Surge Suppressors; Scr Diodes - HindlePower SCR series Operating Instructions Manual

1. These diodes are a part of an SCR-diode module(s) in which the SCR's 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 and 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 be one which requires 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 and SS2 should also be checked to determine that they
are operative (refer to Section III, 2, e).
e. Surge Suppressors, SS1, SS2
1. The metal-oxide varistor (MOV) type of surge suppressor is used throughout these battery
chargers instead of the older selenium type. This is because of personnel safety. When a selenium device
fails and arcs or burns, very toxic selenium fumes are given off which are very poisonous, if inhaled. The
MOV-type have a much sharper limiting "knee" characteristic and perform a better protection function. If
an MOV surge suppressor fails during a high energy transient, it may explode. This is an obvious failure
and the part must be replaced.
2. If the suppressor is shorted, an ohmmeter check will indicate continuity. When the suppressor
shows an infinite resistance in both directions measured with it disconnected from the charger and has a
normal appearance it can be presumed to be in good operating condition.
f. SCR Diodes, SCR1, SCR2 (also see Section III, 2, d)
1. These devices are part of the SCR-diode module(s) described in Section III, 2, d 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, A1 Assembly. 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. (See Section III, 2, g).
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.
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