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SSR vs Mechanical Issue

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BridgeSparky

Member
Location
USA
Occupation
Electrician
Have a control cabinet that controls road signs for lane closures and cautions. Has a green, red, yellow LED output. 120v input voltage from PLC controller. So the issue is the SSR we swapped over to due to the mechanical relays sticking due to inrush voltage and they had to be flicked to stop the double sign ghosting. Now experiencing and just this one gantry a flickering, strobing effect using the SSR. They worked fine for awhile, however now they have a line side voltage of the 120v but load side voltage of about 40v when the wire to the sign is landed on the load side of the SSR. However when you disconnect the wore from the sign on the SSR the load side goes back to 120. We also swapped out back to the old mechanical relay and had no voltage issues on the load side as it was showing 120v. Not sure if it's a cable/connection issue or relay issue.
 
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BridgeSparky

Member
Location
USA
Occupation
Electrician
petersonra said:
Ssr do not have a completely open state like a relay does. You will always have some leakage.
Click to expand...
But 80v of leakage seems high. Comparing to the other gantries with the same set up and control box with the SSR installed we get 120 on both sides. Just got done troubleshooting one of the signs. When disconnecting the controller in the sign the voltage on the SSR reads 112v, however when you hook the controller from the sign back up it drips down to 40v. We tried the old mechanical relays doing the same theory and still had 120v both ways.
 
petersonra

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
BridgeSparky said:
But 80v of leakage seems high. Comparing to the other gantries with the same set up and control box with the SSR installed we get 120 on both sides. Just got done troubleshooting one of the signs. When disconnecting the controller in the sign the voltage on the SSR reads 112v, however when you hook the controller from the sign back up it drips down to 40v. We tried the old mechanical relays doing the same theory and still had 120v both ways.
Click to expand...
Look at the specified leakage current of the ssr. Add a dropping resistor across the output that will allow the leakage current to all flow thru the resistor. It is also possible the load characteristics are different or even that a different ssr was used.

These are common issues with ssr and you have to learn to deal with them if you use them in these type of applications.
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
BridgeSparky said:
Have a control cabinet that controls road signs for lane closures and cautions. Has a green, red, yellow LED output. 120v input voltage from PLC controller. So the issue is the SSR we swapped over to due to the mechanical relays sticking due to inrush voltage and they had to be flicked to stop the double sign ghosting. Now experiencing and just this one gantry a flickering, strobing effect using the SSR. They worked fine for awhile, however now they have a line side voltage of the 120v but load side voltage of about 40v when the wire to the sign is landed on the load side of the SSR. However when you disconnect the wore from the sign on the SSR the load side goes back to 120.
Click to expand...
BridgeSparky said:
But 80v of leakage seems high. Comparing to the other gantries with the same set up and control box with the SSR installed we get 120 on both sides. Just got done troubleshooting one of the signs. When disconnecting the controller in the sign the voltage on the SSR reads 112v, however when you hook the controller from the sign back up it drips down to 40v. We tried the old mechanical relays doing the same theory and still had 120v both ways.
Click to expand...

From that description the problem is 80 volts of drop across the SSR when it's supposed to be turned ON, and not leakage current when it is turned OFF. Your mechanical relays sticking is likely due to a high inrush current when they are turned ON. That inrush current may have damaged the SSR which is having the problem.

What kind of lighting does the sign have? For example, LED lights could have a driver with an internal rectifier that draws a high peak current when it's first turned ON to charge internal capacitors. The transformer used in magnetic low voltage lighting can have a large inrush current. Incandescent lights of course can have relatively high inrush currents.

Here are some different types of SSRs :
A common "random turn-on" type of SSR will turn on immediately as soon as its input is enabled, which could happen anywhere during a cycle of the AC voltage waveform.

A zero-cross SSR will turn on at the first zero-crossing of the AC waveform after the SSR input is enabled. This can significantly reduce the worst-case peak current when capacitive loads are turned ON, such as with some LED drivers. However, they would be bad for inductive loads such with magnetic low voltage lighting, because by turning on at ~0V they have a large initial voltage ramp that could saturate a transformer and cause a high inrush current.

Peak switching SSRs can help with magnetic loads, but they would be the worst kind for capacitive loads as far as inrush currents. They are less commonly available and tend to be more expensive.

