Retrovert
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Why not use the TRIAC mod elsewhere detailed on AK?
Eico HF-81 Transformer Help
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Why not use the TRIAC mod elsewhere detailed on AK?
Never heard of the TRIAC mod, can you send me the link to the detailed post or explain what that is?
I'll definitely keep this in mind when I work on my Eico later on, backed up on a few projects. I also remembered that you could also use a thermistor to do the same job, is this true? If so, would you do the TRIAC mod or use a thermistor? I learned about it from Blueglow:
Retrovert
Addicted Member
No, the thermistor is not equivalent, although you should consider adding one of those, as well.
Remember what an NTC thermistor does: it has a high initial resistance which decreases over time as the thermistor heats. So the thermistor slows the inrush current caused by cold filaments (essentially dead shorts) and, to some extent, initial capacitor or inductor charging (very much like dead shorts)—this process is commonly and consequently called a "soft-start"—but at turn-on the switch nonetheless carries a high current. As the contacts make and break—at a microlevel the contacts bounce open and close whenever the switch is moved—microarcs form, pitting and damaging the surface. That additional roughness worsens the bounce.
Plus the switch still suffers from inductive flyback as the transformer primary flux collapses leading to high dv/dt.
The TRIAC modification removes the bulk of the arcing damage by instead using the switch only to turn a TRIAC on and off, an operation which requires very low current, typically about tens milliAmps. The TRIAC thus carries the current for the equipment, and, being solid state, it is essentially immune from the effects.
NB: High-voltage arcing from flyback may, of course, damage the TRIAC just as any semiconductor junction may be damaged by excessive voltage. A snubber or TVS diode nicely resolves this issue. An MOV is the wrong tool for this, or, indeed, any protection task. We now have far better solutions which do not short out and start fires.
The turn-on current is thus sufficiently low that the contacts do not suffer the same amount of arcing damage.
An RC snubber or TVS diode may be additionally added to reduce the arcing as the switch contacts make-and-break at turn-on and turnoff, even though this is much smaller.
So the TRIAC does not identically function to a thermistor.
Remember what an NTC thermistor does: it has a high initial resistance which decreases over time as the thermistor heats. So the thermistor slows the inrush current caused by cold filaments (essentially dead shorts) and, to some extent, initial capacitor or inductor charging (very much like dead shorts)—this process is commonly and consequently called a "soft-start"—but at turn-on the switch nonetheless carries a high current. As the contacts make and break—at a microlevel the contacts bounce open and close whenever the switch is moved—microarcs form, pitting and damaging the surface. That additional roughness worsens the bounce.
Plus the switch still suffers from inductive flyback as the transformer primary flux collapses leading to high dv/dt.
The TRIAC modification removes the bulk of the arcing damage by instead using the switch only to turn a TRIAC on and off, an operation which requires very low current, typically about tens milliAmps. The TRIAC thus carries the current for the equipment, and, being solid state, it is essentially immune from the effects.
NB: High-voltage arcing from flyback may, of course, damage the TRIAC just as any semiconductor junction may be damaged by excessive voltage. A snubber or TVS diode nicely resolves this issue. An MOV is the wrong tool for this, or, indeed, any protection task. We now have far better solutions which do not short out and start fires.
The turn-on current is thus sufficiently low that the contacts do not suffer the same amount of arcing damage.
An RC snubber or TVS diode may be additionally added to reduce the arcing as the switch contacts make-and-break at turn-on and turnoff, even though this is much smaller.
So the TRIAC does not identically function to a thermistor.