Prevent an AC controlled SCR from turning off at zero voltage of AC wave.

[Alireza1201]

Hi guys,
I have used the following image circuit to control the load.
AS you see the SCRs are controlled by AC signal and The opto-triac is always ON. If the load is a resistor, When the AC signal is positive the current passes through the SCR1 and when it's negative it passes through the SCR2 and when the AC voltage is around the Zero both of the SCRs block the current. It's clear because the gate cathode current is Low.

my problem is that if I use an inductive load instead of the resistor there will be 90 degree phase difference between current and voltage. Now does the SCRs block the current at zero voltage ? If yes how can I prevent it. (The load is inductive and blocking voltage at zero voltage leads to switching at the maximum current so there will be inrush current.)
Thanks
ALIREZA

 

[Lucien Nunes]

Your circuit is connected in series with the power supply and the load, so the voltage across the thyristors is neither in-phase with the supply voltage nor with the load voltage; it is the instantaneous difference between the two. If the opto-isolator is always on, then as soon as there is a voltage difference that would cause a current to flow, the thyristors will fire and then conduct until the current-zero one half-cycle later. I.e. current will flow when the load allows it to flow. The fact that the supply voltage passes through zero half-way through the conduction period is unimportant, as the thyristors can't 'see' that voltage. So in that sense the circuit works with an inductive load.

Problems arise if you want to control the power of an inductive load by modulating the conduction angle with the opto-isolator. The circuit cannot trigger the thyristors at a particular point relative to the supply voltage waveform as only the difference voltage is present on a 2-terminal circuit in series with the load. In that case it is necessary to get a firing phase reference from the supply.

Another aspect to note when triggering thyristors in series with inductors, is that any tendency to resonate can cause the thyristor to turn back off due to the sudden reversal of voltage. Should not be a problem with this circuit though.

Finally it's worth mentioning that many so-called inductive loads in the real world are not pure inductors; they are often mainly resistive so the current lags the voltage by much less than 90 degrees.

 

[pc1966]

Your suggested circuit looks OK though of course your opto-triac has to be rated for the maximum peak blocking voltage you expect to see.

The issue of triggering problems on high dV/dt is sometimes reduced by a snubber network in parallel to the thyristors (something like 10-100R and 0.1-0.47uF in series) and they can often be bought as a single component with suitably 'class X' capacitors used, etc.