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Why are SSR so slow?

H

Hammy

Jan 1, 1970
0
Another reason that wasn't mention was the price.
They are too expensive anyway. As relay replacement
their advantage is the infinite number of on-off
cycles. And most are made as zero crossing switch
anyway.
When we're talking about power, don't forget the
rather hefty voltage across them, in the order of
1.5V at least. There is nothing like 10mOhm or so.

rene

I haven't looked at relays for a while but I don't remember any (at
least in the reasonable price range) that had a 10moHm contact R. The
other thing about relays is the Ron is for initial resistance; over
use it will increase then there's contact bounce.

Both have pro's and cons it depends on the application and the budget.
 
J

Joerg

Jan 1, 1970
0
Rene said:
Another reason that wasn't mention was the price.
They are too expensive anyway. ...


Usually but not always. This is a very low qty app where having a block
with four screw terminals and a big mounting bolt in the middle would
have been much better that assembling a circuit board. Even if it cost
$50-$100.

... As relay replacement
their advantage is the infinite number of on-off
cycles. And most are made as zero crossing switch
anyway.
When we're talking about power, don't forget the
rather hefty voltage across them, in the order of
1.5V at least. There is nothing like 10mOhm or so.

Not for the MOSFET versions. They have only their Rdson.
 
J

Joerg

Jan 1, 1970
0
John said:
Another reason that wasn't mention was the price.
They are too expensive anyway. As relay replacement
their advantage is the infinite number of on-off
cycles. And most are made as zero crossing switch
anyway.
When we're talking about power, don't forget the
rather hefty voltage across them, in the order of
1.5V at least. There is nothing like 10mOhm or so.

rene


The photo-fet types switch in about a millisecond. They look purely
resistive at modest currents and have zero offset, so can be used for
things like thermocouple muxing, autozeroing, or gain switching in
opamp circuits.

This is impressive:

http://www.cel.com/parts.do?command=load&idRootPart=311#

The Ron * Coff figure of merit is about [1] 6 picoseconds.

Cool. The datasheet doesn't want to download but that 80mA limit would
cause a major phut ... *BANG* in my case.

Anyhow, we just ordered the parts for the usual good old discrete solution.
 
R

Rene Tschaggelar

Jan 1, 1970
0
Joerg said:
Rene said:
Joerg said:
Hey guys, [snip]
I have to do a 100Hz PWM at a couple of amps and with this sort or
"performance" SSR relays don't cut it, plus would probably go phut
... *BANG*.


Another reason that wasn't mention was the price.
They are too expensive anyway. ...


Usually but not always. This is a very low qty app where having a block
with four screw terminals and a big mounting bolt in the middle would
have been much better that assembling a circuit board. Even if it cost
$50-$100.

... As relay replacement
their advantage is the infinite number of on-off
cycles. And most are made as zero crossing switch
anyway.
When we're talking about power, don't forget the
rather hefty voltage across them, in the order of
1.5V at least. There is nothing like 10mOhm or so.

Not for the MOSFET versions. They have only their Rdson.

Yes, it is only the resistive part, but the mosfets
are not of the most advanced sort. A standard SO8
cased 40V NFet is available with about 10mOhms.
A 40V Photomos Relay in DIL8 is having in the order
of 600mOhms. While the Fet is specified at above 10A,
the relay is for 500mA. Similar the voltage drop at the
rated current, the fet drops 100mV, the relay drops
300mV. This tells me, the FET technology within the
relays is at least 5 year back.
If it wasn't for the, related to the case, astonishing
isolation voltage, the two spare pins on the LED side
could have been used to supply some electronics on the
secondary side.

My comment about the 1.5V across were for a 25A part
where we measured somewhat above 1V at rated current.
A line unit in a beefy case.

Rene
 
J

Joerg

Jan 1, 1970
0
Rene said:
Joerg said:
Rene said:
Joerg wrote:
Hey guys, [snip]
I have to do a 100Hz PWM at a couple of amps and with this sort or
"performance" SSR relays don't cut it, plus would probably go phut
... *BANG*.


Another reason that wasn't mention was the price.
They are too expensive anyway. ...


Usually but not always. This is a very low qty app where having a
block with four screw terminals and a big mounting bolt in the middle
would have been much better that assembling a circuit board. Even if
it cost $50-$100.

... As relay replacement
their advantage is the infinite number of on-off
cycles. And most are made as zero crossing switch
anyway.
When we're talking about power, don't forget the
rather hefty voltage across them, in the order of
1.5V at least. There is nothing like 10mOhm or so.

Not for the MOSFET versions. They have only their Rdson.

Yes, it is only the resistive part, but the mosfets
are not of the most advanced sort. A standard SO8
cased 40V NFet is available with about 10mOhms.
A 40V Photomos Relay in DIL8 is having in the order
of 600mOhms. While the Fet is specified at above 10A,
the relay is for 500mA. Similar the voltage drop at the
rated current, the fet drops 100mV, the relay drops
300mV. This tells me, the FET technology within the
relays is at least 5 year back.
If it wasn't for the, related to the case, astonishing
isolation voltage, the two spare pins on the LED side
could have been used to supply some electronics on the
secondary side.

My comment about the 1.5V across were for a 25A part
where we measured somewhat above 1V at rated current.
A line unit in a beefy case.

Anyhow, after a perusal of this SSR market ended in utter disappointment
and lots of yawning I just rolled my own. Like usual ;-)

I might check back in 10 years, maybe the SSR manufacturers have learned
a thing or two by then :)
 
J

Joerg

Jan 1, 1970
0
JosephKK said:
Something to remember, the typical SSR is designed to be a lower power
replacement for a mechanical relay in a "low voltage" power control
application. Where response in 1/2 60 Hz cycle (8.33 ms) is quite
adequate. That is where the volume is.

That's where they hope the volume is. Those things are too expensive for
such mundane jobs. At one client I was called in to reduce the cost of
one of their systems. Found over half a dozen SSRs. All of them were out
of there, instantly.
 
J

Joerg

Jan 1, 1970
0
Jim said:
My !@#$%^&* pool equipment controller is full of SSR's :-(

What? Did a rookie design that? The thing must have cost a fortune.
Either way the manufacturer has left money on the table. If it
absolutely has to be electronic this kind of stuff is done with triacs.
I mean, in qties you can get a BTB24-600 (600V, 25A) for well under a buck.

The main pump SSR failed, so I bypassed it with a conventional clock
mechanism for now. When it cools off ;-) I'll replace ALL the SSR's
with small relays, and add a box full of contactors... everything is
on there, main pump, cleaner, spa (and its heat pump), patio misters,
lights :-(

Yep, real men build it with Potter&Brumfield. Or I guess nowadays Tyco.

What always surprises me is why the sprinkler timer guys don't offer one
with a program conducive to pools. Ok, you can program 4-6 zones to max,
hang the filter pump on the master output and the sweep on zone 3, but
it's a bit clumsy and also tricky to explain if one ever sells the
house. All you'd have to do is hang contactors on it instead of valves.
I always wanted to do that but the stone-age dial timers just run and run.
 
K

krw

Jan 1, 1970
0
My !@#$%^&* pool equipment controller is full of SSR's :-(

The main pump SSR failed, so I bypassed it with a conventional clock
mechanism for now. When it cools off ;-) I'll replace ALL the SSR's
with small relays, and add a box full of contactors... everything is
on there, main pump, cleaner, spa (and its heat pump), patio misters,
lights :-(

Maybe Joerg should redesign the controller for them. ;-)
 
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