P
P E Schoen
- Jan 1, 1970
- 0
I have used SSRs with peak firing for inductive loads (transformers), but
they are generally non-stock items and are rather expensive:
http://www.wolfautomation.com/products/30434/peak-switching-ssr-single-phasebrcarlo-gavazzi-rm1c
http://www.galco.com/buy/Crydom/PSD2450
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:
http://www.newark.com/crydom/cwd2490-10/ssr-panel-mount-280vac-32vdc-90a/dp/80K5802?Ntt=cwd2490-10
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:
http://www.digikey.com/product-detail/en/TLP748J(TP1,F)/TLP748JTP1FCT-ND/2021328
and simply using the anode voltage of the main SCR to drive its gate through
the opto-SCR. But I am worried that the sudden application of the peak
voltage (up to 375 volts) to the gate might cause damage, although when the
gate triggers, the anode voltage will drop almost instantly to 1-2 volts.
And at that voltage, I'm not sure the opto-SCR can maintain gate drive after
the initial firing. With the opto-SCRs on for the remaining period of time
determined by the control signal, there will be times during the zero
crossing where there will be no voltage to trigger, and there may be some
distortion until enough voltage is present to cause the main SCR to conduct.
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
they are generally non-stock items and are rather expensive:
http://www.wolfautomation.com/products/30434/peak-switching-ssr-single-phasebrcarlo-gavazzi-rm1c
http://www.galco.com/buy/Crydom/PSD2450
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:
http://www.newark.com/crydom/cwd2490-10/ssr-panel-mount-280vac-32vdc-90a/dp/80K5802?Ntt=cwd2490-10
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:
http://www.digikey.com/product-detail/en/TLP748J(TP1,F)/TLP748JTP1FCT-ND/2021328
and simply using the anode voltage of the main SCR to drive its gate through
the opto-SCR. But I am worried that the sudden application of the peak
voltage (up to 375 volts) to the gate might cause damage, although when the
gate triggers, the anode voltage will drop almost instantly to 1-2 volts.
And at that voltage, I'm not sure the opto-SCR can maintain gate drive after
the initial firing. With the opto-SCRs on for the remaining period of time
determined by the control signal, there will be times during the zero
crossing where there will be no voltage to trigger, and there may be some
distortion until enough voltage is present to cause the main SCR to conduct.
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