SSRs to BS2: Zero Volatge Turn-on vs. Random Valtage Turn-on?

Archiver

Could someone please explain what the difference is between these two
varieties of solid state relays: Zero Volatge Turn-on vs. Random
Voltage Turn-on?

I have a project which uses a BS2 Stamp Microcontroller to control 6
12amp/120VAC solenoids via an 6 SSRs. What advantage or drawback is
there between using Zero Volatge Turn-on vs. Random Valtage Turn-on in
such an application? The pulse time for the solenoids will be approx.
100ms.

These SSR's are fairly expensive for my budget, and I have a good deal
on the Random Turn-On variety. Any reasons why I shouldn't go with
them?

Thanks.
Cheers,
Lovegasline (-n.)

Archiver

At 17:25 05/27/01, lovegasoline@y... wrote:

>Could someone please explain what the difference is between these two
>varieties of solid state relays: Zero Volatge Turn-on vs. Random
>Voltage Turn-on?

Zero Crossing devices turn on and off at the instant the AC cycle is
passing thru a zero voltage point. This assures no current surges above
and beyond what you get as the normal AC cycle rises and falls. For
devices turning on at random times - meaning whenever you tell them to
without waiting for the next zero crossing - can turn on at a point in the
cycle when voltage is high. This can cause some heavy current surges, RF
interference, etc. The extent of any problems varies depending on the
nature of the connected devices - whether they are purely resistive, or
whether they present a complex impedance that is highly inductive or has
appreciable capacitance, as well as other factors, such as wiring layout in
the case of RF interference.

>I have a project which uses a BS2 Stamp Microcontroller to control 6
>12amp/120VAC solenoids via an 6 SSRs. What advantage or drawback is
>there between using Zero Volatge Turn-on vs. Random Valtage Turn-on in
>such an application? The pulse time for the solenoids will be approx.
>100ms.
>
>These SSR's are fairly expensive for my budget, and I have a good deal
>on the Random Turn-On variety. Any reasons why I shouldn't go with
>them?

Assuming the 12-amp rating is the solenoid's switching capacity and not the
current draw of the actuating coil, I see no real reason not to use the
cheaper SSRs. On the very slim chance it takes 12 amps at 120 VAC to
actuate the coil, I'd almost surely go with a zero crossing device.

Jim H

Archiver

--- In basicstamps@y..., Jim Higgins <HigginsJ@s...> wrote:
> At 17:25 05/27/01, lovegasoline@y... wrote:

Hi ya Jim,

Thanks for thre explanation.

> This can cause some heavy current surges, RF
> interference, etc. The extent of any problems varies depending on the
> nature of the connected devices - whether they are purely resistive, or
> whether they present a complex impedance that is highly inductive or has
> appreciable capacitance, as well as other factors, such as wiring layout in
> the case of RF interference.

The circuit will be hardwired in a residence. The circuit will only be
on at occasional times, not continually run.
RF is new to me. Would this interfere with an X10 remote control
system, for ex.?
Regarding surges, would this present a problem if the circuit is fused
correctly and the coil on the solenoids have diode or MOV protection
for spikes?
What, theoretically, is the real world downside of such surges?
Would it interfere with the functioning of the circuit or the Stamp's
ability to control the circuit?

> Assuming the 12-amp rating is the solenoid's switching capacity and not the
> current draw of the actuating coil, I see no real reason not to use the
> cheaper SSRs. On the very slim chance it takes 12 amps at 120 VAC to
> actuate the coil, I'd almost surely go with a zero crossing device.

Hmm.
I'm not certain what is meant by "switching capacity" vs. "current
draw". Would you mind elaborating?

The data sheet on the solenoid states the following:
-"amps seated" value of 0.66'amps
-"amps at 13/4" value of 12.00 amps (the highest amperage draw).

Thanks for any clarification.
Cheers,
-lovegasoline (-n.)

Archiver

Real world considerations:

If there is only one such switching device on the same AC line in a
residence, it doesn't matter much whether you use random or zero-cross
devices. If you use a number of randoms, the noise from gating the thyristor
in one device intentionally can fire the gate of one of the others , which
is usually undesirable. When switching multiple devices, it's always good to
use zero-cross switching to avoid this.
Also, as of present, it's impossible to pass the CE EMF test with random
firing or gating. If you should need to fire a random device frequently,
such as on every AC 1/2 wave, you may see some effect on nearby PC monitors.

