Help Needed: Building 18V Power Supply for Power Tools

Hey guys,
I'm very new to understanding electronics, and I don't speak equation, but I'm hoping I can get some help with my project. I’m building a power supply to replace the 18Volt batteries on my Ryobi Power Tools; I’ve successfully completed it, but have ended up creating a 29V power supply, instead of 18-20V. I need to know what resistors I need to take my rectified voltage down to 20VDC so I can smooth it thru the capacitor for final use. I understand that I will need to wire them in series, but don’t know how to calculate the sizes I need that will maintain 20amps without burning up. Total voltage drop should be 8 or 9V. Equations welcome if each component is explained..
BACKGROUND INFO: I bought a 120-to-20VAC 400VA Toroidal transformer (2x coils at 20V 10A each), and two bridge rectifiers (50V 20A rated), as well as a 10,000µF AlumElec capacitor (20V rated). My original assumption from everything I found online was that this would give me a smoothed 18.x Volts DC around 20Amps, after full-wave rectification and smoothing. Upon completion, I got 18.x VDC after rectification, but then discovered that I get 29.x V DC after connecting capacitor. A few thousand equation-riddles search results later, one kind person explained that the 18.x VDC I was reading was the RMS voltage, but that the capacitor will keep my final voltage near the Peak voltage of the rectified DC, something more like 29V. That finally makes sense, however, when I plug my power tools in, the motors run way higher than they should, and my 20V-rated capacitor would likely explode under sustained load. I need to know what size of resistors I can wire in series to drop my 29Volts down to 20V without losing (much) amperage. After bringing the voltage down to 20, I will pass it by my capacitor in parallel, and it’s ready for use. I am also aware that under full load, my final RMS voltage will drop down around 17V, but the tools are 18V, so this is acceptable.
Any help is appreciated, but I ask that if you starting speaking equation, you explain each component of the formula so I can understand the ‘why’ behind it. Thanks!

 

Thanks for the help.

I used the amp function on my multimeter to see what kind of draw a standard cordless drill pulled on the original battery (Ryobi 18V ONE+ and Lithium battery); just on motor ramp up, with no load, it blew the 10 amp fuse in my multimeter. I found a 120V drill with 2Amp motor which performs comparably to my cordless drill, therefore it would consume 240Watts; based upon this, i figure at 18V mine would pull 13-14 amps, so i sized the transformer up to 20Amps to handle inrush.

While still searching for a solution, I found a diagram where someone dropped voltage by passing through a series of diodes, each one dropping by ~0.6V due to Forward Voltage. Theoretically, i would assume i could pass thru a few more bridge rectifiers like the ones i bought already ($3 each), instead of trying to use a bunch of heavy-duty resistors, and consequently step my voltage down in ~1.2V increments.. would this be a possible solution, or am i maybe missing something inadvisable here?

 

It is a manufactured toroidal transformer, purchased it for about $60 from Antek, so i'm not quite comfortable with taking it apart, let alone know what i'm doing..
I don't know anything about switching regulators, where would i get one for this kind of application? Would it be better to install it on the AC side to lower the input voltage, or the DC side?
I assembled this whole project inside of a 16-inch toolbox, to act as a portable power supply, so i have room to drop a few fans inside of it if that would help cool off the components.. it also has an on/off switch on the AC side, as well as one on the DC side which can power it down when not in use..

In testing, i discovered that my smoothed voltage was around 29V, but it dropped to 24-25 while using the drill, my assumption was that rapid capacitor discharge brought the RMS down by 4-5V under load; therefore i don't need to take the supply voltage all the way to 18, i'm comfortable with it around 21V with no load, and let it drop to 17-18 under load. A voltage test of the original batteries under load looks very similar..

I will only be operating one tool at a time, such as a 'sawzall', drill, or circular saw; but the same battery fits Ryobi's ONE+ series of tools, so my goal was to make it compatible with all of them (i have maybe 20 of about 40 diff tools).

 

Laurence,

I finally got mine working; rather than try going down the switching power supply road, I just bought a new 15V AC toroidal transformer. After bridge rectification (i use a separate bridge for each of the two transformer coils), and capacitor smoothing, I get about 21V smoothed DC, with no load.
As soon as I apply a small load, like a drill on low speed, it drops down to 19-20V range. Under a heavy load, like a leaf blower, it drops to about 17V. I used my flashlight, which has an 18V incandescent bulb in it, and it ran for 15+ mins without burning out or having any issues.

