I am looking for the schematic to an electronic circuit to meet the
following criteria:

General Description:
This circuit is to be used as a go between a PLC with a relay or TTL
output and a phase angle Solid State Relay that is controlled by a
4-20ma input signal. The game plan is that every time this circuit is
true, the output will be a variable resistance. That is shunted across
the leads of the 4-20 ma input of the SSR. Over time the resistance
will increase, hence increasing the current across the 4-20ma input.
This will simulate a soft start effect on the load side of the SSR.
Input: switch closure or voltage signal 	
Output : Variance in resistance

It looks like an n-channel FET (Field Effect Transistor) could serve
as the shunt resistor. Connect the transistor gate to a series RC
network, using a potentiometer for the resistor, for fine tuning.
Also, put a large shunt resistor across the capacitor so that it
unloads between activations. The FET will probably require some
current limiting.

If I understand correctly, your solid state relay incorporates current
controlled phase switching, 4 to 20 ma corresponding to 0 to 180
degrees?

FET resistance is a very indeterminate thing, highly non-linear and
not really suitable as an open ended variable current dump. Better to
make an op-amp based current sink or source ramp circuit -- what ramp
time do you require? Not difficult but can?t post diagrams on
?answers? I?m afraid.

Best

I require a ramp time 0.75-1.0  seconds

Without going into a lot of complicated circuit building (which is
difficult to draw in text only format), you might try placing a
'resistor-electrolytic capacitor' series connected branch as a shunt
across the load and ground, at the input of the SSR. Initially, there
will be a large current diversion into this branch, until the
capacitor completely charges, at which time all the current will go to
the SSR. Try 250 ohms for the resistor, and 1000 uF for the capacitor.
If these values give you approximately what you are looking for, then
adjust the resistor until you get what you want. Also, place a 10 Meg
resistor in parallel with the capacitor only, with one end connected
to ground. This is to discharge the capacitor between activation
periods. Observe the polarity of the bipolar capacitor, with - side to
ground.

This is the 'crow bar' approach to your problem ... not elegant, but
easily achieved.

Bit busy at moment -- will get back to it ASAP. But can you confirm
the current / phase relationship of the SS relay, or better still post
the device number. Also, just in case, what kind of load do you intend
driving?

Best

...and do you want a linear ramp?

BTW dave73, ?crow-bar? is a specific term in electronics -- though
perhaps you know this.

Best

A linear Ramp is preferred, but not required.
Thanks

The term 'crow bar' was probably not the best descriptive term to use. 
In retrospect, I think I would have somehow employed the phrase 'shock
absorber' to convey the general concept....

If somebody else has a way to to this, let's see it.

Really need device specs to advise more, or component type so I can
look it up. But do you already have the SSR? There are a few which
accept 4-20ma for phase control but also voltage control (easier) or
potentiometer. Lot of them feature configurable ?soft-start?, and
would this not be just what you want without further frigging? All
you?d need to do is enable it with the PLC and it would run up to
phase. Fill us in.

Hey dave73, feel free to pull me up too :-) I have a penchant for
accidental ?crow-barring? with expensive chips. Smokin!


Best