I am trying to produce a triac power controller for a large mains solenoid which is for a lab demonstration of electromagnetic induction. I have not had much success!
My initial attempt used a simple 2-wire design as shown in figure 1 of ST's AN308 application note. I found often got power glitches supplied to the solenoid at power-on even with the power control pot turned down to minimum (maximum-resistance, well below the triac's normal triggering threshold). I believe this was due to the highly inductive nature of the solenoid. I also have a 100W mains bulb paralleled across the solenoid to confirm the presence of the glitches with a flash.
My second attempt was to try the 3-wire circuit shown in figure 4 of AN308, which is recommended for inductive loads, where the triac is triggered by the main voltage rather than the voltage across the load (solenoid). The problem is the power/voltage delivery as a function of pot rotation is no longer smooth. At a certain point there is a sudden Jump In Power/voltage (JIP) supplied to the solenoid+lamp. At this point I disconnected the solenoid and used just the lamp as a non-inductive load. The JIP problem remained.
I have tried using both a 'standard' BTA26-600BRG triac and a 'snubberless' type BTA24-600BWRG in case 4th quadrant triggering etc. was a factor but it made no difference.
I also tried simulating the circuit in Spice, using ST's models for the triac and diac. The simulation of the power delivered to the 100W lamp as a function of control pot resistance showed a smooth curve with no discontinuities/jumps.
Observing what the circuit is doing when the discontinuity / JIP occurs with a scope is difficult without either a mains isolation transformer to power the circuit or a battery powered (and therefore not grounded to mains earth) scope.
I did get a very basic single channel battery powered DSO and monitored the triac gate voltage with it. What it showed as the power was adjusted from just below the discontinuity / JIP to just above it was interesting. With the control down at minimum and the triac untriggered the gate voltage is just a 50Hz sinewave. Once the triac starts to trigger and the lamp comes on dimly a single dip appears e.g. in the +ve & -ve going mains 1/2 cycles of the gate voltage sinewave. The control pot is advanced until the JIP (lamp suddenly become brighter) occurs, then the triac gate voltage acquires a 2nd dip in voltage during the +ve/-ve going 1/2 cycles.
This observation made me wonder if the problem could be related to the triac holding current. However the datasheet gives I_L as 70mA to 80mA whereas the JIP occurs at significantly higher currents, e.g. 250mA.
I am at a loss on how to proceed.
Any thoughts?
Thanks
Tom Crane
