Hi again Steve.

Better late than never... [Linked Image]

Well, Dspark covered the reasons for using Hot Cathode emission rather well, and the primary reasons for the heater windings in the ballast.

The older T12 Fluorescent lamps with Bipins will be continually heated [the cathodes, that is] during startup and during operation.
The Heater Windings found in the Ballast supply the lamp ends with around 1.0 amps at around 2.5 to 4.0 VAC. The current which the lamp uses to create light, flows through the Heater Windings without any Impedance [minimal Impedance], so during lamp operation, there's actually two separate currents flowing out the ends of a lamp.

Take notice to the series connection between the two lamps, via the secondary heater winding's taps [the yellow circuitry]. That's why when one lamp is removed, the other quits operation and just flickers. The flickering is due to some small lamp currents being able to flow into the secondary heater winding, or through Capacitive Coupling to the grounded conductor / ballast inputs.

As Dspark said, the Hot Cathode method is most efficient for Rapid Start Fluorescent lamps, operating at low frequencies [less than 1 KHz].
These lamps will produce/consume very near their rated wattage [40 watt lamp will come close to consuming 40 watts and producing a resultant light output to that 40 watts].

The T8 lamps are Cold Cathode Instastart lamps, connected in parallel [most of the time]. The frequency is close to 40KHz, so they run a little more efficiently than if at 60Hz.
They still only consume/produce 90% of their ratings [32 watt lamp will consume maybe 28 to 30 watts max, and produce light accordingly to the consumed power].
Their greatest advantages are no hot cathodes [no wasted power, longer cathode service life] and parallel connections [1 lamp fails, the rest can continue to operate].

To overcome the "dead" starting problem of cold cathodes, a very high starting, or open circuit voltage is used. It's like 2 times the OCV of the hot cathode emission lamps [750 VAC as compared to 300-450 VAC].

To answer your question regarding the HO / VHO lamps, this is the Ballast circuitry for the T12 size HO / VHO lamps.
The only difference is they draw 800 milliamps per each HO lamp, or 1500 milliamps per each VHO lamp, as opposed to 460 milliamps for a standard T12 lamp [such as F40T12CW].

Newer HO / VHO setups use the high Hz cold cathode electronic ballast circuitry, but they would be T8 sized lamps too [I believe...]

Let me know if this answered your Q's.

Scott SET


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!