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Solid State Relay (SSR) with peak firing SCRs for inductive loads

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
 
L

Lasse Langwadt Christensen

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


how about an FETs/IGBTs?

I have been thinking about a short circuit protected master switch for a triac controller board

A sil dc-dc converter, optocoupler, ir2127 and a pair of FETs or IGBTs back to back


-Lasse
 
P

Phil Allison

Jan 1, 1970
0
"P E Schoen"

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

** Very nice, not really that expensive IMO.


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

** Looks a bit wimpy for use with large SCRs.

This is one of the better ones:

http://www.vishay.com/docs/83689/83689.pdf

Despite the term "optotriac" in the title, the output uses SCRs - the
various dv/dt figures are all 5 or 10kV /uS.


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,

** You need a series resistor of about 220 ohms to limit peak amps.


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.

** It is desirable for the trigger device to turn off ( ie fall below
holding conditions) as soon as the main SCRs have fired - so they can
commutate off alone, unimpeded by the trigger having to do like wise.


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,

** Huh ?

With an inductive load, zero current will not coincide with zero volts -
SCRs and Triacs remain on until the current is below holding conditions.


and there may be some
distortion until enough voltage is present to cause the main SCR to conduct.

** Usually a very small issue.

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.

** That is how it works, long as you keep driving the LED.

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?

** Bet ya they do.

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.

** Really ? Why?


.... Phil
 
P

P E Schoen

Jan 1, 1970
0
"Lasse Langwadt Christensen" wrote in message
how about an FETs/IGBTs?
I have been thinking about a short circuit protected master switch for a
triac controller board
A sil dc-dc converter, optocoupler, ir2127 and a pair of FETs or IGBTs
back to back

I have thought about something like this, but SCRs are generally less
expensive and better suited to high power non-PWM applications. After
another search I found the following:
http://www.crydom.com/en/Tech/Tips/SSR the inside story.pdf

It shows two SCRs connected in anti-parallel and a simple switch closure
between the gates causes the module to turn on. Then they show the use of a
rather complex opto-transistor driving another SCR and a FWB for
bidirectional operation of the switch. But it seems that this would require
several volts to maintain gate drive after turn-on. It may not be too
critical, but it seems that it may be enough just to use a resistor to limit
the current to the gate at turn-on.

I considered using a small inductor so that the current would not have such
a high peak at initial turn-on. For some 55A 1kV SCRs I may use:
http://www.mouser.com/ProductDetail...=/ha2pyFadujd9vKNrwSrCFk3fegDfrcvo9vWU/YdEow=
gate turn-on time is about 2-3 uSec, and peak gate current is 4 amps, so I
would need an inductor of about 200 uH. Probably a small one like the
following:
http://www.mouser.com/ProductDetail...GAEpiMZZMsg%2by3WlYCkU9E4kXPnClt9kZd7tCtDMeQ=
220 uH 100 mA 10 ohms and only about $0.35.

Thanks for the suggestions.

Paul
 
E

Ecnerwal

Jan 1, 1970
0
P E Schoen said:
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.

At the risk of depleting the supply, they are often (but unreliably)
available by each, at short leadtimes and low cost if you peruse a
certain popular action site that rhymes with "Be Prey." You have to do
your own lookup most of the time - the sellers often don't know much
about them. Some of the buyers may not either. Keep an eye out, and you
might find something that works for you...
 
P

P E Schoen

Jan 1, 1970
0
"Ecnerwal" wrote in message
At the risk of depleting the supply, they are often (but unreliably)
available by each, at short leadtimes and low cost if you peruse a
certain popular action site that rhymes with "Be Prey." You have to do
your own lookup most of the time - the sellers often don't know much
about them. Some of the buyers may not either. Keep an eye out, and you
might find something that works for you...

Thanks for the idea. I found a possible source for 30A 240 VAC random fired
units here:
http://www.ebay.com/itm/SSR-Solid-S...ndom-Switching-Turn-On-Mode-30A-/321114452036

And there is a website for the manufacturer with higher current models:
http://www.autonics.com/products/products_detail.php?catecode=02/13/01&db_uid=251

This is for an OEM product so the onesey twosey items won't work for us, but
the sources above seem to be able to supply in quantity.

Still, I think it may be best to roll our own, and avoid any possibility of
shortages and long lead times.

Paul
 
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