How Off-Delay Relays Keep Timing After Power Dies: The Capacitor Secret 

When the Power’s Gone, The Timer Keeps Ticking

The motor stops. The power’s cut.

But your cooling fan needs to run for another 60 seconds to prevent bearing damage from residual heat. With a standard electronic timer, the instant you cut power to the relay, the timing circuit dies and the fan stops immediately. Three minutes later, you’re looking at a seized bearing and an $8,000 motor replacement—all because your “smart” electronic timer couldn’t outlive the power supply by 60 seconds.

So how do you get reliable timing when the power source is already gone?

The Power Paradox: Why Electronic Timers Need What They’ve Lost

Here’s the irony: Electronic timing relays are supposed to be smarter than their pneumatic predecessors—smaller, cheaper, more precise. And they are, right up until the moment you need them to work without power.

Standard electronic off-delay relays require continuous input voltage throughout the entire timing period. The microprocessor or RC timing circuit needs electricity to count. The output relay coil needs electricity to stay energized. Cut the power, and the entire system collapses instantly—timing stops, relay opens, your load shuts off.

It’s like a digital watch that stops working the instant you unplug it.

Pneumatic timers didn’t have this problem. When you cut power to a pneumatic timer’s solenoid, the contacts remained in their changed state while compressed air slowly bled through an adjustable orifice—no continuous power required. The timing mechanism was mechanical, driven by air pressure, not electronic logic. They were bulky, expensive ($200-400), and limited to fixed timing ranges, but they worked when the power died.

The 1970s brought solid-state timing relays with RC circuits and later microprocessors—massive improvements in size, cost, and flexibility. But replacement applications hit a wall. Engineers specifying retrofit replacements for pneumatic timers discovered their sleek new electronic units failed in the exact scenario where pneumatics excelled: timing after power removal.

The market demanded a solution. Manufacturers needed electronic precision with pneumatic-style “post-power” operation.

Enter the “true off-delay relay”—also called the “Ghost Power Timer.”

The Ghost Power Timer: Three Ways to Store Energy After Power Dies

True off-delay relays solve the power paradox by carrying their own energy supply onboard. When input power is removed, the relay doesn’t die—it switches to stored energy and continues timing as if nothing happened.

There are three methods to achieve this, each with different tradeoffs:

Method 1: Capacitor Discharge (Most Common)

A capacitor charges to the supply voltage while power is applied. When power is cut, the capacitor discharges slowly through the relay coil and timing circuit, keeping everything alive for the preset delay period.

Think of it as “The Capacitor’s Last Breath”—that stored electrical charge exhales gradually, powering the relay coil just long enough to complete the timing cycle.

A 2200μF capacitor at 12V stores approximately 0.16 joules of energy. That doesn’t sound like much—it’s less energy than lifting a paperclip one meter—but it’s enough to keep a 12V relay coil (typical 85-ohm resistance, 140mW power consumption) energized for 5-10 seconds, depending on the relay’s dropout voltage.

Scale that up to a 10,000μF capacitor, and you’re looking at 30-60 seconds of timing without any external power.

Method 2: Latching Relay + Small Capacitor (Most Efficient)

Instead of continuously powering a standard relay coil, use a latching (bi-stable) relay that mechanically locks in position when energized, requiring no holding current. When power is cut, a small capacitor only needs to provide enough energy to unlatch the relay after the preset delay—maybe 50-100ms of pulse energy instead of 60 seconds of continuous current.

This approach requires roughly 1/10th the capacitor size for the same timing duration. A 470μF capacitor can achieve what required 4700μF with Method 1.

The tradeoff? Latching relays cost 2-3x more than standard relays, and the unlatch timing circuit is more complex. You’re trading component cost for capacitor size.

Method 3: Small Battery (Longest Hold-Up)

For timing periods beyond a few minutes, or for applications requiring years of standby reliability, a small lithium coin cell (CR2032 or similar) can power the timing circuit indefinitely.

