4-25-2011: The fix wasn’t good enough. This is not a good modification. The weed eater killed over again. If your battery operated weedeater dies, either buy a new battery or just chunk it. A 120v AC motor would be better than trying to rectify for a DC motor.
The following is for archival only…it was a fail in the end.
My cordless Black and Decker weed muncher has gotten to the point that it no longer holds a charge. It’s amazing that ten pounds of battery can’t hold a charge. This time I had an idea to remove the battery and just put a 12 volt transformer in it. I went into my workshop and peered into the junk bin looking for an old extension cord. I had a three prong one. Good stuff. I also grabbed a proto board. Inside I had some 1n4003 diodes and a 220uF capacitor.
I went into this mod knowing that it wouldn’t last long. I’ll build a better circuit later. I just wanted to see this work for a second. The tranny is rated to 3 amps, and the weedeater can pull about 3 amps under a heavy load. I expected that the transformer would be ok. The diodes are only rated for 1 amp. I knew this would be trouble, but I decided to continue just to see what would happen.
I constructed a full bridge rectifier and used the capacitor as a smoothing filter, although highly underrated for this job. Without calculating I knew that it would not be enough, but that probably allowed the circuit to be a little soft as maximum power could not be delivered. The voltage would droop as the current level increased.
I know for the final product I’ll need a 5A rectifier. I’ll be shopping at the shack so they’ll probably only have a 10 amp. I’ll also pick up at least a 6400uF cap. I figured this using Hayes and Horowitz “The Art of Electronics” textbook. On a side note, that is probably the best book I’ve ever purchased. If you love electronics, hit up half.com or amazon.com and get a used copy. Sometimes you can find this book for sale for under $20! Ok, so I used this as my target:
- 2.5V ripple
- 2A I_out
- dT = 8mS
I just checked the price of caps from the shack online. I’ll be using a compilation of parts to achieve that. I can’t see paying over ten bucks at the shack for caps to add up to 6400uF! I have a 35V 4700uF cap and a 2200uF cap laying around that should do the trick.
Remove the battery and clip off the charging LED circuit. Solder the extension cord wires to the back leads on the primary side of the transformer. Connect the secondary leads to the AC inputs on the rectifier bridge. On the DC side of the bridge, solder two wires to connect to the switch of the weedeater. Also connect the capacitors to the positive and negative tabs of the bridge. Connect the wires to the switch. Install the circuit in the machine were the battery was. Put it back together and go eat some weeds.
The circle in the picture above shows the test circuit and where it was mounted. It fried and smelled like burned stuff. The next circuit will be a bigger bridge mounted on a piece of aluminum with some large caps hanging off it. Below is a picture of what happens when you use ill rated components.