Crydom solid state relays

[buffalonymann]

Question: I have crydom SSRs in my facility - I see in the specs turn on voltage is 90VAC and turn off voltage is 10VAC. Would like to gain better understanding of exactly what is happening to turn these off.

 

[Jraef]

Question: I have crydom SSRs in my facility - I see in the specs turn on voltage is 90VAC and turn off voltage is 10VAC. Would like to gain better understanding of exactly what is happening to turn these off.

It means that your control signal (the equivalent of a “coil”) must be a minimum of 90VAC, and if in the “off” state there is more than 10VAC on the signal, ie “leakage” in solid state control devices, it may not turn off.

 

[petersonra]

It means that your control signal (the equivalent of a “coil”) must be a minimum of 90VAC, and if in the “off” state there is more than 10VAC on the signal, ie “leakage” in solid state control devices, it may not turn off.

You may have meant "less".

BTW, these are min and max numbers. There is no real way to tell on a specific relay where it will operate. However the spec is telling you if the input control voltage is less than 10 V it won't turn on, and if it is greater than 90 V it will. Between those points it is hard to tell what will happen.

 

[Jraef]

You may have meant "less".

BTW, these are min and max numbers. There is no real way to tell on a specific relay where it will operate. However the spec is telling you if the input control voltage is less than 10 V it won't turn on, and if it is greater than 90 V it will. Between those points it is hard to tell what will happen.

No, 10V is what’s referred to as the “must turn off” voltage, meaning if it is more than 10V, it might not turn off.

 

[GoldDigger]

:thumbsup:
Mechanical relays have an unavoidable hysteresis because of the moving armature. Solid state relays will generally design in similar hysteresis, even when it is not an inherent feature of the gate elements themselves.

 

[buffalonymann]

It means that your control signal (the equivalent of a “coil”) must be a minimum of 90VAC, and if in the “off” state there is more than 10VAC on the signal, ie “leakage” in solid state control devices, it may not turn off.

I understand what the terms mean, I want to understand how they're turning these off and why the large difference in turn-on and turn-off

 

[buffalonymann]

I want you guys to think about this, when we do turn off the control voltage there remains an induced voltage of about 18VAC on the control circuit. The SSR does stop conducting full current, but we see high leakage voltage at the output. When we drive that 18VAC down to 7VAC, we see the leakage voltage disappear.

 

[buffalonymann]

:thumbsup:
Mechanical relays have an unavoidable hysteresis because of the moving armature. Solid state relays will generally design in similar hysteresis, even when it is not an inherent feature of the gate elements themselves.

Not sure what your saying in the latter half of your comment

 

[GoldDigger]

Not sure what your saying in the latter half of your comment

I am saying that some solid state relay designs could easily be made without hysteresis, but it is designed in anyway.
Other designs, like the Triac circuitry in a two wire dimmer, cannot avoid hysteresis.

Sent from my XT1585 using Tapatalk

 

[buffalonymann]

I am saying that some solid state relay designs could easily be made without hysteresis, but it is designed in anyway.
Other designs, like the Triac circuitry in a two wire dimmer, cannot avoid hysteresis.

Sent from my XT1585 using Tapatalk

These are 3 pole AC relays - shall I assume they're using triacs?

 

[gar]

181023-1939 EDT


buffalonymann:

Your SSR probably uses a TRIAC, or back to back SCRs as the power switching element. You need to understand how these devices work. Whether TRIAC or SCR the results are quite similar. Both switches have similar turn on or off characteristics. The devices consist of anode, cathode, and gate electrodes.

The TRIAC or SCR is largely nonconductive from anode to cathode in its normal operating range when there is no current to the gate. In this state there is some leakage current, but it is very small.

When triggered or held on by the gate the TRIAC conducts from anode to cathode in both directions. The SCR conducts in only one direction, positive current from anode to cathode.

The threshold triggering to the gate that causes anode to cathode conduction is quite variable from one device to another. Thus, part of the determination of guaranteed turn on of an SSR. The specification has to include the worst production value and temperature range.

A TRIAC or SCR will remain turned on after being triggered so long as the anode cathode current remains above a holding value. If the current drops below the holding value, then conduction stops if there is not adequate gate current. Conduction remains off until there is adequate anode cathode voltage AND there is again gate current above the trigger value.

Turn off occurs after the first current zero crossing after gate current is removed.

To provide electrical isolation between some control signal and the gate cathode junction some form of optical coupler is typically used. There has to be some power to the output of the optical device to supply current to the gate. This will generally come from the anode cathode circuit when the device is not conducting. With this type of gate drive there has to be some voltage drop from anode to cathode to be the power source. Thus, turn on at the start of a new cycle will not occur at the voltage zero crossing, but at a slight angular delay.