The battery doesn’t power the output relay coil—that would drain it in hours. Instead, it powers only the microprocessor and timing logic, which consume microamps. When the timing period expires, the battery-powered microprocessor releases a small capacitor-stored pulse to drop the output relay.

Advantages: Extremely long timing capability (minutes to hours), no capacitor degradation over time.

Disadvantages: Battery replacement requirement (every 3-5 years), higher initial cost, regulatory considerations for battery disposal.

For the remainder of this article, we’ll focus on Method 1—capacitor discharge timing—as it’s the most common, most cost-effective, and mechanically simplest solution.

How a Capacitor Becomes a Clock: RC Time Constants Explained

Understanding how stored charge becomes precise timing requires understanding capacitor discharge through a resistor—the fundamental RC circuit.

The Charging Phase: Storing the Ghost Power

When power is applied to a true off-delay relay, two things happen simultaneously: the output relay energizes (closing or opening contacts per the application), and the storage capacitor charges through a charging resistor to the supply voltage.

The energy stored in a fully charged capacitor follows a simple formula:

E = ½CV²

কোথায়:

• E = energy (joules)
• C = capacitance (farads)
• V = voltage (volts)

For a 2200μF capacitor charged to 12V:

E = ½ × 0.0022F × (12V)² = 0.158 joules

That’s enough energy to keep a 12V/85Ω relay coil (power = V²/R = 1.69W) energized for about 0.094 seconds… if you discharged it instantly at full power.

But you don’t. The capacitor discharges gradually through the relay coil resistance, and that’s where the timing magic happens.

The Discharge Phase: The 37% Rule

When input power is removed, the capacitor begins discharging through the relay coil resistance. The voltage across the capacitor doesn’t drop linearly—it follows an exponential decay curve governed by the RC time constant:

τ (tau) = R × C

কোথায়:

• τ = time constant (seconds)
• R = resistance (ohms)
• C = capacitance (farads)

Here’s the beautiful part: After exactly one time constant (τ), the voltage will have decayed to precisely 37% of its initial value.

Not 40%. Not 35%. Exactly 37% (actually 36.8%, or more precisely, 1/e where e ≈ 2.718).

This isn’t arbitrary—it’s baked into the exponential function that governs RC discharge:

V(t) = V₀ × e^(-t/τ)

At t = τ: V(τ) = V₀ × e^(-1) = V₀ × 0.368 = 37% of V₀

কেন এটি গুরুত্বপূর্ণ: Every additional time constant drops the voltage by another 37% of the remaining voltage.

• At 1τ: 37% remaining (63% discharged)
• At 2τ: 13.5% remaining (86.5% discharged)
• At 3τ: 5% remaining (95% discharged)
• At 5τ: <1% remaining (>99% discharged)

For our 12V relay with 85Ω coil and 2200μF capacitor:

τ = 85Ω × 0.0022F = 0.187 seconds

After 0.187 seconds, the voltage across the capacitor (and thus across the relay coil) will be 4.4V. After 0.374 seconds (2τ), it’ll be 1.6V. After 0.56 seconds (3τ), just 0.6V.

But here’s the critical question: At what voltage does the relay coil actually release?

The Dropout Trick: Why Real Timing Is Longer Than Math Predicts

A 12V relay doesn’t need 12V to stay energized once it’s pulled in.

দ্য pickup voltage (voltage needed to initially energize a de-energized relay) is typically 75-85% of rated voltage—call it 9-10V for a 12V relay. But the dropout voltage (voltage at which an already-energized relay releases) is much lower: typically 20-30% of rated voltage, or 2.4-3.6V for our 12V relay.

This happens because of the magnetic circuit’s hysteresis. When the relay armature is touching the pole piece (fully energized position), the air gap is zero, magnetic reluctance is minimized, and much less magnetomotive force (and thus less coil current/voltage) is needed to maintain the magnetic field holding the armature in place.

This means your timing extends well beyond the naive RC calculation.

