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Zeners in series...

Hey there, could somebody take a quick look at this circuit and try to explain me what's the purpose of the 2 zeners near the interrogative point ? Here's the schematic:

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If they are working as voltage stabilizer, shouldn't there be a resistor between them and the rectifier?? To be honest those 2 zeners is what I've found on the circuit but.... the parts list say that D615 and D614 should be a 120V zener and a 31V 100pF varactor diode (1S3150A)... this makes no sense to me :S

Reply to
Nox
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t ?

Maybe they found two zeners that would split the voltage fine, which would be cheaper, and forgot to update the parts list.

Reply to
Sansui Samari

A varactor diode there makes even less sense. They probably needed a 270 V zener as protection against excessive voltage at that location and there was no single zener that was 270 and adequate power. C603 and the other component upstream of it limit the current.

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Reply to
Samuel M. Goldwasser

Looks like a 270V clamp to me.

Reply to
WangoTango

It appears to me as if they are, as you say, working as a voltage stabilizer / regulator.

There is a resistor present, to limit the current flowing through them and through the rectifier. If you trace out the inputs to the rectifier bridge, you'll see that one side goes back through connection 15 and then to the "0V" side of the lowest secondary winding. The connection to the other input goes to the heavy horizontal bus (which appears to serve as a ground reference), and this bus is connected to the "850V" end of the high-voltage secondary winding through R47 (which is a 3.3M resistor).

So, the winding develops a high AC voltage, which is fed to the bridge in a current-limited fashion (via R47), rectified to create high-voltage DC, and the DC voltage is clipped/limited by the series-connected zeners.

A 31-volt varactor? DOesn't make much sense to me either.

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Reply to
Dave Platt

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point ?

70 V
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If the little blobs at crossing points are to be trusted, C603 isn't in series there, and it doesn't limit the current. Winding resistance and rectifier series resistance are all I see on this winding leg.

It appears that the whole rectifier/zener assembly is there to limit AC excursions of that leg of the transformer winding (the OTHER leg of the transformer winding has the high impedance).

Reply to
whit3rd

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270 V
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her

The two zeners in series are there to protect TR607 and C604 in case the HV regulator circuit fails (or line voltage is excessive) and TR607 goes into cutoff where HV DC would appear across its C-E terminals. They clamp the DC to 270V. Two diodes were probably used since a single 270V device is harder to find or to split up the power dissipation. Under normal operation, they do not carry any current. TR607 is probably a 300-400V device.

The high voltage for the CRT cathode (-1kV) is sensed through R631 and fed to error amplifier TR608. TR608 then controls TR607 inside the DC loop of bridge rectifier MR3. This allows the transistor to control (and regulate) the AC voltage delivered to rectifier strings D616-D618 and D621-623. The +3kV is then cross-regulated since it derived from the same AC voltage as the -1kV. This allows brightness and focus to be stable under varying AC line voltage.

-Scott

Reply to
swohlfarth

ha scritto nel messaggio news: snipped-for-privacy@z11g2000yqz.googlegroups.com...

Thanks everyone for the kind explanations! :D

Reply to
Nox

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