26 Aug
2014
26 Aug
'14
7:45 a.m.
On 08/25/2014 1:14 PM, Chuck Guzis wrote:
On 08/25/2014 12:33 PM, Marc Verdiell wrote:
This was called a "Keep-Alive", and not only does it protect the drive
transistor this practice also increased the life of the lamp by
pre-heating the filament so it lasted far longer than it would have
otherwise. The thermal shock of being turned on from the cold state is
the time when filaments normally fail - we've all seen that - turn a
house light on and it glows extra bright and pops open...
John :-#)#
I use 10 Ohm 10 or 25W resistors, 2 or more of
them in parallel,
depending how much current you want to draw. They look like white
rectangular bars. They have them on the shelf at my Fry's electronics
store. For what it's worth, on the latest power supply I tried, just
one resistor was not enough to start the supply, I needed 2 to draw
at least 1A from the 5V and then it was OK.
A wirewound power resistor in series with an incandescent lamp is
perhaps a better solution than the incandescent alone, particularly if
you're loading close to the overcurrent limit of the PSU.
An incandescent (tungsten) lamp can have a hot-to-cold resistance
change of 10 to 1 or more. When incandescents were used for panel
indicators and such, one technique was to add a resistor to keep the
lamp faintly glowing "warm" to avoid the shock on the driving transistor.
Globar resistors are probably the best loads. Negative temperature
coefficient, non-inductive; gives a nice "soft start"
characteristic--and used extensively as RF dummy loads. Sandvik
Kanthal seems to be big vendor of these nowadays.
--Chuck