Let’s recalculate for our 12V relay (85Ω coil, 2200μF capacitor) assuming a dropout voltage of 2.8V (23% of rated):

Using V(t) = V₀ × e^(-t/τ), solve for t when V(t) = 2.8V:

2.8V = 12V × e^(-t/0.187s)

0.233 = e^(-t/0.187s)

ln(0.233) = -t/0.187s

-1.46 = -t/0.187s

t = 0.273 seconds

So our 2200μF capacitor keeps the relay energized for 0.273 seconds, not the <0.1 seconds suggested by naive energy calculations.

That’s The Dropout Trick in action.

Want 5 seconds of hold-up time? Work backwards:

t_desired = 5 seconds, τ = RC = 0.187s (from earlier)

How many time constants is 5 seconds? 5s / 0.187s = 26.7 time constants

At 26.7τ, the voltage would be essentially zero—way below dropout. We need to solve for when voltage reaches 2.8V:

2.8/12 = 0.233, so we need: e^(-t/τ) = 0.233

-t/τ = ln(0.233) = -1.46

For t = 5s: τ = 5s / 1.46 = 3.42 seconds

Therefore: C = τ/R = 3.42s / 85Ω = 0.040F = 40,000μF

A 40,000μF capacitor at 12V? That’s physically large (roughly the size of a D-cell battery) and costs $15-25. Doable, but not elegant.

This is why latching relays (Method 2) or longer timing periods often use microprocessor-based designs with small batteries—the capacitor size becomes impractical beyond 30-60 seconds of continuous relay holding.

Sizing Your Capacitor: The 3-Step Method

Let’s work through a real-world design example: You need a 12V relay to stay energized for 10 seconds after power removal.

Step 1: Know Your Relay’s Specs

What you need:

• Coil voltage: 12V DC
• Coil resistance: Measure with a multimeter or check datasheet (let’s say 80Ω)
• Dropout voltage: Either test empirically or estimate at 25% of rated = 3.0V

If you don’t have the dropout voltage, test it: Apply rated voltage to the relay coil. Once energized, slowly reduce voltage with a variable power supply while monitoring the contacts. Note the voltage at which the relay releases. That’s your dropout voltage.

Pro-Tip #1: The dropout voltage is your friend. Most relay coils hold at 20-30% of rated voltage, giving you 3-5x more timing than naive energy calculations suggest.

Step 2: Calculate Required Capacitance

Use the dropout trick formula derived earlier:

t = -τ × ln(V_dropout / V_initial)

Where τ = RC, so:

t = -RC × ln(V_dropout / V_initial)

Rearrange to solve for C:

C = -t / [R × ln(V_dropout / V_initial)]

For our example:

• t = 10 seconds
• R = 80Ω
• V_initial = 12V
• V_dropout = 3.0V

C = -10s / [80Ω × ln(3.0V / 12V)]

C = -10s / [80Ω × ln(0.25)]

C = -10s / [80Ω × (-1.386)]

C = 10s / 110.9

C = 0.090F = 90,000μF

That’s the theoretical minimum.

Step 3: Account for Real-World Factors

Theory meets practice here. Three factors will steal your timing:

Factor 1: Capacitor Leakage Current

Real capacitors aren’t perfect insulators. Leakage current provides a parallel discharge path, effectively reducing timing. For electrolytic capacitors, leakage can be 0.01CV to 0.03CV (μA per μF-V) at room temperature.

For our 90,000μF/12V capacitor: Leakage ≈ 0.02 × 90,000μF × 12V = 21,600μA = 21.6mA

Compare that to the relay coil current at dropout (3V / 80Ω = 37.5mA). The leakage current is consuming more than half as much current as the relay coil!

সমাধান: Use low-leakage film capacitors (polypropylene or polyester) for critical timing applications, or add 30-50% capacitance margin for electrolytics.

Pro-Tip #2: Capacitor leakage current steals your timing. Use film capacitors (polypropylene/polyester) for delays >10 seconds, not electrolytics.

