16V Intercom and 12V Card Reader Diode Isolation for Door Strike

User question

What size diode and type to use for two different voltages of 16 volt intercom and a 12vokt key card reader that unlocks the intercom ‘s 12volt door strike with out shorting either the intercom or card reader when either access component is triggered

Artificial Intelligence Response

Direct answer to the question

Do not connect the 16 V intercom output and the 12 V card-reader output directly together.
The correct solution depends on what the intercom output actually is:

• If the intercom output is 16 V AC or 16 V DC power: use an isolation relay, not just a diode.
• If both outputs are DC and both are actually powering outputs: you can use a diode-OR arrangement.
• If either device provides a dry contact relay output: use the contacts to switch a single 12 V supply to the strike. That is the best practice.

Practical recommendation

For most access-control systems, the safest and most professional method is:

• Use the card reader’s 12 V supply to power the 12 V door strike
• Use the intercom output only to energize a relay
• Let the relay contacts switch the same 12 V strike circuit
• Add a flyback diode across the strike coil if the strike is DC

If you insist on diode isolation and both sources are DC

Use:

• Two series isolation diodes, one from each source to the strike
• Prefer 1N5404 to 1N5408 class diodes, not small signal diodes
• Add a flyback diode across the strike: 1N4007 minimum, preferably sized similar to the strike current

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Detailed problem analysis

Your problem has three distinct electrical issues:

1. Backfeeding

   • When the intercom activates, you do not want its voltage feeding backward into the card reader.
   • When the card reader activates, you do not want its voltage feeding backward into the intercom.

2. Voltage mismatch

   • Your strike is 12 V
   • Your intercom is 16 V
   • Your card reader is 12 V
   • A 12 V strike usually should not be driven directly from 16 V for long periods

3. Inductive kick from the strike

   • The electric strike coil is an inductive load
   • When switched off, it generates a reverse voltage spike
   • That spike can damage relay contacts, reader outputs, or diodes if not suppressed

Why a simple diode answer is sometimes incomplete

A diode only solves reverse-current isolation.
It does not solve:

• AC versus DC incompatibility
• overvoltage to a 12 V strike
• relay-contact versus powered-output differences
• coil kickback suppression by itself, unless separately installed as a flyback diode

That is why many quick answers saying “just use 1N4007” are only conditionally correct.

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Current information and trends

In practical access-control work, the preferred architecture is:

• one regulated supply for the lock/strike
• multiple control devices connected through dry contacts, relay isolation, or controller inputs
• suppression devices across inductive loads
• avoidance of directly combining two different powered outputs

Current field practice strongly favors:

• using relay isolation when voltages differ
• using 3 A rectifier diodes instead of 1 A parts when driving locks or strikes
• checking whether the intercom lock release is 16 VAC, which is common in older systems

This matters because a 16 VAC intercom release output is fundamentally different from a 12 VDC reader output.

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Supporting explanations and details

1. Best solution: relay isolation

If the intercom output is 16 VAC or 16 VDC, and the strike is 12 VDC, do this:

• Power the strike from the 12 VDC supply
• Use the card reader output to switch the strike normally
• Use the intercom output to energize a relay coil
• Use the relay’s NO/COM contacts in parallel with the card reader release contact

Conceptual wiring

Preferred arrangement:

• 12 VDC supply powers strike
• Card reader relay contact closes to unlock strike
• Intercom energizes separate relay
• That relay contact also closes to unlock strike

So both devices act like switches, but only one voltage source actually powers the strike.

Why this is better

• No backfeed path
• No 16 V applied to a 12 V strike
• Works whether intercom is AC or DC
• Cleaner and more reliable

2. If both outputs are DC power outputs: diode-OR method

If, and only if, both outputs are DC and both are intended to source power to the strike, use two diodes:

• one diode from intercom output to strike +
• one diode from card-reader output to strike +
• diode cathodes tied together at the strike positive terminal

Orientation

The banded end of each diode goes toward the strike.

Schematic

Intercom +16VDC ----|>|----+
D1 |
+---- Strike +
Card Reader +12VDC -|>|----+
D2
Strike - -------------- common return / negative

This blocks one source from feeding into the other.

3. Diode type and size

For a door strike, I would recommend:

Better choice

• 1N5404 / 1N5406 / 1N5408
• Rated about 3 A
• Much better margin for strike inrush than 1N4007

Acceptable only for low-current strikes

• 1N4007
• Rated 1 A average
• Often works, but it is a lighter-duty choice

If voltage drop is critical

A Schottky diode has lower forward drop:

• Example class: 3 A Schottky
• Advantage: less voltage lost, especially important on the 12 V branch

But be careful:

• choose enough reverse-voltage margin
• use a part with current rating comfortably above strike current

4. What current rating should the diode be?

Use this rule:

\[ I{diode} \ge 2 \times I{strike} \]

at minimum for practical margin.

