P
Paul E. Schoen
I am testing my smart SCR firing circuit with a simple transformer load and
a lamp on its output. The test circuit is about as follows, except I have a
50 VA isolation transformer supplying the ACHI and ACLO. (My first ASCII
diagram!)
CB
ACHI--[1A]---- SCR1 >|--------|
| | | |-------
--|< SCR2 ---- )|( |
120V )|( 16V O Lamp 28V
)|( |
ACLO--------------------------| |-------
I was able to program the PIC to turn the SCRs on and off, adjust phase
angle to dim the lamp, and check for primary voltage across the SCRs before
allowing initiation.
Then, I wanted to check for an open gate, which I figured would cause a
large DC current in one direction, and would probably trip the 1 amp
circuit breaker. I assumed I would read a large voltage across the SCR that
was not triggered, and that I could use this to cause the firing circuit to
shut down.
Much to my surprise, when I removed either of the gate wires, the circuit
seemed to perform almost the same as when both were connected. I did read
about 2 VDC across the SCRs, and the AC voltage (at about 50% modulation)
changed from about 64 VAC to 66 VAC, when one gate was disconnected.
I am thinking that possibly the ungated SCR is being triggered by dV/dT
from an inductive kick from the transformer, and possibly it is related to
the high impedance of the source and the load. I will need to look at the
waveforms with a scope, and also try a lower impedance source. It may also
help to put a snubber across the SCRs.
Your thoughts will be appreciated, and I will post my additional test
results.
Thanks,
Paul
a lamp on its output. The test circuit is about as follows, except I have a
50 VA isolation transformer supplying the ACHI and ACLO. (My first ASCII
diagram!)
CB
ACHI--[1A]---- SCR1 >|--------|
| | | |-------
--|< SCR2 ---- )|( |
120V )|( 16V O Lamp 28V
)|( |
ACLO--------------------------| |-------
I was able to program the PIC to turn the SCRs on and off, adjust phase
angle to dim the lamp, and check for primary voltage across the SCRs before
allowing initiation.
Then, I wanted to check for an open gate, which I figured would cause a
large DC current in one direction, and would probably trip the 1 amp
circuit breaker. I assumed I would read a large voltage across the SCR that
was not triggered, and that I could use this to cause the firing circuit to
shut down.
Much to my surprise, when I removed either of the gate wires, the circuit
seemed to perform almost the same as when both were connected. I did read
about 2 VDC across the SCRs, and the AC voltage (at about 50% modulation)
changed from about 64 VAC to 66 VAC, when one gate was disconnected.
I am thinking that possibly the ungated SCR is being triggered by dV/dT
from an inductive kick from the transformer, and possibly it is related to
the high impedance of the source and the load. I will need to look at the
waveforms with a scope, and also try a lower impedance source. It may also
help to put a snubber across the SCRs.
Your thoughts will be appreciated, and I will post my additional test
results.
Thanks,
Paul