Thanks all for the wonderful input and confirming cathode current limiters
is a valid replacement for passive anode current limiting, even though the
latter is far more common. Despite the active cathode current limiters
requiring more parts and cost, the benefits are worth it for the projects
If you want to make this rock-solid, the pulldown resistor can be
calculated knowing the max tristate leakage current (usually 10uA) and
ensuring that 10uA of leakage does not produce enough base-current to turn
on the transistor.
Worst-case, you will have a transistor with infinite
One other thought Allen...
What I believe is a simulation schematic shows 1 meg pull-downs on the base
lines. These may not be necessary depending on other elements of your
circuit in the real world.
My current design uses 595 shift registers and the outputs are high-Z when
disabled (Output
Individual cathode current limiters work great... Stole the idea from
Greg's B7971 design. Note the transistors and current set resistors lining
the display in the photo below. First light today on this Sperry SP-151
Panaplex.
In this case, I'm using individual cathode limiting due to widely
No difference; current is current, and that is what determines how bright
the glow will be. However, the circuits you posted are quite different. The
anode-resistor provides current-limiting, but no current regulation: If the
supply voltage or the tube's voltage drop change, so will the anode
