Germanium vs Silicon Transistor interchange

rstehle

rstehle

Active member
I need some info from one of our experts. Can an old germanium transistor (circa 1967), such as may have been used in an auto ignition circuit, be replaced with a modern day Silicon transistor with similar specs? Thanks in advance for your input.
 
rstehle said:
I need some info from one of our experts. Can an old germanium transistor (circa 1967), such as may have been used in an auto ignition circuit, be replaced with a modern day Silicon transistor with similar specs? Thanks in advance for your input.
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Yes, but ...
The bias requirements will be different and need to be modified. Silicon transistors require around .7 V between base and emitter to turn it on while germanium require around half of that - if I remember correctly.
If your old circuit used a pair of germaniums then I would recommend a pair of silicon transistors, or even mosfets - depending on the difficulty of the circuit modifications.

Is that what you are doing, modifying an old ignition system?
 
Another cautionary thought, be aware of the type of transistor, often the "Old" Germanium transistors were "PNP" types wheras "most", although not all, of the modern ones are of the "NPN" configuration.
 
LightUp said:
Yes, but ...
The bias requirements will be different and need to be modified. Silicon transistors require around .7 V between base and emitter to turn it on while germanium require around half of that - if I remember correctly.
If your old circuit used a pair of germaniums then I would recommend a pair of silicon transistors, or even mosfets - depending on the difficulty of the circuit modifications.

Is that what you are doing, modifying an old ignition system?
Click to expand...

One of the members of a classic car forum I belong to is working on one, and the question came up.
 
rstehle said:
One of the members of a classic car forum I belong to is working on one, and the question came up.
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Ahhh.... yes CD ignition systems.
I had a homebuilt Cd ignition using the original ignition coil - back in the 70's. I had also added 2 toggle switches to revert to the original configuration. After leaving home and halfway to work on the busy highway my CD ignition died. Flicked the 2 toggle switches and I was good to continue to work. I stopped playing with the idea then when I couldn't keep the reliability up. I was using a regular small power transformers (120VAC - 12VAC) instead of a proper switching transformer.

I still have the Radio Electronic magazine articles on CD ignitions and can look there to see what was used - if needed.
 
Silicon transistor need .65v bias between base and emitter. Germanium is .35v. If it is only shorting to ground then yes, it should work. If it's part of an amp then the design of the amp will need mods.

And wow - I cannot remember the last time I saw a Ge transistor!
 
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