Quote:
Originally Posted by polyphase
I have an extra unit like the one shown in my original post, and I have parts for a few more. Send a PM if any interest.
If anyone is or is thinking of building one of these, the value of the 30ohm 5W resistor has been changed to 20ohms 5W. 30ohm is fine, but it was running hot and I didn't have room for a higher power one in the can. It seems the only purpose of this resistor was to limit charging currents through the original selenium rectifiers. With silicon diodes, it is unnecessary, but I'm more comfortable having something there.
Incidently, the reactance of the 10uf capacitor in this design limits current to less than .5A per string, even under short circuit conditions. Adding fuses or thermistors in this circuit would be pointless.
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The reason for adding a fuse is to account for the eventual possibility of a substantial shunting of the reactive element by an unexpected resistive (leakage) element, which could change the impedance vector substantially from the desired angle of approximately 45 degrees and allow considerably more than 0.5A to flow in such a fault-compromised circuit. Replacing the original selenium rectifiers which had a forward voltage drop on the order of 5V with 1N4007 silicon diodes having a forward drop of only 0.7V and the fact that modern household power specifications are 120V instead of the 115V of 60 years ago will mean that the B+ could be as much as 15V above OEM specifications with the original 37-ohm resistor and almost 30V too high with that element of the Ballast omitted. The design we have been testing and are in the process of miniaturizing as much as possible to fit inside a container of size comparable to the Motorola original Ballast Tube was specifically designed to address such a "worst-case" scenario as an eventual component failure inside the Ballast itself.