Re: [tesla] Soft transformer turn on without a variac

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <Mddeming-at-aol-dot-com>

In a message dated 7/24/03 10:17:17 AM Eastern Daylight Time, 
tesla-at-pupman-dot-com writes:




> > , I need to have a single switch for turn on preferably without a Variac
> > to keep things simple for the staff.  The present system has solid state
> > relays but is complicated and unreliable. I need to turn on the motor first
> > then a couple of seconds later soft turn on the power to the transformer.
> > Even switching on power to a normal Variac gives a spike that will trip my
> > lower current breakers. The whole system will have to be electrically
> > quiet.  Does anyone have experience with inrush limiters or have other
> > suggestions?
> >
> > Cheers
> > Peter (Tesla Downunder)


Hi Peter,

        I seem to remember a couple of tricks we used back about 3 years 
before The Flood to ramp up power with. A pair of very heavy-current VT 
diodes were connected in parallel but reversed to each other. It took about 
15 sec for the filaments to come up to temp, which provided a nice slow 
ramping of the output power. VTs are analog devices and intrinsically slow, 
which is what you want, while solid-state devices are essentially 2-state 
with rise times many orders of magnitude too fast. This method did put some 
stress on the tubes, but had the advantage of no moving parts.  Another 
technique was to use a pair of triodes hooked up as above, but biased to 
cutoff. A small actuator motor, like the type used on retractable 
automobile antennas, was used to drive the bias pots, providing the 
necessary ramp-up over quite a few seconds. It is very difficult to get 
things to happen gradually with SS devices.
        I don't know if these ideas are directly applicable to your needs, 
but they may jog the memories of other oldsters** to respond with something 
you can use.

Matt D.
**People older than nano-second devices, MTV, etc. ;-)))