I have used SSRs with peak firing for inductive loads (transformers), but they are generally non-stock items and are rather expensive:
So we thought we might use some random firing SSRs which can be triggered for phase angle modulation or with externally generated peak firing for initiation:
We have quite a few of these or similar units in stock which were bought for another product that has now been redesigned. I thought it should be easy to make a small circuit that would be powered from the 5V to 12V DC control
signal and sense zero crossing or peak of the 80-265 VAC load signal across the output terminals to determine the peak point and fire the SSR at that time. However, these random-firing SSRs are also mostly special order with long lead times, minimum quantities, and high cost. Crydom seems to be the only manufacturer for this type SSR, but there may (and should) be others.
So, I have thought that it may be better to build the entire SSR from discrete devices or dual modules, which are easily available in many ratings, and we use them in many products, but with a trigger board that has two separate, isolated gate drive circuits, and associated circuitry, which results in a rather large, complex, and expensive board.
I would like to duplicate the drive circuits of the peak firing SSRs, which must be fairly simple. I have considered using optoisolators with SCR outputs, such as:
Since it is a highly inductive load, there will be current in the SCR at the time of the applied voltage zero crossing, and if the gate is not triggered the current will keep it in conduction. If it does stop conducting and voltage once again appears on the anode, the opto-SCR will trigger the gate.
Are these peak-firing (and the ubiquitous zero-crossing firing) SSRs really so simple as to use opto-SCRs to drive the gated from the anode voltage? Or do they use some sort of trigger pulse transformer for the gates? We have found that it is necessary to maintain DC gate drive on both SCRs throughout the entire cycle of the sine wave. The other possibility is that the main SCRs could be LASCRs, but I have been unable to find any commercially available in the size I need (40-150A, 120-265 VAC and possibly up to
500 VAC).Thanks,
Paul