Examples:

• If the strike draws 250 mA, a 1N4007 may survive
• If the strike draws 500 mA to 1 A, use 1N540x
• If startup/inrush is unknown, use 3 A class diode

A lock or strike can have:

• steady-state current
• higher pull-in current
• transient spikes

So sizing the diode only to nominal current is not best practice.

5. Voltage drop matters

A normal silicon rectifier diode drops roughly:

\[ V_f \approx 0.7 \text{ to } 1.0 \text{ V} \]

So:

• 12 V source through diode → strike may see about 11.3 V
• 16 V source through diode → strike may see about 15.3 V

That leads to two concerns:

On the 12 V side

11.3 V is usually acceptable for a 12 V strike, but not always.
Some strikes get weak if cable runs are long.

On the 16 V side

15.3 V on a 12 V strike is often too high for continuous duty.
It may be acceptable for a very short unlock pulse, but I would not design it that way unless the strike datasheet permits it.

6. Flyback diode across the strike

If the strike is 12 VDC, install a flyback diode directly across the strike terminals.

Connection

• diode cathode to strike +
• diode anode to strike -

Part choice

• 1N4007 minimum for many small strikes
• 1N5408 if strike current is substantial

Purpose

When power is removed, the coil generates reverse voltage.
The flyback diode clamps that spike and protects:

• reader output transistor
• relay contacts
• blocking diodes
• controller electronics

Important

If the strike is AC, do not use a standard DC flyback diode the same way.

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Ethical and legal aspects

For door-entry systems, there are practical safety and compliance concerns:

• Access systems may be part of life-safety egress arrangements
• Fire code or building code may restrict how strikes and release circuits are wired
• Fail-safe versus fail-secure hardware must match the application
• Improper wiring can cause:
  • lock malfunction
  • nuisance unlocks
  • failure to release during emergency
  • damage to listed access-control equipment

If this door is on a commercial building, multifamily building, or fire-rated egress path, verify compliance with:

• local electrical code
• fire alarm interface requirements
• access-control hardware listing requirements

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Practical guidelines

What I recommend you do

Case 1: Intercom output is 16 VAC

Use a relay.
This is the most likely correct answer if it is a traditional intercom release output.

• Relay coil rated for 16 VAC or universal 12/24 VAC/DC
• Relay contacts rated above strike current
• Relay contact parallels the card reader release contact
• Strike powered only from 12 VDC

Case 2: Intercom output is 16 VDC power

Still better to use a relay, because:

• it prevents overdriving a 12 V strike
• it isolates systems cleanly

Case 3: Both intercom and reader outputs are dry contacts

No blocking diodes needed between outputs.
Just wire both contacts to switch the same 12 V strike supply, provided their commons are arranged correctly.

Best practices

• Measure the intercom output with a meter:
  • AC range
  • DC range
• Check the strike nameplate or datasheet:
  • 12 VDC or 12 VAC?
  • current draw?
  • inrush current?
• Check the card reader output type:
  • powered output?
  • transistor output?
  • dry relay contact?

Minimum hardware I would choose

If using diode isolation on DC:

• 2 × 1N5408 isolation diodes
• 1 × 1N4007 or 1N5408 flyback diode across strike

If using relay isolation:

• 1 × appropriate relay
• 1 × flyback diode across DC strike
• optionally a suppression diode across a DC relay coil

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Possible disclaimers or additional notes

• Many intercom “door release” terminals are 16 VAC, not DC.
• A 12 V card reader often does not output power directly; it often provides a relay contact.
• A 12 V strike may draw more current than expected, especially at pull-in.
• A 1N4007 may work in some installations, but it is not my first choice unless the strike current is known to be low.
• If wire runs are long, voltage drop in the cable plus diode drop may make the strike unreliable.

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Suggestions for further research

You should verify these four items before final wiring:

1. Intercom lock-release output: AC, DC, or dry contact?
2. Card reader door output: dry contact, open collector, or powered output?
3. Door strike rating: 12 VDC or 12 VAC?
4. Door strike current draw: steady and inrush

A useful next step is to post or read the labels/specifications for:

• intercom release terminals
• card reader relay/output terminals
• strike voltage/current rating

With that, the circuit can be specified exactly.

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Brief summary

• Do not tie 16 V and 12 V outputs together directly.
• If voltages differ, the best solution is relay isolation and a single 12 V supply for the strike.
• If both outputs are truly DC power outputs, use two isolation diodes, one per source.
• For door strikes, use 1N540x series rather than relying on 1N4007, unless current is definitely low.
• Add a flyback diode across a DC strike.
• If the intercom output is 16 VAC, a simple blocking diode is not the proper solution.

If you want, I can draw the exact wiring diagram for:

1. 16 VAC intercom + 12 VDC card reader + 12 VDC strike, or
2. 16 VDC intercom + 12 VDC card reader + 12 VDC strike.

Disclaimer: The responses provided by artificial intelligence (language model) may be inaccurate and misleading. Elektroda is not responsible for the accuracy, reliability, or completeness of the presented information. All responses should be verified by the user.