What I thought was a simple AC circuit with a Refrigerator & SSR

In summary, a timer switch was created that turns off an appliance for a set amount of time. The switch works for a short time on the larger refrigerator but then something continuously cycles on and off. The ground and the box are warm but not hot. There is something Circuitly wrong with the refrigerator's inductive load. A switch in series with the Hot lead, right after the fuse, is suggested to fix the issue.
  • #1
nickmanc86
10
0
I am half way through my Electrical Engineering program and have only just started to learn about AC circuits and inductive loads but I did a little reading and went off to try a simple project on my on. A company I work for wanted a switch to turn off their large refrigerator for certain periods of time. Most countdown timers you can buy will turn something ON for the set amount of time. I needed something that turned OFF for a set amount of time and ON the rest of the time. I created a timed switch using easy to obtain components:

A Normally Closed SSR (SNC-R2025-511)
http://www.alliedelec.com/images/products/datasheets/bm/NCC/70059698.pdf
An off the shelf timed switch (Sylvania SA 160)
http://www.smarthome.com/56217/Sylvania-SA-160-Dual-Mode-24-Hour-60-Minute-Digital-Auto-Shut-Off-Timer-20-Amp/p.aspx
The Refrigerator

I can't get much info on the Sylvania part other than the 120VAC, 20A rating for inductive and resistive loads and the fact that it is a SPST device. I assume its just essentially another SSR.

Included are two circuit diagrams of my box and of the refrigerator and one picture of the actual device. Sorry if they are messy and/or incorrect.

Anyway I put the timer and SSR into a box with a male and female plug. I made two of these for two different refrigerators. One a smaller 6A, 120VAC, 60Hz and the other a 9.6A, 120VAC, 60Hz model. They both work fine on the smaller fridge and other small appliances.However, on the larger fridge they will work for a little while and then something (perhaps the SSR) continuously cycles on and off. It will remain on for 15 seconds or less then off briefly then on again. The relay and the box are warm but I wouldn't say hot and the refrigerator is well within the acceptable load range of the device. Is there something I am missing here. Is there some kind of blowback voltage from the refrigerators inductive load causing the issue? Do I need to isolate the timer or install a protection circuit? Is it just heat? I know I must be overlooking something really simply hahaha, any help would be appreciated.

Also sorry if the formatting of my question is incorrect this is only my second time posting here.
 

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  • #2
I'm not of any help on the operation questions, but how are you handling the safety/Earth ground? And to avoid a fire hazard, you probably should have an input fuse in the Hot lead right as it enters your junction box.
 
  • #3
Ya sorry the diagram was not explicit about the ground. Both ends are typical 3 prong Male/Female (respectively) plugs. The ground lead from the male plug, which plugs into the outlet, runs into and is attached to a grounding screw in the box. The same goes for the female plug which runs to the fridge. The ground wire from the fridge plug goes into the female plug which runs into the box and grounds on the same screw as the male side. Is that okay/safe? The fuse is an excellent idea, I will have to add that in.
 
  • #5
nickmanc86 said:
Ya sorry the diagram was not explicit about the ground. Both ends are typical 3 prong Male/Female (respectively) plugs. The ground lead from the male plug, which plugs into the outlet, runs into and is attached to a grounding screw in the box. The same goes for the female plug which runs to the fridge. The ground wire from the fridge plug goes into the female plug which runs into the box and grounds on the same screw as the male side. Is that okay/safe? The fuse is an excellent idea, I will have to add that in.

The grounding sounds reasonable. I'd add a switch in series with the Hot lead, right after the fuse. With the fuse and switch, you are coming close to being UL-approvable...

I like dlgoff's suggestion as well. :smile:
 
  • #6
Do you have any heatsink besides the 4x4 box? The specs for free air use are a maximum current of about 7.5 amps. For any current above that you should use a heatsink. I don't think you have enough contact with the box (and didn't use thermal compound). The specs call for a 1/8" 6" x 6" as the next size from free air (i think if you would use a 6"x6"x4" box with no knock outs where you are mounting the ssr, or as a test try putting a fan blowing on the 4"x4" box with the cover off to see if it works longer before cycling).
I would try the fan first before investing in any more parts.
 

What is an AC circuit and how does it work?

An AC circuit is a type of electrical circuit that uses alternating current (AC) to power devices. Alternating current is a type of electrical current that flows back and forth in a circuit, constantly changing direction. This is in contrast to direct current (DC) which flows in only one direction. In an AC circuit, the current alternates between positive and negative values, creating a sinusoidal waveform. This type of circuit is used in most household and commercial electrical systems because it is more efficient for long-distance power transmission.

What is the role of an SSR in an AC circuit with a refrigerator?

A Solid State Relay (SSR) is an electronic switch that uses solid-state components to control the flow of electricity. In an AC circuit with a refrigerator, the SSR acts as a switch to turn the refrigerator on and off. This is important because the compressor in a refrigerator requires a lot of power to start, and an SSR can handle this high current without wearing out like a mechanical relay would. The SSR also helps to protect the refrigerator from power surges and fluctuations.

Why is it important to have a dedicated circuit for a refrigerator?

A dedicated circuit is a separate electrical circuit that is used exclusively for one device, such as a refrigerator. It is important to have a dedicated circuit for a refrigerator because refrigerators require a lot of power to operate, and sharing a circuit with other devices can cause overloading and tripped breakers. A dedicated circuit also helps to ensure that the refrigerator has a consistent and reliable power supply, which is important for preserving food and preventing damage to the appliance.

What are some common issues that can arise in an AC circuit with a refrigerator and SSR?

Some common issues that can arise in an AC circuit with a refrigerator and SSR include power surges, faulty wiring, and SSR failure. Power surges can damage the SSR and other components in the circuit, while faulty wiring can cause the SSR to malfunction or fail. It is important to regularly check the wiring and connections in the circuit to ensure they are in good condition. If the SSR fails, it will need to be replaced in order for the refrigerator to function properly.

How can I troubleshoot problems in an AC circuit with a refrigerator and SSR?

If you are experiencing issues with your AC circuit and refrigerator, the first step is to check the power supply and make sure the circuit breaker has not been tripped. If the power supply is fine, then you can troubleshoot the SSR by checking the connections and testing the device for proper functioning. If the SSR is functioning correctly, then the issue may be with the refrigerator itself and you may need to consult a professional for repair.

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