Not everything in life needs fast start! Some things in life
are meant to have a slow start, like the tube filament! The inrush current
surge is one of tube killers!
This solution mentioned here are for DC filament only where
one is using the very common 3-pin regulator, LM317 and the likes, to power the
tube filament.
 |
| LM317HV regulator |
Picture above shows a normal / fast start LM317 regulator.
As the reference voltage is available almost instantly, the output is
programmed to the preset (R1/R2) value swiftly.
 |
| LM117 with slow start |
In this slow start configuration above, the inclusion of an extra
transistor (2N2905), R3 and C1 makes it sure that the output of the circuit
turns ON gradually after an input supply is applied. The output switch on
ramp-up will depend on the value of R3 and C1. Increasing the values will
produce higher time delays and vice versa.
Almost all small transistors
will work in this circuit. BC327 (pnp) was tried for LM317 and BC337 (npn) for
LM337. For LM337, use npn transistor and turn all the electrolytic, instead of
pnp transistor for LM317.
What if I bought readily made
modules without slow-start? Shown below is a standard module sold by some vendors online. It has
to be modified slightly to have the slow-start function.
R3 of 50k and pnp + npn
transistors need to be added. D7, D8, C5 and C6 pins connecting to ADJ pins
have to be disconnected and re-connect to gate pin of transistor. R3 should be
added as per the original slow-start schematic.
It’s not difficult to modify but
the ROI is very high.
 |
| LM317 regulator without slow start |
 |
| LM317 regulator modified with slow start |
J&K Audio Design
30/12/2013
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30/12/2013
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