Exide Technologies JA5009-00 - Manual (Page 12)

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e. Rectifier Diodes (SCR1/SCR2) - (also see Section III, 2, f)
1. These diodes are a part of an SCR-diode module(s) in which the SCRs and diodes are contained in
a module(s) mounted by an electrically insulated heat sink plate. Three connections (plus one SCR gate
terminal) are available for checking. The diode portions should be checked by disconnecting one terminal
and measuring their forward & reverse resistance, in either of two ways. In using an ohmmeter, use the R x 1
scale for the forward resistance and this should show approximately 5 to 10 Ohms. The reverse resistance
should be up in the thousands of ohms (probably 50,000 or more) measured with a higher scale of the meter.
The diodes may also be checked by connecting them in series with a 6 or 12 Volt lamp across a DC source
of the same voltage. The lamp must require at least one-quarter Ampere. The lamp should light with nearly
full brilliance with current flowing in the forward direction and not at all in reverse. If it lights in both
directions, the diode is shorted. If in neither direction, the diode is open.
2. Note that semiconductors usually short in pairs in bridge circuits, seldom as single units, and it is
rare that all four semiconductors in a bridge are found defective. When diodes fail it usually is because of
surge voltages. Therefore, surge suppressors (SS1/SS2) should also be checked to determine that they are
operative. Refer to Section III, 2, d on page 7.
f. SCR Diodes (SCR1/SCR2) - (also see Section III, 2, e)
1. These devices are part of the SCR-diode module(s) described in Section III, 2, e above. Power
regulating devices SCR1 and SCR2 are silicon-controlled rectifiers, which cannot be checked using the
same method as used with rectifier diodes by forward and reverse resistance checks, since the SCR will
always show a high resistance until triggered.
2. The operation of the SCR can be checked with an oscilloscope. The gate-firing voltage signal is
produced by the control module assembly (A1). The oscilloscope should be GROUND ISOLATED for these
tests. This is normally done by using a line isolation transformer in which the secondary that powers the
oscilloscope is UNGROUNDED. Alternatively a battery powered portable oscilloscope can be used. The
gate signal may be checked on the control module (see Section III, 2, g) or where the twisted-pair trigger
leads terminate on the SCR-diode module. The same signal should appear at both points. Absence of the
signal indicates that the control module is defective, not the SCR.
3. If the charger AC input breaker trips immediately, and a shorted SCR is suspected, a simple check
with an ohmmeter can be made. Turn off CB1 and CB2. Connect to the anode and to the cathode and adjust
to the direct reading scale of the ohmmeter. If a low resistance is observed, reverse the leads and again check
the resistance. If this reading is also low, the SCR is shorted and should be replaced.
4. An SCR can also be checked for operation with a Simpson Model 260 voltohmmeter. With the
black lead in common and red lead in (+), put polarity switch to +DC. Connect the red lead to the anode and
the black lead to the cathode. The meter should now indicate high resistance above 50,000 Ohms (when on
the R x 10,000 scale). With the leads connected as above, set the function switch to R x 1 and touch the gate
to the anode. This should fire the SCR and give a reading of approximately 5-20 Ohms. This shows the SCR
has been turned on. On very small SCRs this reading will hold after removing the gate lead. This is latched-
on and can be unlatched by opening the cathode lead. Larger SCRs will not stay on with the current
available with Simpson meter. The Simpson may not have enough current to gate or turn on extremely large
SCRs (400 Ampere and up).
5. If the charger output is too high, unplug the control module and turn on the charger. With no gate
signals the charger should have zero output. If there is still current output, one or both SCRs are defective.
6. The above checks can be used to confirm that a suspected SCR is indeed bad. However,
occasionally an SCR might check OK in all these tests and still break down or fail in the charger circuit
during normal operation. Any SCRs suspected should be replaced.