Strange replacement for NiCds with caps and zeners (HP 3420B)

[perdrix]

I just removed a couple of perf board circuits from inside an HP 3420B.

The intention I am sure was to use these to control the voltages fed to the circuits after the NiCd batteries had been removed.

The 24V supply has a 2200uF cap, a 22k resistor and a 1N5359 24V 5W zener all in parallel

The floating 12-0-12 supply has a 2200uF cap between the +/- 12V rails, and then a 12k resistor in parallel with a 1S3012A 1W zener between each rail and 0V.

Now it has to be said that the drive circuits was designed to charge the battery packs so will be high impedance or current sources.  Still I'm puzzled.

Did whoever fitted these get confused and fit the resistors in parallel with the zeners when they should be in series, or it this correct?

Thanks
Dave

[bitseeker]

I'm not sure what the original intent of the add-on circuits was, but those sound like bleed resistors to empty the caps after the power is cut. Is the 24V supply unregulated, relying on the battery to smooth the power, hence the zener diodes?

[perdrix]

Both the 12-0-12 and 24V circuits are unregulated battery charging circuits (current source).

Dave

[edpalmer42]

If nicds were removed, the voltage to any downstream circuitry would rise, possibly to a level that would result in problems due to the voltage rating of the components and/or power dissipation.  The zener would clamp the source to an acceptable level at the expense of dissipating perhaps a significant amount of power.

The resistor sounds like a bleed resistor, but I don't really see the point.  It's unlikely that it was supposed to be in series.  22K in series would limit the current to about 1 ma.

Ed

[bitseeker]

Both the 12-0-12 and 24V circuits are unregulated battery charging circuits (current source).

Thanks for the confirmation. Yep, so bleed resistor, filtering cap and rudimentary voltage regulation (zener diode). As Ed said, the bleed resistor is odd. Usually you'd put that in for draining high voltage caps in a power supply.