Interposing relay or direct drive: will your PLC relay output survive the contactor coil?

Every panel I inherit falls into one of two camps. Either there is an interposing relay between every PLC output and every coil in the cabinet, pilot lights included, or there are none at all and the contactors hang straight off the output card. Both camps are sure they're right. Usually neither one did the math.

And the math is short. A contactor coil has two numbers, pickup and hold-in. A relay output has one rating, written in one of two languages. Line them up and you know whether an interposing relay earns its spot, plus the thing nobody asks about: how long the output contact is going to last.

A coil pulls two currents, and the contact sees both

An AC coil energizes with the magnet still open, so it grabs a slug of current until the armature seals, typically 30 ms or less. Then it drops to hold-in. The output contact has to make the big number and break the small one.

An ordinary clamp meter won't catch the inrush. It is over before the meter settles, and the hold-in reading looks harmless. Use the coil data.

Pickup and hold-in for common contactor coils. On the 100-C the ratio climbs with frame size.

The "about ten times" everybody quotes only holds in the middle. On one product line it runs from under 8 to almost 17, worst on the big frames.

Pilot duty and AC-15: one relay, two ways to rate it

Relay ratings come in two languages. A code like B300 or C300 on the side of a relay or output card is NEMA pilot duty. AC-15 with a current or VA figure is IEC.

In pilot duty the letter sets the current class and the number is the maximum voltage. Each letter comes with a make VA and a break VA, and the check is direct: pickup at or under make, hold-in at or under break.

AC-15 is the category for contactor coils, valves and magnets. The rated current is what the contact breaks, and the category assumes the load closes at ten times that. Small magnet loads get AC-14, which assumes six times. So on the IEC side my check is this: take the hold-in VA, take one tenth of the pickup VA, and whichever is bigger has to fit under the AC-15 figure.

Same contact, two ratings. Only the IEC side leaves the make number for you to work out.

One trap on 24 V AC control: don't divide the make VA by 24. Below 120 V the make current stays at its 120 V value, and break current is break VA over the actual voltage, capped at the continuous current. So a B300 contact on 24 V AC is good for 30 A make and 5 A break, not the 150 A the division gives you.

Same four coils, two PLC outputs

Take two relay outputs I run into constantly. The ControlLogix 1756-OW16I is pilot duty C300, good for one operation every 3 seconds at rated load. On the Modicon M221 side, a TM221C16R has 2 A relay outputs, and for inductive loads at 120 V AC its AC-15 figure is 60 VA for 100,000 operations or 18 VA for 300,000, with no more than 20 operations a minute at maximum load.

For the TM221C16R, the VA to fit is the larger of hold-in and one tenth of pickup.

On paper the ControlLogix card takes all four. The M221 passes three and drops the CA6-180, and not because of hold-in: 50 VA is under 60. It's the pickup. 650 VA is more than ten times the rating.

A CA6-180 coil against a TM221C16R relay output at 120 V AC. Hold-in fits under the 60 VA line, but pickup is more than ten times that.

Put a 700-HLT relay between them and the picture changes. Its contact is B300, 3600 VA make and 360 VA break at 120 V, so the CA6-180 coil is a light load for it, and the M221 output now switches a 0.2 W relay coil.

The column that actually decides it is life, though. Say a booster pump cycles 40 times an hour, around the clock. That's 350,400 operations a year. An output in the 100,000 class burns through its rated life in about three and a half months, and even the 300,000 class is done in under a year. On a brick controller like the M221 those relays are built into the controller. One wears out, you're rewiring to a spare output or buying a PLC.

A plug-in relay is a consumable. An embedded output is not. That, more than any single rating, is why I put an interposing relay in front of anything that cycles hard.

Direct drive or interposing relay: the checks I run

Five checks, and any one in the right column is enough.

The noise row isn't theoretical. An AC contactor coil throws high-frequency interference when it lets go, and on an M221 that can show up as an I/O bus error. For TM3 relay expansion outputs driving AC contactors, Schneider wants an RC suppressor or an interposing relay on every one of them.

Voltages bite on compact controllers. On the TM221C16R, Q0 to Q3 share one common and Q4 to Q6 another, so a 24 V DC valve and a 120 V AC coil can't sit in one group. The 1756-OW16I isolates every output, and that reason goes away.

Big DC contactors are their own story. A CA6-180 with a conventional 24 V DC coil pulls 540 W at pickup, about 22 A. A 700-HLT is rated 1 A at 24 V DC. For that coil the interposing relay has to be a CA7-30 contactor or bigger.

When an interposing relay is the wrong answer

Electronic coils changed this. A CA6 contactor with an EI electronic coil takes a 24 V DC, 15 mA signal straight from a PLC output. Switch that coil's supply at A1 through an interposing relay instead, and the electronics can pull 40 A for 1 ms and 20 A over 20 ms, about 80 ms in all before they settle. The manufacturer recommends against it, and I agree. The relay you added to protect the PLC ends up taking a hit the PLC output never would have.

On the 100-C, Allen-Bradley's answer is coil code EJ, a 24 V DC electronic coil with an integrated diode. A 100-C09 with it picks up on 10 W average, 17 W peak, and holds in on 1.7 W, around 70 mA. Check that peak against a transistor output before you skip the relay. A 100-C with a 110 to 240 V AC coil can take a 100-JE instead: 18 to 30 V DC in from the PLC, AC out to the coil, no additional coil suppression needed. It does the interposing job, but it mounts on the contactor and needs no extra suppressor.

The suppressor goes at the coil

Direct or interposed, the coil still kicks when it lets go. Whatever you use goes across A1 and A2 at the coil, not at the PLC terminal, and that includes the interposing relay's own coil.

Sizing rules for the three common coil suppressors.

Direct drive, an interposing relay, or an interface on the contactor. Wherever a suppressor is needed, it sits across the coil.

Interposing relay questions I get asked

What is an interposing relay?

A relay between the PLC output and the real load. The output drives only the relay coil, 0.2 W on a 24 V DC 700-HLT, and the relay contact takes the inrush, the arc and the wear. It also keeps the PLC's output voltage apart from the load's.

Can a PLC relay output drive a contactor coil directly?

Often, yes. The coil has to fit the output's make and break numbers, the contact life has to cover how often the contactor cycles, and the coil gets a suppressor across it either way.

What does C300 mean on a relay output?

NEMA pilot duty. C is 2.5 A continuous with 1800 VA make and 180 VA break, 300 is the maximum voltage. At 120 V AC that works out to 15 A make and 1.5 A break.

Do I need an interposing relay for an electronic coil contactor?

Depends on the coil. A CA6-EI takes a 15 mA signal, so the output can drive it directly. A 100-C with an EJ coil peaks at 17 W, so check that against the output first. Either way, switching an electronic coil's supply through a relay loads that contact far harder than the PLC output ever was.

Parts for either answer

Run the three numbers before ordering anything: pickup, hold-in and the output's rating. If the coil fits and the load barely cycles, skip the interposing relay and keep the panel space. If it doesn't, or the machine cycles hard, we stock both sides of it: the 700-HLT1Z24 and 700-HLT1U24 terminal block relays, the 1756-OW16I isolated relay output module, 100-C09 contactors including the 100-C09EJ01 with a 24 V DC electronic coil, and the 100-FSC280 RC suppressor for 110 to 280 V AC coils on the 100-C.