Ecler PAM2000, Service Manual

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5
Q123, together with the resistances which make the base divider, configure the current detector (this
divider takes its voltage from one of the source resistances of a N MOSFET, which is proportional to the current
circulating through itself).
The threshold separating zone A from zone B is determinated by the D125 zener. When this zener stops
working and there is no current circulating through it because the Vds voltage is lower (let's remember this circuit
is also in parallel with this voltage) or, what is the same, the load voltage grows because it is not 0
O
anymore and
has a given value, like 0.5 Ω to 1 Ω , and the help given by D126 stops so more current will be needed for the shot.
We have climbed the first stair of the stairway of the SOA graphic.
When the zeners D124 and D118 stop working because the load voltage goes on growing (values higher
than 1
Ω
) or -what is the same- the Vds decreases, the Q125 transistor does not receive current anymore in its
base and so it is shorted, allowing Q124 to enter conduction. This way R172 stays in parallel with the base-emitter
of Q121, making up a voltage divider with R173. This divider will climb another stair of the stairway and enter the
ZONE C.
The link between the module's protection circuit and the relays' control circuit is made through IC103 and
IC104 which are, as mentioned earlier, opto-couplers, just to insulate the existing high voltages at the power
amplifying module,
" 63V in the case of the PAM2600, and the power supply voltage of the existing logic circuits in
the relays' control card.
Once the pulse generated by the protections is detected, the control circuitry resident in the
protection
card, appart from opening the corresponding relay, returns the signal A.O. SUPPLY CONTROL to the module,
which cuts by means of Q119, Q120 and IC102 the operational's power supply.
This is the way to insure a fast and safe cut of the Id current in the MOSFETs (around 80
µ
s time),
because they stop receiving their respective reference voltages and, consequently, their Vgs polarization voltages
so they are cut. The circuit is shown in figure 9 and its operation is very simple.
When the A.O. SUPPLY CONTROL (+10V) signal appears, the Q119 transistor starts conducting,
shortcircuiting to ground the positive power supply of the operational. On the other hand, the signal is also applied
to the IC102's LED (opto TIL112 (4N35)), which puts its internal transistor and Q120 into conduction, connecting
the negative power supply of the operational to ground.
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