atomic_fungus (atomic_fungus) wrote,

#5167: Well, that was educational

So, I wanted to get the grass cut, and--spoiler!--I did manage it. Getting there was half the fun!

Tractor needs a new battery, and an inactive winter in the garage did not help matters any. I'm not eager to do that today; I just wanted to cut the dang grass. So I hooked it up to the charger and started working on getting the tires filled, thinking that I wouldn't need to charge it for long to get it running.

Little charger wasn't making any headway--one revolution and nothing--so I got the biggie out and tried it. And then after letting it go for several minutes at the 10 amp setting, I tried it.

And after two or three slow revolutions, it stopped, went BUZZ, and didn't move.

Me: "Buzz"?

Now, look: it's purely a DC system. The starter is a DC motor. What you should not hear from a stalled DC motor is a sixty cycle AC buzz. No.

So I went and got my cheap Harbor Freight multimeter, and--sure enough!--it reported a 32v AC voltage. I unplugged the big charger and reconnected the little one, then tore into the big charger, thinking there was something wrong with it.

I had expected, perhaps foolishly, that there would be some capacitors in the thing; either a smoothing cap had gone, or I had a faulty diode, right? Well, when I got the lid off I was dismayed to see there were no caps in the thing, and where the hell was the rectifier? This thing was not pumping full-wave AC into a battery; that wouldn't charge it., on the transformer, four diodes, mounted to a plate which was both the anode and a heat sink.

Diodes all checked out. Okay--so the battery charger supplies rectified AC voltage and the 32-volt AC the DMM saw was just because the waveform isn't smoothed:

The battery charger outputs a voltage that looks like the dotted line. That's fine, because batteries don't really care all that much. I was expecting something more like the red line, but that doesn't happen when there aren't any capacitors in the circuit.

Capacitors would probably fail too quickly, anyway.

Regardless, I figured out that there was nothing wrong with the charger--guess I learned something useful from DeVry!--so I put it back on, cranked it to 50 amps, and got the mower started. And of course the grass got cut.

Go, me.

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