The noise produced from random firing of a triac or SCR is, I believe, too
fast for X-10 devices to catch. I have a number of X-10 cameras and other
devices being switched constantly - that work flawlessly during the
development and testing of all kinds of gate firing circuits nearby,
including phase angle firing of multi-kVA loads.

Chris

Would this interfere with an X10 remote control
> system, for ex.?
> Regarding surges, would this present a problem if the circuit
> is fused
> correctly and the coil on the solenoids have diode or MOV protection
> for spikes?
> What, theoretically, is the real world downside of such surges?
> Would it interfere with the functioning of the circuit or the Stamp's
> ability to control the circuit?
>
> > Assuming the 12-amp rating is the solenoid's switching
> capacity and not the
> > current draw of the actuating coil, I see no real reason
> not to use the
> > cheaper SSRs. On the very slim chance it takes 12 amps at
> 120 VAC to
> > actuate the coil, I'd almost surely go with a zero crossing device.
>

Archiver

At 19:14 05/27/01, lovegasoline@y... wrote:

>--- In basicstamps@y..., Jim Higgins <HigginsJ@s...> wrote:
> > At 17:25 05/27/01, lovegasoline@y... wrote:
>
>Hi ya Jim,
>
>Thanks for thre explanation.
>
> > This can cause some heavy current surges, RF
> > interference, etc. The extent of any problems varies depending on the
> > nature of the connected devices - whether they are purely resistive, or
> > whether they present a complex impedance that is highly inductive or has
> > appreciable capacitance, as well as other factors, such as wiring
> layout in
> > the case of RF interference.
>
>The circuit will be hardwired in a residence. The circuit will only be
>on at occasional times, not continually run.
>RF is new to me. Would this interfere with an X10 remote control
>system, for ex.?
>Regarding surges, would this present a problem if the circuit is fused
>correctly and the coil on the solenoids have diode or MOV protection
>for spikes?

Fusing does nothing to reduce surges unless the surges exceed the fuse
rating, then you don't just suppress the surge; you kill the whole
operation. And you can't put simple diodes across a coil powered by AC -
they'll conduct more than just surges. You could use a MOV, but MOVs can
"wear out" when hit repeatedly. Each surge they take takes a bit of life
out of them. Someone more up on this than I could advise how long it takes
to kill one. I'm sure it depends on how big the surges are. Back to
diodes, I suppose you could use something like back-to-back 250-volt (wild
guess on the voltage) zeners, which will pass spikes above 250 volts, but
won't pass the 170-volt peak AC voltage normally seen on a 120 VAC nominal
line. Please note that actual selection of zener value is a bit out of my
comfort zone so the above is only generally illustrative, not definitive.

>What, theoretically, is the real world downside of such surges?
>Would it interfere with the functioning of the circuit or the Stamp's
>ability to control the circuit?

If the spikes get back to the Stamp either via it's power supply or via any
of it's data or logic connections then it's operation could be affected or
it could be damaged. It's not as scary as it may sound, but you should
design adequate isolation and protection into your project. The specs on
your SSRs should tell you how much isolation they provide between the
driving source and the power source. If they are intended to be driven by
logic devices they are probably OK. But it pays to check.

> > Assuming the 12-amp rating is the solenoid's switching capacity and not
> the
> > current draw of the actuating coil, I see no real reason not to use the
> > cheaper SSRs. On the very slim chance it takes 12 amps at 120 VAC to
> > actuate the coil, I'd almost surely go with a zero crossing device.
>
>Hmm.
>I'm not certain what is meant by "switching capacity" vs. "current
>draw". Would you mind elaborating?

Sure - and maybe I assumed something I shouldn't have. When you said
"solenoid" I read that as "really big relay" such as the solenoid in your
car which switches current to the starter. Now I'm guessing that with what
seems to be a 1.75" travel this solenoid may actually be an actuator rather
than an electrical relay. Whichever it is, I think Chris' answer covered
it completely.

>The data sheet on the solenoid states the following:
>-"amps seated" value of 0.66'amps
>-"amps at 13/4" value of 12.00 amps (the highest amperage draw).

I'd tend to agree with the "real world considerations" Chris provided. You
may want to consider the content of future projects when designing this
one. If nothing more sensitive than what you have at present is planned
your SSRs are probably OK.

Jim H