Solid State Relays

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StarCat

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Imdustrial Engineering Technician - HVACR Electrical and Mechanical Systems
I do not have a lot of experience with these. Do they exist in DPDT forms with both N.O. and N.C. switches?
Like something that could replace the function of a typical Octal base Ice cube relay?
 
kwired

kwired

Electron manager
Location
NE Nebraska
I think there are even three pole SS relays that can control a three phase load.

Guess techincally a VFD is sort of one of those - but I think there are some much simpler versions that have similar operationfunction to a three pole contactor
 
StarCat

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Imdustrial Engineering Technician - HVACR Electrical and Mechanical Systems
Russ Interesting, as an example there are 2 interlock circuits:
Relay 1 switch handles 24 VAC and is held open during normal run cycle, Pilot duty to another 24 VAC relay, [ verylight load]
Relay 2 switch handles 120 VAC and is held closed during the normal run cycle and powers downstream to a small steam solenoid valve [ slightly larger load than the above but still small, .6A]
Any thoughts are helpful.
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
201029-1403 EDT

StarCat:

Why are you considering a solid-state relay?

Rapid cycling?
Very large number of cycles, 10s of millions?
Zero current turn off?
No electrical noise at turn on?
Low input power?

Some disadvantages.
Failure from electrical transients.
Contact voltage drop.
May need minimum load current, but many get a sustained input signal.
Need to drop below a holding current to drop out.
A single device type may work only on DC or AC, but not both.
Contact leakage current.

.


.
 
StarCat

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Imdustrial Engineering Technician - HVACR Electrical and Mechanical Systems
Good information.
This is an unusual control panel which was built to be just outside the classified area, but still sort of required to have either SS or Seal Switch type relays. The 2 main operating relays which are not part of the interlock circuit are SSR and cycle often. The safety interlocks were added later and in a hurry as is often times the case. I was not able to order the " best choice " product when that was done at that time.
It looks like I have all figured out and the hermetic octal relays along with this timer relay will do the job:

Many thank for all the input.

Star
 
Jraef

Jraef

Moderator, OTD
Staff member
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Electrical Engineer
kwired said:
I think there are even three pole SS relays that can control a three phase load.

Guess techincally a VFD is sort of one of those - but I think there are some much simpler versions that have similar operationfunction to a three pole contactor
Click to expand...
Those are called "Solid State Contactors", also Soft Starters" are the same thing, but with ramp functions and OL protection.

VFDs are different, they convert AC to DC then back to AC (sort of). Much much more complicated than an SSR or SSC
 
P

paulengr

Senior Member
Jraef said:
Those are called "Solid State Contactors", also Soft Starters" are the same thing, but with ramp functions and OL protection.

VFDs are different, they convert AC to DC then back to AC (sort of). Much much more complicated than an SSR or SSC
Click to expand...

A solid state relay/contactor is just that. It is not a soft starter. A basic AC solid state relay uses TRIACs just like a soft starter but it merely does on and off while a soft starter does waveform chopping to reduce output voltage like a lamp dimmer. They are mostly used for motor starting.

The DC solid state relay is a transistor output, usually a MOSFET. You need this since the SCR/TRIAC can’t turn off on DC.

They have somewhat fallen out of favor due to their somewhat limited design in that you must power then to turn off but the UJT transistor has the bizarre property that you fire the gate to turn it off rather than on...an “NC” device but I confess I’ve never seen one in a catalog as an SSR.

SSR pricing is all over the place. You can buy smaller ones that are 3-5 mm wide in DIN mount that are equal to ice cube relays at about $10 each. Lots of good things but the best is nearly infinite switching cycles,
 
R

RumRunner

Senior Member
Location
SCV Ca, USA
Occupation
Retired EE
VFDs are SCALABLE while solid state relays are not.
Hence, we can say that solid state relay doesn’t act like a soft-start.

Solid state relays are just exactly what they are. They are dummies that do what is commanded to do.
Turn OFF when not needed— turn ON when needed.

However, a much-improved technology to overcome this latching problem -- that is typical of SCR—the IGBT (isolated gate bipolar transistor) and the GTO (gate turnoff transistor) took care of this problem.

SCRs are still indispensable in some applications and cheaper . Most notable application is controlling output voltage to regulate heat with resistance heaters. . . pretty basic actually.
Also, unlike SCRs --Solid State Relays don’t require Holding Current Resistor -- while SCRs do need to turn them to turn on and stay ON.

Some call them “parasitic” resistors

There are ways to turn them off though.

