Generator Question

A generator can be flashed to work as positive or negatine ground. Tractor manufacturers were switching to negative ground somewhere around 1958.

Gerald is right on with the poop on the Transistor radio's. Intolerant is the magic word in that particular case. I remember reading many years ago about Transistor radio's and the need to run them Neg. Grd. and on 12V's. As Doug mentioned, Gennies can be flashed to work pos or neg ground. I've also built a few pos grd Alternators for customer's special applications. Once in a great while, I'll still do a 6V Alternator for one of our Antique Auto customers. Preferably for the Model A guys. It was a nice simple reliable upgrade for those types of old systems.
Steve@B&B

Actually transistor equipment could be designed around positive ground systems, it's just how they started doing it that pushed negative ground as the standard. Could it be because they designed the NPN transistor before they designed the PNP, or vice versa? I dunno. Vacuum tubes require the plate to have a positive charge, relative to the cathode, so I'd think negative ground would be easier to design around back in the good old days. I just go with what they do because that's what they do.

Edited by DougS - 12 Jul 2017 at 9:34am

Making all vehicles being consistent negative ground certainly makes jumping safer especially for drivers not noticing the ground difference. Jumping a positive ground vehicle with a negative ground vehicle and hooking ground to ground and none ground to none ground ends up shorting both batteries, likely melting the jumper cables with a kiloamp + current and causing a fire if the copper wires don't melt. A hazardous situation.

There have been high speed fuses that claimed to protect transistors, but in my experience they are only reliable indicators that the transistors have shorted from the reversed polarity.

PNP transistors take negative voltage on the collector, the output terminal of the transistor. The earliest transistors were germanium and PNP was predominant. The current gain in those early germanium transistors was highly variable and somewhat temperature sensitive. The collector to base leakage current was very temperature sensitive. Those properties made it a challenge to design a linear amplifier circuit (like for audio) to get the circuit parameters as predicted. But a transistor circuit using PNP or NPN transistors can be designed for either supply polarity, though the supply polarity opposite the transistor polarity requires thinking upside down. Silicon transistors were more predominantly NPN but also available in PNP for circuits that used both. Still intolerant of reversed polarity. Germanium transistors had lower voltage drop when saturated and for base bias that made them handier in low voltage applications.

Vacuum tubes require positive on the plate but are not damaged by reversed polarity. Many a vacuum tube car radio used a vibrator to convert the DC supply to AC to feed to a step up transformer and then used a rectifier tube to make DC again. Those radios worked the same with either battery polarity. Some two way radios used a more complex vibrator that accomplished the high voltage rectification as well as the AC generation. Many of those were built so they could be plugged into their socket in two positions, one position for each polarity.

Transistor radios became more popular for broadcast and two way because they were significantly more compact and more tolerant of shock and vibration that tended to break vacuum tubes. And because vacuum tube life was not infinite they were always in sockets for easy replacement. Transistors were also generally cheaper to purchase than vacuum tubes though a transistor radio might take half again as many active parts as a vacuum tube radio. A few transistor radios were made with switchable polarity, essentially internally made without circuit grounds, then if the power and ground were connected properly they tolerated the vehicule's polarity. If not connected correctly they fried instantly. So for jumper safety and survivability of radios (and incoming vehicular computers) SAE decreed the end of positive ground.

Its not the regulator that is affected by flashing. Its the residual magnetism of the field poles. And that residual is what causes the generator to build voltage when its turned and sets the polarity of the generator output. Fundamentally the works in the regulator work the same for either polarity, though some regulator contacts are not symmetrical and wear much faster when used for a different polarity than specified on the regulator. More modern transistor regulators are intolerant of wrong polarity, even my design for the steam turbine generator on the Chinese locomotive on the Boone and Scenic Valley Railroad.

When starting out with vacuum tube circuits, it definitely is easier to think negative ground for electronics than positive ground, but I have worked both ways. Some high power transmitters from Collins Radio (where I worked three years) used both positive and negative ground power for the power amplifier tubes.

Gerald J.