Typically, rectifier diodes of the so-called button diode type are employed in automotive and other vehicle battery chargers. It is standard practice to seat a pair of such diodes in pockets of a diode holder (or housing) with the cathodes of the diodes exposed and the anodes engaging brass tab terminals connected to an AC source. The diode holder is conventionally riveted to a conductive plate (referred to as a heatsink plate) with the diode cathodes pressed in firm contact therewith. The plate conducts the DC output of the diodes as desired, for example, to the bolt of a circuit beaker.

A button rectifier diode is basically a semiconductor chip bonded between two nickel-plated copper heat sinks with an encapsulating material of thermal-setting silicone. Button rectifiers commonly employed in automotive battery chargers include 50 amp rectifier diodes, such as type AR 5002 from Electronics Industry (USA) Co. (d/b/a EIC Semiconductor) and SR 4355 from Motorola, Inc. (Semiconductor Products Division).

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Note added at 27 mins (2016-08-11 14:36:33 GMT)
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Техническая характеристика генератора Г- 221 - Устройство и ...
www.transpostand.ru/rantas-675-1.html
35. Изолирующая пластина;. 36. Провод фазного вывода обмотки статора;. 37. Положительный диод;. 38. Держатель положительных диодов;. 39.

изолирован?

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Note added at 30 mins (2016-08-11 14:40:06 GMT)
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Testing rotating rectifiers with an ohmmeter - page 15-16
Rotating rectifiers (diodes) are illustrated in Figures 2 and 3. Compare the forward and reverse resistance of each diode. If one side of each diode is disconnected, a defective diode will be very easily detected.
Each diode should show high resistance in one direction and a low resistance in the opposite direction.

The diodes may also be tested without disconnecting them from the heat sinks or the exciter armature. The generator field lead wire, however, must be disconnected from the (+) and (-) terminals on the heat sink. The normal resistance of a surge suppressor is high enough so it can be left in the circuit. When testing diodes that are connected to the heat sink and exciter armature, a single shorted diode will give the same indication as if all diodes on the heat sink were shorted. An open diode will not, however, give a very obvious indication since current can flow through diodes in the circuit that are in good condition and give a comparatively low resistance. Therefore, when testing for an open diode without disconnecting the diodes from the heat sink, watch carefully to see whether the meter stabilizes immediately or whether there is a short delay in its swing, possible only a fraction of a second, due to the inductance in the armature winding. It may be helpful to compare all three diodes on the heat sink and look for small differences among them in repeatability of response of the meter.
http://www.emersonindustrial.com/en-en/documentcenter/electr...