27-01-2017, 11:34 AM
(27-01-2017, 10:32 AM)pwdrive Wrote: ... it is in part irrelevant, the temperature I mean.
It may be more relevant than you think, Lawrence.
Have you ever considered why incandescent light bulbs virtually always fail at switch on?
Quite simply, the cold resistance of the filament is much lower than it is at its normal working temperature, thus the inrush of current when cold is much higher than the bulb's rating might suggest. When the wire for the filament is manufactured there will always be some small part of it that is marginally thinner than the rest so, at switch on, the voltage drop - and current - will be higher than other sections with the same unit length. Thus there is a tendency for a minuscule portion of the wire to vaporise, rendering that part to get progressively thinner with use and increasing the unequal current distribution during every switch on cycle. Eventually the switch on surge is so high that the filament melts and you hear the characteristic 'ping' as the coiled filament springs apart.
A valve heater acts in the same way but, as the ratio between the cold and operating temperatures is much lower than with the white hot heat of a light bulb, the inrush current is much better controlled and open circuit valve heaters are very rare indeed. However, whilst this is fine when heaters are fed in parallel, thus controlling the current with the fixed voltage, the same is not true of series heater chains.
No two valves will exhibit identical switch on characteristics and without precautions, one valve would always receive the lion's share of the excess current. So a thermistor is added to the equation.
Having a high resistance when cold and a low one when hot it reduces the current flow at switch on to a very low level, gradually increasing as its temperature rises during which time the valve heaters are brought gradually up their nominal operating point without any excess current or voltage drops.