Factor 2: Temperature Effects

Capacitor leakage current roughly doubles for every 10°C increase in temperature. A capacitor with 20mA leakage at 25°C might have 40mA at 35°C, 80mA at 45°C.

Relay dropout voltage also changes with temperature—typically increasing slightly as coil resistance increases with temperature (positive temperature coefficient of copper). This helps slightly, but not enough to compensate for capacitor leakage.

Factor 3: Capacitor Tolerance

Electrolytic capacitors commonly have -20%/+80% tolerance. That 90,000μF capacitor might actually be 72,000μF (at -20%). Film capacitors are tighter, typically ±5-10%.

Apply Safety Margin:

Given these factors, multiply your calculated capacitance by 1.5 to 2.0x for reliable operation across temperature and component tolerance:

C_actual = 90,000μF × 1.75 = 157,500μF

Round up to a standard value: 2 × 82,000μF = 164,000μF in parallel, or use a single 150,000μF capacitor if available.

At 12V, a 150,000μF electrolytic capacitor is physically about 35mm diameter × 60mm tall, costs $8-15, and stores approximately 10.8 joules.

Inrush Current Limiting: Don’t Forget the Charging Resistor

When you first apply power, that large uncharged capacitor looks like a short circuit. A 150,000μF capacitor charging from 0V to 12V through zero resistance would theoretically demand infinite current.

In practice, wiring resistance and power supply impedance limit this, but you’ll still see inrush currents of 10-50A for the first few milliseconds, potentially damaging contacts, fuses, or the power supply itself.

সমাধান: Add a charging resistor (R_charge) in series with the capacitor to limit inrush current, with a parallel diode to bypass it during discharge:

[Power In] → [R_charge] → [+Capacitor-] → [Relay Coil] → [Ground]
                ↑              ↓
                └─── [Diode] ──┘ (cathode to power, anode to cap-)

The diode allows the capacitor to discharge directly through the relay coil (no series resistance) while forcing charging current through R_charge.

Size R_charge to limit charging current to a reasonable level (0.5-2A):

R_charge = V_supply / I_charge_max = 12V / 1A = 12Ω

This adds 12Ω to the RC time constant during charging only, extending charge time to about 5τ = 5 × (12Ω + 80Ω) × 0.15F = 69 seconds to fully charge.

If that’s too long, reduce R_charge but accept higher inrush (say 6Ω for ~2A inrush, 35-second charge time). The tradeoff is yours.

Pro-Tip #3: RC time constant (τ = RC) is just the starting point—real hold-up time depends on relay coil resistance matching your capacitor discharge curve.

Capacitor Selection: Why Type Matters More Than Size

You’ve calculated the capacitance. Now you need to choose the actual component. Capacitor chemistry dramatically affects performance in timing applications—size isn’t everything.

Film Capacitors vs Electrolytics: The Leakage War

Electrolytic Capacitors (Aluminum or Tantalum):

সুবিধাদি:

• Highest capacitance per unit volume (critical for large values)
• Low cost per microfarad ($0.05-0.15 per 1000μF)
• Readily available in high voltages

অসুবিধা:

• High leakage current (0.01-0.03 CV spec, worse in practice)
• Polarity-sensitive (reverse voltage = instant death)
• Limited lifetime (electrolyte dries out over 5-10 years)
• Temperature-sensitive capacitance and leakage

এর জন্য সেরা: Timing delays <30 seconds where size and cost dominate, or where you’ve added 1.5-2x margin for leakage.

Film Capacitors (Polypropylene, Polyester, Polycarbonate):

সুবিধাদি:

• Very low leakage current (<0.001 CV, often 10-100x lower than electrolytics)
• চমৎকার তাপমাত্রা স্থিতিশীলতা
• Long lifetime (20+ years)
• No polarity restrictions (can handle AC or reversed DC)

অসুবিধা:

• Much larger physical size for same capacitance
• Higher cost ($0.50-2.00 per 1000μF)
• Limited to lower capacitance values (practically <50μF for reasonable size)

এর জন্য সেরা: Precision timing >30 seconds, high-temperature environments, or applications where long-term drift is unacceptable.

