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For keeps: Schauer TB10012 battery charger

01 Apr 2011

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This charger worked fine for 30 years. When it stopped working a couple of years ago, I pried into its interior to see what was wrong. It turns out that the failure was purely mechanical. I must have dropped the unit, which dislodged the copper PCB from its isolated mounting and allowed it to short-circuit the case. In addition, the movement of the heat sink caused a pin to break off the output transistor that was mounted on it. I repaired the unit by pushing the heat sink back onto the insulated boss and re-soldering the wire to what remained of the transistor pin. It seemed that the charger was still broken until I remembered that it was designed to have no output until it was hooked to a battery. Once I biased up the control board with a few volts from a dead motorcycle battery, the charger worked fine, including the adjustable output voltage on the control board. An important factor in the longevity of the unit is that the designers did not use electrolytic capacitors.

1. The design uses puck-style diodes similar to those in automotive alternators. Manufacturers solder them directly to the copper heat sink. The two wires to the diodes are the ends of a centre-tapped transformer's secondary winding.

2. A thermal circuit breaker limits output current in case the output transistor short-circuits. The breaker is wired into the negative side of the output.

3. The mounting of the control PCB allows an adjustment potentiometer to extend from the back of the case. The board regulates the output voltage and prevents output unless an attached battery is present.

4. The output pass transistor has two thick wires soldered to each of its pins. The wire from the control PCB caused stress, which broke off the transistor's base pin.

5. A thin copper sheet serves as a heat sink. A square hole in the bottom snaps over a plastic mounting boss in the outer case. The heat sink operates at 15V, the raw unregulated rectified output of the transformer.

6. The charger's main transformer features primary and secondary windings on separate bobbins, similar to the high-isolation transformers that medical equipment uses. The case includes a ground wire in accordance with UL (Underwriter Laboratories) requirements.


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