Solid State Relay: Odd Failure

In summary, the conversation discusses an issue with a solid state relay (SSR) in a circuit for testing sensors. The SSR is used as a high side switch and is rated at 100V/40A, but is only being used with a 5V/5A power supply. After a while of operating normally, the voltage measured at the Vcc line starts to decrease, and during the 80 minute cycle, it drops to 0.7V and then to 0.45V in an exponential decay pattern. The problem seems to be with the SSR, as measuring the voltage across the output temporarily fixes the issue. Possible causes discussed include damage to the parts, a misconfigured circuit, a damaged SSR, or
  • #1
Jiggy-Ninja
309
1
I'm not that familiar with the inner workings of solid state relays, and I'm seeing an unusual anomaly in my circuit that I'm not quite sure how to explain.

For my job, I'm running tests on some sensors. One of the requirements for the test is to turn the power to the parts on and off periodically. One full power cycle (ON/OFF) is done every 2 minutes for 50 minutes, then the parts are left completely off for 30 minutes. The whole 80 minute cycle is repeated a specific number of times or until something breaks. To do this switching, I am using a solid state relay as a high side switch on the line coming out of the power supply.

The power is a 5V/5A basic bench top power supply. There are 18 sensors connected to the supply, each consisting on a pair of Hall sensors, 2 opto-isolator channels, and 1 small EEPROM. The SSR is a Crydom D1D40 (100V/40A rating, WAY overspecced for this job). It's DC input and DC output. Crydom website specifies that it has "MOSFET output". The input of the SSR has a 10k-ohm resistor across it, to prevent residual voltage across the input, thought this may be superfluous.

After a while of operating normally, the monitoring equipment starts to measure that Vcc isn't making it all the way up to 5V, and is staying down closer to 4.5V. After a while, it gets a bit more unusual.

At the beginning of the 80 minute cycle (50 minutes ON/OFF cycling), the Vcc line is being measured at only 0.7V!. That level remains stable for about 8-10 minutes, then over the next 40 minutes the level falls to 0.45V in what appears on the graph to be an exponential decay, or something quite similar. The parts then have power removed (SSR turned off) for 30 straight minutes, and the next cycle repeats the pattern EXACTLY (Within margin of measurement error, of course).

If it was just that, I could accept that the SSR was going bad without too much puzzlement. However, there is one final observation.

When I saw that the relay was ON (and giving power to the parts) but the measuring equipment was only recording a value of 0.7V Vcc, I measured across the output of the SSR with a voltmeter (set to voltage, not ohms or continuity; I know better than that), and read just a hair above 0V, which is what it should be if the switch is fully on. When I looked back at the other measuring equipment, it was recording full 5V Vcc! Somehow, just measuring the voltage across the SSR output had gotten rid of whatever gremlin was causing the problem.

Every observation given in this post has been verified with an oscilloscope monitoring the power supply above and below the switch. The voltage level above the relay remains stable, so the power supply does not seem to be causing issue. It's below the relay that the issues appear.

I have set up a similar testing circuit like this several times before, so it is not completely new to me. I have also double and triple checked the polarities of the input and output; they are correct. I have also done several other tests on the same parts using the same equipment; this is the first time I have ever seen this issue.

Has anyone ever had a similar problem using an SSR? The evidence seems to point clearly to the SSR as the root of the problem, and I have replaced it in the test circuit. However, I am responsible for running this test and determining if the parts pass or fail. I have already had one problem where my original reasoning led me to believe it was part failure, but further in-depth troubleshooting showed it was actually an error with our custom made and programmed testing boards. I wish to have a deeper understanding of this issue before I write it off as just equipment failure.

What kinds of things could cause these symptoms?

1) Damage to one or more of the parts? They are taking significant abuse (mostly thermal) to determine their robustness. Could failures of one or more sensors cause issues?
-----Point against this, all the parts are showing identical symptoms at the same time. This suggests that something external and common to all the parts is causing the problem, ie the SSR.
2) Misconfigured circuit? I've checked the wiring a few times, but there's still the possibility of overlooking something or some other bone-headed mistake.
3) Damaged SSR? What kind of damage would cause this? And why would appear to fix itself (at least temporarily) when measured with a voltmeter?
4) Something else that I'm overlooking entirely?

Those that have read this far, thank you. :)
 
Last edited:
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  • #2
"I measured across the output of the SSR with a voltmeter (set to voltage, not ohms or continuity; I know better than that), and read just a hair above 0V, which is what it should be if the switch is fully on."
Sounds like you have an open circuit and when the capacitance charges up something switches.
After checking the circuit, you may need to add a resistor where the voltmeter was.
 
  • #3
Capacitance of what? The sensors do not have any significant amount of resistance. Do you mean the capacitance of the DMM? Or internal capacitance in the SSR somewhere?

What is the "something" that switches, and why wouldn't it switch before? Why immediately when a DMM is hooked up?
 
  • #4
Double check the specs on your SSR, there may be a minimum turn on voltage. I would not be comfortable running a 100V SSR at 5V. Would recommend that you get a SSR speced at 30V or less.

Good luck.
 
  • #5
Integral said:
Double check the specs on your SSR, there may be a minimum turn on voltage. I would not be comfortable running a 100V SSR at 5V. Would recommend that you get a SSR speced at 30V or less.

Good luck.
100V is the maximum voltage on the output.

The input range is 3.5-32V, so 5V is okay.

The problem hasn't shown up since I replaced the relay (though it has only been 1 day), so it seems that was what was bad after all. I would still like to know if anyone has more information about these symptoms, so I know what to look out for in the future.
 

Related to Solid State Relay: Odd Failure

What is a Solid State Relay?

A Solid State Relay (SSR) is an electronic switching device that uses solid-state components, such as transistors and thyristors, to control the flow of electricity. It is commonly used in place of traditional electromechanical relays due to its faster switching speed, longer lifespan, and quieter operation.

How does a Solid State Relay work?

A Solid State Relay works by using a control signal, typically from a low voltage source, to trigger a semiconductor device, which then allows current to flow through the relay's output circuit. This process is known as opto-isolation, as the control signal and output circuit are electrically isolated from each other.

What are some common failures of Solid State Relays?

Some common failures of Solid State Relays include short circuits, open circuits, and thermal failures. These failures can be caused by overvoltage, overcurrent, or overheating, and can result in the SSR not functioning properly or completely failing.

What is an "odd failure" in a Solid State Relay?

An "odd failure" in a Solid State Relay refers to a failure that is unexpected or uncommon. This could include failures that do not fit into the typical categories, or failures that occur due to unusual circumstances or conditions.

How can odd failures in Solid State Relays be prevented?

To prevent odd failures in Solid State Relays, it is important to carefully select the appropriate SSR for the application, ensuring that it can handle the voltage and current requirements. Proper installation, regular maintenance, and monitoring for any unusual behavior can also help prevent odd failures. In some cases, adding protective measures such as fuses or thermal management systems may also be necessary.

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