The Hybrid Approach: Best of Both Worlds

For timing in the 30-60 second range, consider parallel combination:

• Large electrolytic (80% of calculated capacitance) for bulk energy storage
• Small film capacitor (20% of calculated capacitance) for low-leakage precision

Example: 120,000μF electrolytic + 30,000μF film = 150,000μF total

The film cap compensates for electrolytic leakage, extending timing closer to theoretical calculations. Cost increase is moderate (~30% more than all-electrolytic), but timing accuracy improves significantly.

Common Mistakes and Fixes

Mistake #1: Using capacitors rated below supply voltage

A 12V supply needs 16V-rated (or higher) capacitors for reliability. Voltage transients, ripple, and component tolerance mean a “12V system” might see 14-15V under certain conditions. Operating a capacitor near its voltage rating accelerates failure and increases leakage.

Fix: Use capacitors rated at least 1.3x supply voltage (16V for 12V systems, 25V for 18V, etc.)

Mistake #2: Ignoring ESR (Equivalent Series Resistance)

Capacitors have internal resistance (ESR) that appears in series with the ideal capacitance. High ESR reduces available discharge current and creates voltage drop under load, effectively reducing hold-up time.

Large electrolytics might have ESR of 0.1-1Ω. For a relay coil drawing 150mA at dropout, 1Ω ESR means 0.15V lost to internal resistance—enough to reduce your margin.

Fix: Check ESR specs. For timing applications, prefer low-ESR types (0.1Ω or less).

Mistake #3: Parallel connection without current balancing

Connecting multiple capacitors in parallel (say, four 10,000μF caps instead of one 40,000μF) works great in theory but can cause problems if capacitors have mismatched ESR or leakage. The “better” capacitor does more work, ages faster, and fails first—then the remaining caps are suddenly undersized.

Fix: Use matched capacitors from the same manufacturing batch when paralleling. Add small series resistors (0.1-0.5Ω) to each capacitor to force current sharing.

Pro-Tip #4: The latching relay trick gives you 1/10th the capacitor size for the same timing by using mechanical memory instead of continuous power.

VIOX টাইম রিলে পণ্য

The Ghost Power Timer: Timing That Survives the Power Loss

True off-delay relays solve a fundamental paradox: how do you measure time when the clock’s power source disappears?

The answer lives in The Capacitor’s Last Breath—stored electrical energy that exhales gradually, powering relay coils and timing circuits for seconds or minutes after input power vanishes. It’s ghost power: enough juice to complete one last task before fading to zero.

Three methods achieve this:

1. Capacitor discharge (most common)—RC time constants turn energy storage into precise timing
2. Latching relay + small capacitor (most efficient)—mechanical memory needs only pulse energy
3. Small battery backup (longest hold-up)—microamp consumption enables hours of timing

The physics is elegant: The 37% Rule governs exponential RC discharge, but The Dropout Trick extends practical timing by 3-5x beyond naive calculations by exploiting relay hysteresis.

A $2 film capacitor and a $5 relay can achieve what once required a $200 pneumatic timer—smaller, cheaper, more reliable, and field-adjustable.

Modern control systems demand timing that survives power interruptions. Whether it’s cooling fans preventing bearing damage, process valves completing shutdown sequences, or safety circuits maintaining protection during transients, the true off-delay relay delivers timing insurance when standard electronics would fail.

VIOX ELECTRIC offers a complete range of electronic timing relays including true off-delay models with capacitor-based energy storage, suitable for motor control, process automation, and safety applications. Our timing relays meet IEC 61810 standards and provide reliable operation across industrial temperature ranges (-25°C to +70°C ambient).

For technical specifications and selection guidance, contact our application engineering team. We’ll help you size the right timing solution for your application—no ghost power required on our end.