Battery charger innards

Today’s dissection was a dead battery charger for an 18V Black and Decker FireStorm battery that goes into my sander. The charger died a few years back and I came across it while cleaning out the junk box. The label says it takes 120V AC output and puts out 21.75V DC output. Putting the DMM across the terminals only gave about 3.8VDC.

Inside the part that plugs into the wall is a large transformer (right), like the previous wall wart. At the other end is the part that clips onto the battery. Inside I found a simple circuit with some diodes, resistors and an LED to indicate charging. Any time I see 4 diodes, I think “rectifier”, so I drew out the circuit based on the circuit board traces. Sure enough, I found a full wave bridge rectifier.

Checking the transformer, there was no continuity on the primary side, but the secondary side was fine. Made me wonder how I was getting the 3.8V at the charger terminals.

Based on this, I have a feeling the innards of the next wall wart on the dissection tableworkbench is going to be similar: big transformer with some rectifying circuitry to produce a “DC” output.