1. Inject negative pulse to the anode.

2. Remove power (of course) lol at the source

3. Reverse the polarity at the source.

On IGBTs, you don’t need complicated triggers (on most designs) to turn them off.. . . high speed switching is a bonus.

Also, on GTOs--they can be turned off by the GATE lead.
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
201030-1719 EDT

Any switched circuit containing an inductance will have to somewhere dissipate the energy stored in the inductance at the time of switching off. If the circuit is a simple series circuit of voltage, switch, and inductor with current flowing at the time of switching, then a large enough voltage will be produced across the switch or whatever is around the switch at the time of turn off to maintain the existing current. Basically this means arcing across the switch or destruction of the switch.

Since an SCR or TRIAC continues to conduct current until current drops below the device's minimum holding current, not an external holding current to the gate, but a load holding current thru the device from anode to cathode, the switch is essentially self protecting.

A mechanical switch will arc between the contacts at turn off until the inductive current dissipates enough. So a mechanical switch also somewhat switches off at a current zero crossing. But an SCR or TRIAC maintains an anode to cathode voltage drop to around 1 V before current drops low enough to terminate conduction.

.
 
M

MD Automation

Senior Member
Location
Maryland
Occupation
Engineer
Star, sounds like you have it figured out.

Regarding you first link above...we use a lot of those kinds of DIN rail mounted Phoenix relays, both the electro-mechanical (relay module is white) and solid state (module is black) in our control cabinets. They are not crazy expensive... ~ $35-40 for a single pole. Maybe your industrial / hazardous location type is more?

1604120247740.png

Fun story... About 10 years ago, after deploying some machines across the US, we started working with various sites for troubleshooting. One site kept talking about knowing that outputs are not coming on because they "Don't see the Chiclet lighting up". Of course, I have no idea what they are talking about. But I come to find out that certain techs call that little white Phoenix relay a "chiclet" because it reminds them of a piece of chewing gum 😉

Good luck with your cabinet!
 
StarCat

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Imdustrial Engineering Technician - HVACR Electrical and Mechanical Systems
Ok this is the LATEST find:
The last find was an Allen Bradley unit that cost a Kings Ransom.
This one looks like a strong possibility.
Found some Yaskawa cube sealed relays rated for locaiton with LIGHTS, but no 24 VAC coils. have to go with old school cans on those I guess.
Either way it will be improved. You guys with the details electronics knowledge are appreciated. I am reading and working on retention of key points. Thank you very much.

Star
 
P

paulengr

Senior Member
myspark said:
VFDs are SCALABLE while solid state relays are not.
Hence, we can say that solid state relay doesn’t act like a soft-start.

Solid state relays are just exactly what they are. They are dummies that do what is commanded to do.
Turn OFF when not needed— turn ON when needed.

However, a much-improved technology to overcome this latching problem -- that is typical of SCR—the IGBT (isolated gate bipolar transistor) and the GTO (gate turnoff transistor) took care of this problem.

SCRs are still indispensable in some applications and cheaper . Most notable application is controlling output voltage to regulate heat with resistance heaters. . . pretty basic actually.
Also, unlike SCRs --Solid State Relays don’t require Holding Current Resistor -- while SCRs do need to turn them to turn on and stay ON.

Some call them “parasitic” resistors

There are ways to turn them off though.

1. Inject negative pulse to the anode.

2. Remove power (of course) lol at the source

3. Reverse the polarity at the source.

On IGBTs, you don’t need complicated triggers (on most designs) to turn them off.. . . high speed switching is a bonus.

Also, on GTOs--they can be turned off by the GATE lead.
Click to expand...

The last GTO manufacturer (ABB) stopped production about ten years ago.

The UJT is sort of coming back. It is easier to make one with SiC. There are now “normally open” IGBT-like SiCs but the first ones were “NC”, the opposite of an IGBT.
 
P

paulengr

Senior Member
Both are outrageous.


Roughly half the price. But usually I shop for these on Mouset, Digi Key, Newark, or Allied because they are even smaller and cheaper there. If you need high power like the 10 A+ automation direct ones expect to pay dearly for it. But if you need more modest 1-3 A they can be had for $10 each. That’s about the price of mechanical relays.

As mentioned there are definitely some advantages and disadvantages to be aware of,

As an example right now I’m working on a job that has both hydraulics and a lot of starters. So with the starters for conveyors and hydraulic pumps those are going to cycle maybe a few times a day so with say a 2-10 million cycle relay it will last for years and can handle the large inductive load of the starter coil with ease. But the hydraulics will be constantly turning on and off so lifespans with ice cube relays are months. With an SSR I can expect years if protected properly.
 
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