The 90 V and 10 V values in the specification are to account for variations in the trigger sensitivity of different devices, the need to turn on close to zero crossing, and not turn on late in the AC cycle.

Many commercial SSRs have a snubber circuit built in from anode to cathode to reduce turn off voltage transients across the switching device. This adds leakage current in the SSR off state that is much higher than the inherent leakage of the TRIAC or SCR.

From your post numbered #7

I want you guys to think about this, when we do turn off the control voltage there remains an induced voltage of about 18VAC on the control circuit. The SSR does stop conducting full current, but we see high leakage voltage at the output. When we drive that 18VAC down to 7VAC, we see the leakage voltage disappear.

I can not give you an answer without actual measurements in your circuit. This at least mean some scope measurements.

See https://en.wikipedia.org/wiki/TRIAC

.

 

[buffalonymann]

@gar

thanks for the indepth info. I know how the scr and triac work but I'm not familiar with practical applications. I've confirmed these are back to back SCR, SCR is either on or off (theoretically) (I know there is always leakage current in all semiconductors)

If I read you correctly, you're suggesting we may be seeing voltage at the output due to a snubber circuit?

Here is a link to the schematic http://www.crydom.com/en/products/catalog/53tp-series-ip00-ac-panel-mount.pdf

 

[gar]

181024-0857 EDT

buffalonymann:

In the circuit diagram at the right of each pair of back to back SCRs is a series circuit of two capacitors and one resistor. This is the snubber circuit, and this provides a leakage path around the SCRs. When the SCRs are off this is the dominate leakage path.

The trigger circuit appears to use phototriac devices. See http://www.vishay.com/docs/84780/appnote34.pdf
There should be no backfeed thru these devices to your input control signal terminals.

I want you guys to think about this, when we do turn off the control voltage there remains an induced voltage of about 18VAC on the control circuit. The SSR does stop conducting full current, but we see high leakage voltage at the output. When we drive that 18VAC down to 7VAC, we see the leakage voltage disappear.

If you remove the wires to the control terminals, then do you still see 18 V across the terminals? I doubt that you wiil. If that voltage goes away, then the origin of the voltage is from your control circuit. 18 V to the SSR input terminals may be sufficient to cause current flow thru the SSR output area.

When the drive to the input terminals is zero, then there can be current flow thru the snubber circuit in the output path.

I see nothing obvious that would cause 18 V across the input control terminals.

.

 

[buffalonymann]

I'm in total agreement that the control voltage is external. I'm trying to get a handle on what is happening in the circuit - let me give you a bigger picture. We're using these SSRs in our extrusion process, a PLC receives a signal from a thermocouple and pulses the SSR via an AC output module to energize resistance heaters. I suspect my AC output module is leaking current when the node is off and this is where the 19 VAC comes from. I have been told by a tech that there is about 190 volts across l1 and l2 output when the control circuit is turned off. Furthermore I am told when they disconnect the control voltage the 190 VAC goes away

 

[gar]

181024-1518 EDT

buffalonymann:

I'm in total agreement that the control voltage is external. I'm trying to get a handle on what is happening in the circuit - let me give you a bigger picture. We're using these SSRs in our extrusion process, a PLC receives a signal from a thermocouple and pulses the SSR via an AC output module to energize resistance heaters. I suspect my AC output module is leaking current when the node is off and this is where the 19 VAC comes from. I have been told by a tech that there is about 190 volts across l1 and l2 output when the control circuit is turned off. Furthermore I am told when they disconnect the control voltage the 190 VAC goes away

Almost certainly if your three phase Crydom SSR is driven directly from a PLC AC output, then you may have sufficiently high leakage current to produce the 18 V.

I don't know what l1, l2 are (L1, L2), but I suspect these are some lines, but where?

Shunt the input of your SSR with 5000 ohms 5 W, and see if this lowers the 18 V sufficiently. If not go to a 1000 ohm 20 W which should be more than adequate, just wastes power. A better solution is to use a relay output on the PLC.

Since I don't know what L1 and L2 are the 190 V does not mean much, but I expect that these might be a load resistance, part of your heater. Are L1 and L2 two wires to a three phase delta heater?

.

 

[Russs57]

FWIW, I have had something similar before. In my case it was a PLC with triac outputs trying to energize a motor starter coils. This particular motor starter required an input reference voltage for the control circuit. The triac output was about 1.5 volts lower (IIRC) and that wasn't close enough to let motor starter pull in.

I could have made a voltage divider with a pair of resistors to lower input reference the required 1.5 volts. Instead I wired the PLC to an interposing relay's coil and wired reference voltage to common and motor starter coil to N.O. contact. Added plus was said relay had push to test feature and LED to let me know if PLC was calling for MS to be pulled in. Also went this way because I had previous problems with PLC outputs not coping well with inductive loads and needing snubbers across them.