As Tom ( ptonsparky ) mentioned more heavy duty relays would help, such as lighting contactors.
And if you use a SSR, use ones with a high enough current rating to handle the inrush current.
 
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BridgeSparky

Member
Location
USA
Occupation
Electrician
synchro said:
From that description the problem is 80 volts of drop across the SSR when it's supposed to be turned ON, and not leakage current when it is turned OFF. Your mechanical relays sticking is likely due to a high inrush current when they are turned ON. That inrush current may have damaged the SSR which is having the problem.

What kind of lighting does the sign have? For example, LED lights could have a driver with an internal rectifier that draws a high peak current when it's first turned ON to charge internal capacitors. The transformer used in magnetic low voltage lighting can have a large inrush current. Incandescent lights of course can have relatively high inrush currents.

Here are some different types of SSRs :
A common "random turn-on" type of SSR will turn on immediately as soon as its input is enabled, which could happen anywhere during a cycle of the AC voltage waveform.

A zero-cross SSR will turn on at the first zero-crossing of the AC waveform after the SSR input is enabled. This can significantly reduce the worst-case peak current when capacitive loads are turned ON, such as with some LED drivers. However, they would be bad for inductive loads such with magnetic low voltage lighting, because by turning on at ~0V they have a large initial voltage ramp that could saturate a transformer and cause a high inrush current.

Peak switching SSRs can help with magnetic loads, but they would be the worst kind for capacitive loads as far as inrush currents. They are less commonly available and tend to be more expensive.

As Tom ( ptonsparky ) mentioned more heavy duty relays would help, such as lighting contactors.
And if you use a SSR, use ones with a high enough current rating to handle the inrush current.
Click to expand...
Very informative. Thank you, we used other known SSR from other gantries that work just fine just to compare. We also used a spare sign and ran a piece of MC from the load side of the SSR terminal block and directly from the line side voltage to the entire cabinet, bypassing everything. Here is a picture of the SSR we are using for the entire application of every gantry on the bridge.
 

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BridgeSparky

Member
Location
USA
Occupation
Electrician
These are LED signs, every sign is controlled by a 3 wire PLC driven independent initiative input going to (red/green/yellow) directly to the corresponding relay under said 120v bridged 3 termainal blocks (red,green,yellow)(for a single lane) with a single nutrual headed out to one sign, each sign has it's own neutral which ties into all 3 signals/spliced together. Each signal has its own LED controller which is made up of two separate controller's for redundancy, which then leads to a 24v input on to the load side of the controller and then to the board, one operating as primary the other acting as back up for each input (color)
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
BridgeSparky said:
But 80v of leakage seems high. Comparing to the other gantries with the same set up and control box with the SSR installed we get 120 on both sides. Just got done troubleshooting one of the signs. When disconnecting the controller in the sign the voltage on the SSR reads 112v, however when you hook the controller from the sign back up it drips down to 40v. We tried the old mechanical relays doing the same theory and still had 120v both ways.
Click to expand...
It's not really leaking 80V, it actually "leaks" the full voltage of the circuit. I've had to deal with this issue with 480V soft starters for years; people will put a DMM on the load side and read 480V, it freaks them out. But it only reads that because a DMM has no burden to it. If you read it with a Wiggy or Knopp inductive tester, it reads zero. The problem is, "kids" in our industry don't carry those any more, they all have digital meters only.

The leakage CURRENT is very low, as in milliamps, maybe even microamps. The problem with LEDs is that in the efforts to save energy, the "driver" circuit for the LEDs requires so little energy that it actually charges up from this tiny amount of leakage through an SSR.
 
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BridgeSparky

Member
Location
USA
Occupation
Electrician
Jraef said:
It's not really leaking 80V, it actually "leaks" the full voltage of the circuit. I've had to deal with this issue with 480V soft starters for years; people will put a DMM on the load side and read 480V, it freaks them out. But it only reads that because a DMM has no burden to it. If you read it with a Wiggy or Knopp inductive tester, it reads zero. The problem is, "kids" in our industry don't carry those any more, they all have digital meters only.

The leakage CURRENT is very low, as in milliamps, maybe even microamps. The problem with LEDs is that in the efforts to save energy, the "driver" circuit for the LEDs requires so little energy that it actually charges up from this tiny amount of leakage through an SSR.
Click to expand...
So what would suggest the next step be?
 
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