"john" <x@[EMAIL PROTECTED]
> wrote
> Aristo also has a device to allow the use of digital timers [with a
T-12].
> Is it necessary with the F-Stop timer?
It shouldn't be.
There are two technologies timers use to control
the lamp: electro-mechanical relays and solid-state
silicon controlled rectifiers (SCRs).
Some electronic timers with relays get upset by the
electrical interference generated when the contacts
open with an inductive load. Current through an
inductor wants to keep flowing and can't be stopped
instantly. When relay contacts open the current
creates a small arc across the contacts. A 'snubber
circuit' across the contacts provides a path for this
current and limits the arc. The f-Stop Timer uses
a snubbed relay rated at 12 amps.
Since SCRs only turn off when the current goes
to zero they have no trouble turning off inductive
loads. OTOH, they can have problems with incandescent
lamps. When a light bulb fails it often creates
a dead short - the bright flash/pop - and the current
through the short can destroy an SCR that is not
adequately protected. They are also prey to
high-voltage surges on the power lines. Snubber
circuits are used with SCR's, but this time to
absorb power line spikes when the SCR is off.
Timers that are properly designed and used will
work without problems using either relays or
SCRs.
High-power loads connected to a timer can require
an external contactor to handle the current. Regular
Aristo 4x5 heads are no problem for most any timer,
including the Darkroom Automation f-Stop Timer.
I don't know the inductive characteristics of the T-12
and don't have any experience with this unit. My guess
is it will work, but there is a chance the external
'solid state contactor' may be needed. In any case,
plugging the head in and seeing if it works will cause
no damage to the timer.
--
Nicholas O. Lindan, Cleveland, Ohio
Darkroom Automation: F-Stop Timers, Enlarging Meters
http://www.darkroomautomation.com/index2.htm
n o lindan at ix dot netcom dot com
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