Paul, you are on the right track with this.
Some further experimentation may be beneficial now that the false triggering mechanism is understood.
I'm no electronics technician or engineer so all the chips are stacked against me really, all I have is an inquisitive and sometimes
logical mind, a digital multimeter, lots of patience and tutorials on line which I am currently trawling through to get an understand-
ding of what I'm playing around with here.
What I can do at this point is present the facts based on observations with very limited equipment and experience to move forward
with, so with that in mind, this is the situation thus far:
If the 'Gate' of the SCR is connected to R3 when an instantaneous 12v is applied, the SCR fires. If the Gate is NOT connected to R3
but the Anode and Cathode is still connected to Vcc and Vee respectively then it does not fire. This tells me that applying 12v insta-
ntaneously to the Anode/cathode is not causing this firing, the problem has to reside with the Gate itself.
With the Gate disconnected from R3 and a multimeter connected to Vee and Pin 8 of the MC3423, the multimeter does not seem to
register any kind of voltage spike on Pin 8 when applying 12v to the circuit which might be enough to fire the SCR's Gate, instead
the Pin 8 output seems to creep up to between 40-50mV and hover around there. Obviously this 40-50mV will not be enough to fire
the Gate in itself.
With the circuit running below Vtrip threshold (connecting the SCR's gate AFTER the power has been applied) the extra 100 ohm resistor
between Gate and Vee takes the 40-50mV leak from Pin 8 towards zero so I see what this component is doing.
The fact that Pin 8 on the IC is NOT showing evidence of a voltage spike on 'power up' on the multimeter (I wonder if an oscilloscope would
tell a different story ???) also tells me that the 'power up' should not be causing a momentary problem with the voltage divider part of the circuit
which might activate Pin 8 momentarily......Although !
Considering the equation in the MC3423's datasheet:
The Vref figure of 2.6v - I also wondered if when the IC first powers up that this 2.6 reference voltage used in the formula might take
'X' amount of time to stabilize and may be lower for a certain amount of time. The resistance of R1 and R2 are effectively constants
whereas Vref may not be. With this I used the formula using 1.0v as Vref, with the R1 and R2 values I got around 4.5v which is not
enough to fire Pin 8 into action. The other side of the coin though, Vref COULD be higher momentarily before it stabilizes which might
open a whole new can of worms ! Unfortunately no one can put a probe on the little device inside the IC to really see what it is doing
when I suddenly hit it with 12v, I can only go by what the output Pin 8 tells me.
I don't really want to put a delay into the mix before the IC activates as it seems to destroy the whole purpose of this circuit in the first
place for my desired results although the datasheet does have suggested stuff for a delayed firing of the IC.
So here we are. It seems I need to concentrate the battle at the trench between IC Pin 8 and the dreaded gate. Again, im no technician or
engineer so this is where I need your help guy's.
I will continue this on a separate reply as this wont seem to allow me to upload another image
Cheers
Paul
- - - Updated - - -
Hello All again,
Consider the 'A' and 'B' positions marked either side of the 220R resistor in the following pic:
Options:
1. Connecting a capacitor from Vee to position 'A'
2. Connecting a capacitor from Vee to position 'B'
What effect would each position have on this link to the gate ! Lets use a grossly high value cap here so the effect is massively over exaggerated which
might make it easier for me to grasp.
Another option:
Disconnect the 100R resistor from the cathode and place the same cap between 100R and the cathode creating a resistor/capacitor bridge from the Gate
to the cathode (or Vee if you like)
This is a bit like a voltage divider I believe although not the same obviously, what effect would this have if no cap was to 'A' or 'B' ???
Sorry to be a pain guy's, I'm learning here....Honest !
Many Regards
Paul