Might be worth the trouble for you. Lets you push a tab on the relay and verify MS and heater are working without PLC command. Lets you see (from relay LED) if PLC is outputting a command when you feel it should.

 

[gar]

181024-2029 EDT

Russs57:

You have a good suggestion.



buffalonymann:

I ran a quick experiment with an OAC5 which is a 5 V DC controlled small SSR.

With a resistive load and 120 V I could get the SSR to turn on during a part of an AC cycle. But the input voltage to this was very touchy. Very difficult to maintain a controlled output state.

I believe the OAC5 uses a TRIAC as the switching element. It may not use exactly the same type of optical coupling as your SSR does.

With a scope what I see is 0, half wave, or full wave ac across the resistive load. This makes sense because trigger sensitivity for a TRIAC is not quite the same for + vs - to the anode.

.

 

[buffalonymann]

181024-1518 EDT

buffalonymann:

Almost certainly if your three phase Crydom SSR is driven directly from a PLC AC output, then you may have sufficiently high leakage current to produce the 18 V.

You are referring to the PLC output module leaking sufficient current? This is what I think is happening as well.

I don't know what l1, l2 are (L1, L2), but I suspect these are some lines, but where?

Yes - L1 and L2 output on the SSR

Shunt the input of your SSR with 5000 ohms 5 W, and see if this lowers the 18 V sufficiently. If not go to a 1000 ohm 20 W which should be more than adequate, just wastes power. A better solution is to use a relay output on the PLC.

I've contemplated a resistor across the control input; my first response is to correct the problem if it can be at a reasonable cost. This system, once heated, also maintains the temp so the output cycles on and off about every second or two, which is probably why they didn't design it with relay output on the PLC module

Since I don't know what L1 and L2 are the 190 V does not mean much, but I expect that these might be a load resistance, part of your heater. Are L1 and L2 two wires to a three phase delta heater?

The heaters I've looked at are single phase. I inherited this issue, not sure if the orginal design was meant to be three-phase delta heaters; that's another issue I need to review.

.

SEE ABOVE

 

[buffalonymann]

FWIW, I have had something similar before. In my case it was a PLC with triac outputs trying to energize a motor starter coils. This particular motor starter required an input reference voltage for the control circuit. The triac output was about 1.5 volts lower (IIRC) and that wasn't close enough to let motor starter pull in.

I could have made a voltage divider with a pair of resistors to lower input reference the required 1.5 volts. Instead I wired the PLC to an interposing relay's coil and wired reference voltage to common and motor starter coil to N.O. contact. Added plus was said relay had push to test feature and LED to let me know if PLC was calling for MS to be pulled in. Also went this way because I had previous problems with PLC outputs not coping well with inductive loads and needing snubbers across them.

Might be worth the trouble for you. Lets you push a tab on the relay and verify MS and heater are working without PLC command. Lets you see (from relay LED) if PLC is outputting a command when you feel it should.

Russ - I'm not making the connection between my issue and yours. I have 3 pole SSRs delivering power to resistive heater bands; I'm told by a tech that when the PLC code commands the PLC output module to turn off control voltage to the SSR, he is finding 190 VAC across the SSR out L1 and L2 and L3. He also claims there is 19 volts on the SSR control input.

 

[gar]

181025-0821 EDT

buffalonymann:

I believe Russs57's point is that if you use your PLC output to control an ordinary electromagnetic relay, such as a P&B KUP, with mechanical contacts on its output, then you can obtain an output contact with very low leakage current compared typical SSRs.

I don't know what l1, l2 are (L1, L2), but I suspect these are some lines, but where?

Yes - L1 and L2 output on the SSR

The Crydom drawing shows one output switch terminal pair as A1-A2, another as B1-B2, and the third as C1-C2. I don't believe any one of these pairs is your L1-L2.

Considering only one SSR contact in the Crydom package, for example A1-A2, then is that contact the switch in a simple single phase series circuit consisting of a 240 V source, the A1-A2 contact, and a load resistor? I am concluding that is what you have said. In that series circuit do L1 and L2 refer to the load resistor terminals? If not, then where in the circuit are L1 and L2?

For a high power load on the output of an SSR the snubber leakage current will produce no significant voltage across that load resistance. If you see a noticeable voltage, but not full voltage, across the load, then the SSR is conducting during a portion of the AC cycle. It appears that your SSR is conducting over some portion of the cycle. In my OAC5 test the SSR had three distinct states --- no conduction, looking like a 1/2 wave rectifier, and full conduction, and very touchy input control voltage to get the 1/2 wave rectification. I can visualize in your case that at 18 V there is some partial cycle conduction, and not so touchy as in the case of my OAC5.

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