Galvanic isolation allows a control signal to cross between two circuits without a direct conductive connection. In a conventional solid-state relay, this transfer is commonly handled by an optocoupler: an LED on the input side emits light, and a light-sensitive semiconductor on the output side detects it.
That principle is well established, but it is not the only way to build an isolated industrial interface relay. Delcon uses a pulse transformer to transfer the switching command across the isolation barrier. The distinction matters because the two approaches obtain and process switching energy differently.
How an optocoupler transfers a signal
An optocoupler converts the electrical input into light and then back into an electrical signal. The isolation barrier is optical, so the input and output circuits remain galvanically separated. Performance depends on the complete relay design, including LED drive, detector characteristics, output-stage supply, threshold circuitry and protection components.
Engineers should therefore evaluate an optocoupler relay as a complete device rather than treating the word “optocoupler” as a guarantee of noise immunity or load compatibility. Important questions include minimum load, off-state leakage current, input thresholds, behaviour with long cables and response to transients on the load circuit.
How Delcon pulse-transformer isolation differs
In Delcon plug-in solid-state relays, the control command is transferred as pulses through a transformer. Product-specific datasheets identify the insulation method as a pulse transformer and specify the tested input-to-output isolation. For example, the SLO24CR DC output relay and SLI24CR input relay specify a 4,600 V AC RMS / 1 s input-to-output test voltage, 8 mm air/creepage distance, overvoltage category III and pollution degree 2.
The practical result is a relay architecture designed specifically for industrial signal interfacing. Delcon datasheets also specify input hysteresis and product-dependent protection networks. These details are relevant when control cables are long, cable capacitance is significant or nearby equipment produces electrical interference.
What to compare in a real application
- Isolation rating: use the value and test conditions in the exact product datasheet.
- Input thresholds and hysteresis: check both switch-on and switch-off values, not only the nominal voltage.
- Minimum load and leakage current: these can affect sensitive PLC inputs and low-current loads.
- Transient protection: select a relay and external suppression method that match the actual AC or DC load.
- Temperature and mounting: follow the product derating curve and installation instructions.
Isolation technology is only one selection criterion. The load voltage, continuous current, inrush current, inductance or L/R value, switching frequency, ambient temperature and required approvals must all be verified. A pulse-transformer relay should not be selected solely because its nominal voltage matches the application.
Where pulse-transformer relays are especially useful
The architecture is particularly relevant in PLC and DCS interfaces exposed to long field cables, electrical noise, leakage current or demanding switching duty. Typical applications include solenoid valves, contactor coils, signal conversion and galvanic separation between field devices and controller I/O.
See Delcon pulse-transformer technology for the design principle, then use the exact product datasheet to confirm every electrical limit. If you provide the control voltage, load type, load voltage, current, inrush and switching frequency, Delcon can help identify a suitable relay.