[Amps] Solid State Relays Again

[KA4INM]

On 02/09/14 22:13, Jim W7RY wrote:

> I just had a solid state relay in my 8877 power supply short during the
> RTTY contest this weekend.
> I use 2 relays, one to short out the step start resistor, and the other
> one for the other leg of the plate transformer.  Both are controlled by
> the WD7S triode board.

> The one that shorted is was NOT the step start resistor shorting relay.

> They are both Crydom D2440. They have been around a while because the
> defective one has a date code of 9425.

> Just trying to do some research and thinking of switching to mechanical
> relays.
> I looked at the archives and read a debate between Ian and Paul about
> this exact subject. Any new thoughts?

   There are two kinds of SS contactors (relays) for resistive loads and 
the other for inductive loads, these switch only at the incoming power's 
zero Voltage crossing point, obviously the latter is what you need in 
this application.
The step start contactor (more heavily heat sunk) should bypass the 
initial contactor (and the resistor) and the initial contactor can be 
relaxed and do nothing.
   Neither application should be very hard on a SS contactor and they 
have a long life history as long as the correct type is used.

   SS contactors can easily be home made, using two crossed SCRs with a 
resistor between the two gates in series with a switch to disconnect 
them for off.  The switch needs to be a latch that is SET by an `AND 
gate' comparing the incoming sine wave with zero volts.  (both the same 
only at zero crossing)

   The advantage of SS contactors are much less noise (never any buzz) 
and an instant disconnect if the protection circuit detects too much 
screen current or a sharp increase in (negative) power supply current
(an arc) surely make them well worth bothering with.
-- 
   Ron  KA4INM - Youvan's corollary:
                 Every action results in unwanted side effects.