![Current-Limit Section](/images/new-backgrounds/15349/15349145x1.webp)
Theory of Operation 4
Detailed Circuit Description
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Resistors R313 and R329 are current shunts for the high voltage supplies. These are connected in series with the bridge rectifiers, with the common point between the shunts being the high voltage supply common, HVCOM. The voltage developed across the +400V shunt is negative; the voltage across the
The shunt voltages, +400V IMON and
The outputs of U301A and U301B are open collector, which allows them to be wire- OR’ed with the open drain output of Q309. The output of this logic drives Q308, resetting the high voltage supply switches and signaling the processor via HVCLR. Transistor Q310 monitors the presence of the ±15V supplies. If either of these supplies fail, the high voltage supplies are switched off.
System Supply Section |
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The digital/relay power supply tap (+5V) is routed via J350 through RT350 and bridge rectifier CR351. A positive
The +5V digital supply powers both logic and relays. The power for each routes away from the +5V regulator separately, via lines +5V and +5VK. This prevents relay drive currents from causing drops in the logic supply voltage.
Because the ±20V supplies draw little current, they can share power transformer secondary windings with the ±15V supply with little efficiency lost. This
An accurate +15V supply is obtained by using U351. Regulator U352 regulates the
An accurate +20V supply is obtained by regulating with U353, a 5V regulator, but referencing it to the accurate +15V supply. VR351 forces U353 to be forward biased as the +15V supply comes into regulation. This avoids powering up the +20V supply through CR359 (at the improper voltage), reverse biasing U353.
A discrete regulator makes the negative