I couldn't get a clear picture. It's not hard to figure out though. Desolder the fuse holder and take the leads that were attached to it and solder them to the circuit breaker. One lead from the transformer goes in one of the circuit breaker contacts and the other lead from the power supply circuit goes in the other. It doesn't matter which lead is which on the breaker, either direction will work. I put electrical tape over the circuit breaker leads to prevent them rubbing against something in the case and causing a short circuit.

Slide the power supply circuit into the remaining space and close the case. Your adaptor is now ready to use! If the breaker pops, you just have to hit the button to reset it. Easy as pie!