I haven't yet tested my amp draw under any of these loads, but I regularly feel my wires and components for overheating, and none of them have reached anything over room temp. I haven't yet run my power supply for extended periods or under extended high loads, but I think it will handle pretty well.

I probably have much less experience with electronics than most here, but if any of my tools have a startup current spike of 30Amps, none of my wiring or components indicate an inability to handle it.
As I recall when I took the old NiCad battery apart to convert it into my plug-in adapter, the wiring that connected the battery bank to the battery terminals was surprisingly thin (maybe 16 or 18 gauge, at the most). This suggests to me that any high-amp demand by the tool is so short-lived that the wiring doesn't have time to heat up and therefore is not at risk of burning up.

So unless your power supply has amperage limitations with its onboard components, I wouldn't be too worried about the inrush current. Overall wattage needs to be able to handle 15-20Amps sustained, and you're good..

Steve

 

Kevin,
I solved it by buying a 15v toroidal transformer (originally bought 18v).. I was looking at using a series of bridge rectifiers on each leg to drop the voltage gradually, but the right size ones and the amount i needed was like 40-50 bucks. Turns out a dual 10-amp 15v xformer was only 40 bucks.. My final voltage after my 10,000-farad capacitor was about 20v. My tools don't notice the difference. Under a load, the voltage drops down to a 17v ripply DC, but doesn't seem to have any effect on them over a battery.
Also, as the other Steve posted, there is no feedback wiring two AC leads together, as long as their 120v sources are on the same phase. Also, I didn't wire my two legs together before the bridge rectifier; I put each on its own bridge rectifier, and the output of each of those is basically DC, which doesn't have issues being wired together.
Found a guy that posted a way to use a bridge rectifier or series of them to drop 1.2v each.. You wire the positive AC wire to the negative stud of the rectifier; then you jumper the two AC studs on the rectifier together; then your voltage out goes on the positive stud, 1.2v lower ( after passing thru exactly 2 diodes in the rectifier, each diode dropping 0.6v). It's easier to visualize if you draw it out..
I would have loved to test this out, but was looking for a more efficient solution. Just remember, you have to buy at least twice the amperage load for your rectifiers cuz they count the total amperage of the diodes together, or something stupid. Basically, get at least 40-amp rated bridge rectifiers. You'll have to double the quantity of them and do it on the second transformer leg as well..
Also, someone previously said that the rectifiers would overheat trying to dissipate hundreds of watts of power; I'm no expert, but as I understand it, they only drop voltage, not amperage, since they are wired in series, therefore a collective drop of 7 volts and maybe 0.1 amps is practically nothing. Definitely not alot of wattage, so I think this would work..
Hope it helps. I'll post a wiring drawing if I can remember to update mine, that may help as well.

 

Thanks, other Steve, for the help..

Wish i would've chatted with you before ever buying my first transformer (it was 20V, not 18; i even thought i was doing good by accounting for diode voltage drop). Would've been nice to save 60bucks and hours of head-scratching.. Had to figure out about RMS vs peak the hard way.

and Yes, my bad, its a 10k *micro*Farad capacitor. seems to work just fine for me.

Attached is my completed wiring diagram. This works fine for me so far. BTW, found a nifty little 0-30V DC LED display that shows me the voltage. When i power off my switch on the DC side, it automatically drains the capacitor within about 10 secs. Found several of them on ebay from a seller in China for about $7 shipped. Amazing!

 

Sorry, new to attachments..

Here it is.

 

Suicide is just a slang term that refers to a standard factory-made cable which 'sacrifices itself' to simplify wiring. (Picture attached of the cables i used on my project)

i don't connect 120V to 20V (forgot to modify my drawing for my new 15V transformer, so it's technically 15V AC now before rectification). Terminal Block A is a 120V AC block. power comes in, and goes out to 4 items: 2x Transformer coils, and 2x spare power cords (this whole P/S is housed in a little 16" toolbox, so i just plug it into a wall and carry it around with me, and can plug lights or tools into my spare 120V cords, while using my cordless tools on my 18V power supply).

Also, you're right, AC should be indicated as Line (L) and Neutral (N), or something like that, but not being an electrician, i just drew this the way i learned it. I'll update it for others who might want to do the same thing as me..

 

Updated drawing, correcting some confusing elements, and accounting for my new 15V transformer.

Hope this helps others out there trying to do the same thing as me..

BTW, my total cost, including some 'luxury' additions was less than $200, cheaper than buying 2 new batteries, and MUCH longer-lasting..