Electronic Design

Solid-State Relay Features Load Indicator

Several years ago, I designed a remote energy-control system that split a building into sections and wired the overhead space heaters into "zones." As space was an issue and OAC5s (optically isolated output modules) were way too big, each energy control panel controlled the heaters using homemade discrete solid-state relays (SSRs). While the overall design was fine, a problem arose at 3 a.m. one night. It seemed there wasn't any heat in part of one zone. Someone had flipped off the breaker feeding power to that heater. Apparently it's a common mistake when shutting lights off from a breaker panel. As winter progressed, so did the annoying 3 a.m. calls. A solution had to be found.

Out came the breadboard. After several tries, I came up with a load-indicating SSR. The first idea was to have an LED in the load line with six diodes (1N4004s) connected in series-parallel (see part a of the figure). While it worked in the lab, it failed in the real world. The added voltage drop of about 2 V led to erratic behavior of the gas valves. The worst case was during brownout conditions and long control wires (typically 100 to 200 feet). Back to the breadboard.

The second version presented here, which eliminates the unacceptable voltage drop, has worked for over 15 years in hundreds of panels. While my application involved 24-V ac gas valves and relays, this circuit is easily modified to suit your application.

As shown in part b of the figure, there's a red LED and reverse 1N4004 diode in series-parallel between pin 6 of the MOC3011 optotriac driver and the MT2 pin of the 2N6075A power triac. Also, there's a 10-(omega) resistor from the junction of pin 4 of the MOC3011 and gate of the power triac to the MT1 pin of the power triac. This arrangement of components minimizes the voltage drop of the loop to just the on-state voltage of the triac. That's great news, considering space heaters can easily have 100 to 200 feet of control wire!

Obviously, component values will need to be changed to suit your application—perhaps an R-C snubber across the triac, although I've never needed it. Using a zero-crossing optocoupler might also be required. While not a complicated circuit by any means, the minor expense of an LED, diode, and resistor has eliminated those annoying phone calls and gives peace of mind to both my clients and me.

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