It is the control engineer’s version of the “Tabs vs. Spaces” argument.
You are designing a control panel. You have a 24VDC solenoid valve that controls a pneumatic cylinder. You have a spare output on your PLC card.
Do you:
- A) Wire the valve directly to the PLC output?
- B) Install an “interposing relay” between the PLC and the valve?
If you ask this in a room full of engineers (or on the r/PLC forum), you will start a fight.
The “Old Guard” will tell you that driving a valve directly is reckless malpractice that will cost you thousands. The “Modernists” will tell you that adding relays is a waste of space and money that introduces unnecessary failure points.
Both sides are right. And both sides are wrong.
The answer isn’t about dogma; it’s about Risk Management. Let’s break down the “Sacrificial Lamb” debate and find the Golden Rule that should actually guide your design.
1. The Conservative Argument: “The Sacrificial Lamb”
For the maintenance-focused engineer, the interposing relay is non-negotiable. Their logic is built on financial asymmetry.
- The Asset: An Allen-Bradley or Siemens Output Module. Cost: $300 – $500+.
- The Shield: A simple plug-in relay. Cost: $5 – $10.
The Logic:
Solenoid valves are the “bad boys” of the industrial world. They short out. Coils burn up. Cables get crushed by forklifts.
If you connect that valve directly to your $500 PLC card, a single short circuit in the field could blow the internal transistor of that output channel. In a worst-case scenario, it smokes the whole card.
Now you have a down machine at 3:00 AM. To fix it, you have to power down the rack, unscrew the μπλοκ ακροδεκτών, swap the expensive card (if you have a spare), and rewire it.
But if you use an interposing relay? The relay takes the hit. It is the Sacrificial Lamb.
The PLC stays safe. The technician walks up, pulls the burnt relay out of its socket, plugs in a new $5 cube, and the machine is running in 30 seconds.
Pro-Tip: For the “Conservative” engineer, the goal isn’t just protecting hardware; it’s protecting Mean Time To Repair (MTTR). A relay swap is faster than a module swap every time.
2. The Modernist Argument: “The Complexity Tax”
The other side of the table argues that the “Sacrificial Lamb” strategy is outdated thinking from the 1980s.
1. Modern PLCs Aren’t Fragile
Twenty years ago, output cards were delicate. Today? Most quality transistor output modules have built-in electronic short-circuit protection. If a valve shorts, the card detects it, shuts down the channel, and waits for the fault to clear. It protects itself. It doesn’t need a bodyguard.
2. Mechanical Life Limits
A relay is a mechanical device. It has moving parts. It has a life cycle (maybe 100,000 to 1 million cycles).
If you have a high-speed sorting application where a valve fires every 2 seconds, that relay will mechanically fail in a few months.
A PLC transistor output is solid-state. Its theoretical life is infinite. By adding a relay, you are taking a robust system and adding a “wear part” that guarantees future maintenance.
3. The “Complexity Tax”
Every component you add is a potential failure point. Adding a relay means:
- Adding a socket (Space cost).
- Adding 4+ connection points (Loose screw risk).
- Adding extra wire (Labor cost).
The Modernist argument is: “You are buying a false sense of security by paying a tax in space, labor, and complexity.”
3. The “Real” Killer: The Inductive Kickback
While the Conservatives and Modernists argue about how to switch the load, they often ignore what kills the switch.
The real enemy isn’t the short circuit. It’s the Inductive Kickback (Surge).
A solenoid coil is an inductor. When you turn off the power to a magnetic coil, the collapsing magnetic field generates a massive reverse voltage spike. A 24V coil can generate a -500V to -1000V spike in a microsecond.
- If you use a PLC: This spike punches through the transistor junction.
- If you use a Relay: This spike arcs across the contacts, welding them shut or carbonizing them.
The Verdict: It doesn’t matter if you use a relay or not—if you don’t use Surge Suppression (a diode for DC, or an MOV/RC snubber for AC), you are going to kill your switching device.
Pro-Tip: Never install a solenoid valve without a diode (flyback diode) across the coil. Many modern valve connectors (DIN connectors) come with LEDs and diodes built-in. Use them. They are the πραγματικό cheap insurance.
4. The Golden Rule: “Inside vs. Outside”
So, who wins?
The debate was settled by a pragmatic “Golden Rule” that experienced system integrators use. It decides when to use a relay based on location.
Scenario A: The Load is INSIDE the Cabinet
- Examples: Pilot lights, other PLC inputs, small control relays, VFD enable signals.
- Ετυμηγορία: Direct Drive.
- Why: The environment is controlled. No forklifts are driving through your cabinet. The risk of a dead short is near zero. Using a relay here is a waste of money and space.
Scenario B: The Load is OUTSIDE the Cabinet (The Field)
- Examples: Solenoid valves on the machine, motor contactors, horns.
- Ετυμηγορία: Interposing Relay (or Fused Terminal).
- Why: The field is a war zone. Cables get snagged, water gets into junction boxes, and rats chew wires. The risk of a short circuit is high.
- The Compromise: You don’t need a big, blocky “Ice Cube” relay. Use Slimline Relays (like the 6mm ones from Phoenix Contact, Finder, or VIOX). They take up the same width as a terminal block but offer full isolation.
Scenario C: The “High Current” Exception
- Ετυμηγορία: If the solenoid pulls more than 0.5 Amps (or close to the PLC card’s limit), Always Use a Relay.
- Why: Running a PLC card at 90% capacity generates heat and shortens its life. Let a cheap relay handle the heavy lifting.
Summary: Don’t Be Dogmatic, Be Strategic
The “Direct vs. Relay” debate isn’t binary. It’s a sliding scale of risk.
- Assess the Risk: Is the cable running across a factory floor? Relay. Is it two inches away on the same backplane? Direct.
- Suppress the Surge: Diode-clamp every DC coil. No exceptions.
- Save the Space: If you use relays, use the 6mm ultra-slim interface style.
- Consider Fused Terminals: A fused terminal block is a great middle ground. It protects the PLC wire from a field short without the mechanical wear of a relay.
You don’t need to “sacrifice a lamb” for every output. But if you’re sending 24V out into the wild, wild west of the factory floor, it’s nice to have a $5 bodyguard standing in front of your $500 controller.
Τεχνική Σημείωση Ακρίβεια
Πρότυπα & Πηγές Αναφέρεται: Concepts align with NFPA 79 (Electrical Standard for Industrial Machinery) regarding overcurrent protection and separation of control circuits.
Terminology: “Interposing Relay” refers to a relay used specifically to isolate a low-power controller (PLC) from a higher-power or higher-risk load.
Timeliness: Best practices regarding solid-state output protection and slimline relays are current as of November 2025.
