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DIY Galvanically Isolated Solid State Relay for mains power controlled by 5..20mA current loop

Many of articles on this site might look trivial. So they are. But this is the result of my personal experience: I got tired catching myself re-inventing the wheel again and again. Thus I decided to post whatever proved to be useful for me at least once, in the hope that it might save some time to myself and others in the future.The industry makes it all for you. One can purchase a Solid State Relay (SSR) that would fulfill virtually any imaginable requirements. The only question is money: a decent device capable of handling mains power load of few hundred Watts would cost as from $10 and up. While if we lower the bar and look for an SSR capable of switching say 100mA - it would be possible to get one for less than a dollar.Here is a very simplistic (and relatively inexpensive) SSR implementation that is capable of managing heavy loads. It uses a TRIACas the main power switch and a "telephony" SSR controlling it.

Powerful Solid State Relay with opto-isolated input

"Telephony" SSR = LH1500 (datasheet)


The so-called "snubber circuit" is there in order to avoid an accidental opening of the TRIAC in case of a very steep voltage raise across it (nearly impossible scenario in usual applications like controlling lights and such). But let's keep it anyway - it also reduces EMI. Astute readers are welcome to follow an excellent Application Note AN-3008 RC Snubber Networks for Thyristor Power Control and Transient Suppression by Fairchild Semiconductor.
To be precise we should have added some sort of a transient voltage surges suppression device too, something like a MOV or a power Zener. But usually TRIACs tend to act as crow-bars on their own too. And let's assume we are working behind some mains protection already.

The control circuitry can vary widely. It could be as simple as a TTL or CMOS logic output with the current limiting resistor in series.

Simple driving circuit for opto-isolated SSR

In a rather serious installation with potentially long cables I used the current source controlled by the MCU. LED is there for convenience, without it the current setting resistor should be replaced by 430 Ohm in order to obtain the CCS current around 10mA.

Constant-current driving circuit for SSR

To make my life simpler a bit while wiring a big setup I used diode bridges in order to allow twisted pairs to be twisted 🙂 The additional resistor protects the LED inside the SSR from the overcurrents in case of accidental application of DC voltage to the control wires. Due to the voltage drop across the protection resistor and two diodes in the bridge this variation asks for a relatively high supply voltage (12V or so) and potentially the CCS, for example the one described above.

Foolproof opto-isolated Solid State Relay

Attention: SSR circuits described in this article are NOT SUGGESTED FOR AUDIO. In fact it should not be used anywhere in proximity of a high fidelity audio reproduction system. Despite the snubber circuit and relatively low "on" voltage threshold it still produces distortions very unwanted in audio. By the way the same restriction applies to almost any (costly) industrial SSR that deploy thyristors or triacs.

Below is the citation from the LH1500 document by Siemens:
Application:   Motor, Light, Heat, Solenoid Control
Equipment:    Industrial Controls, Programmable Controllers, Factory Automation Equipment, Appliances

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