Troubleshooting Always-On Relay Issues Caused By Inadequate Driving Voltages

Understanding Relay Coil Voltage Requirements

Relays contain an electromagnet that is energized by applying voltage across its coil terminals. This creates a magnetic field that actuates the relay’s mechanical switch contacts. Always-on relays require the coil voltage to be maintained steadily within the pickup and dropout voltage range specified by the manufacturer in order for the contacts to remain closed.

Typical relay coil voltages range from 3V up to 24V DC and require a few milliamps of current to pickup and hold. Operating outside of the specified voltage range can result in intermittent contacts or failure to actuate the relay properly. An inadequate driving voltage that dips below the relay’s minimum pickup threshold will cause the contacts to flutter or drop out entirely.

Relay Coil Resistance

The coil resistance helps determine how much current is needed from the power source to develop the required magnetic field strength for actuating the relay. Lower resistance coils allow higher current flow and increased magnetic field strength for a given voltage. Typical relay coil resistances range from about 50 ohms to 1500 ohms.

Table of Contents

Pickup vs Dropout Voltage

The pickup voltage defines the minimum coil voltage needed to close the relay’s contacts and complete the circuit. The slightly lower dropout voltage is the threshold where the magnetic field collapses, deactivating the electromagnet and opening the contacts.

For reliable operation, the driving voltage must stay within this voltage window. A voltage dip below the dropout specification can unintentionally deactivate the relay while the circuit is still active.

Voltage Transients

Fast voltage spikes from inductive switching actions or noisy power sources can impair relay operation. Voltage transient suppression using MOVs, TVS diodes, or RC snubbers help make relays more reliable in electrically noisy environments.

Temperature Derating

Increased coil temperatures due to ambient conditions or self-heating will increase the relay’s resistance, requiring a slightly higher voltage to actuatate properly. Many relays list derating curves that show adjusted voltage thresholds over temperature.

Common Causes of Inadequate Driving Voltage

Several issues can potentially lead to supply voltages dropping excessively low for an always-on relay, including:

Resistive losses in power wiring that runs for long distances
Underspecified power supplies unable to source enough current
Excessive loading from additional circuitry connected on the supply
Malfunctioning voltage regulator components
Intermittent connections causing drops or fluctuations in voltage
Noise interference causing glitches or transients below dropout threshold

Resistive Losses in Wiring

The resistance present in power wiring, connectors, PCB traces, and other conductive elements causes a slight voltage drop proportional to the current flow. This effect intensifies over long cable runs with smaller gauge wires. Jumper wires often utilize thin 26 or 24 AWG wire with high resistance per unit length. Replacing long wiring runs with thicker higher gauge wires mitigates excessive resistive losses.

Underspecified Power Supplies

Power supplies have limits on their output current capacity before exhibiting voltage droop. Driving relays along with other loads from an undersized power supply can drag the voltage below dropout threshold under peak load conditions. Consulting power supply datasheets and budgeting adequate headroom prevents running supplies out of spec during operation.

Excessive Loading Conditions

The increased current draw from connecting multiple peripherals and components can similarly overload a power supply past its rated output specs, dragging down the voltage. Assessing the total current demands of always-on relays and other circuit loads ensures overloading conditions do not inadvertently occur during normal or peak operating scenarios.

Malfunctioning Voltage Regulators

Faulty voltage regulator modules, failing capacitors, or destabilized feedback loops can impair a regulator’s ability to maintain steady voltage output as current demands fluctuate. This allows more voltage droop during loading transients. Periodically testing and replacing aging voltage regulator components helps stabilize driving voltage applied to always-on relays over time.

Intermittent Connections

Loose connectors, cold solder joints, and fraying wires create sporadic high resistance junctions in the power feed to relays. The voltage drops across these junctions dips erratically below relay pickup thresholds, causing intermittent contact chatter or dropout. Carefully inspecting wiring and connections identifies any suspect junctions contributing to transient undervoltage events.

Electrical Noise Interference

Noisy power sources with excessive voltage ripple or spikes can drive relays outside of their dropout/pickup specifications. Introducing RC filtering, ferrite chokes, or optical isolation helps mitigate electrical noise interference on the driving voltage that disrupts the magnetic field for always-on relays.

Measuring and Monitoring the Driving Voltage

Quantifying irregular voltage dips and fluctuations helps troubleshoot the root causes behind inadequate driving voltage reaching always-on relays. Voltmeters, multimeters, oscilloscopes, and data loggers offer various capabilities for measuring and recording driving voltage behaviors affecting relay operation.

AC/DC Voltage Meters

Simple AC/DC voltmeters or multimeters connect in parallel to measure the overall driving voltage under steady state conditions. While suitable for general measurements, they lack the sampling rate to capture transients. However, they assist basic testing of power supplies, wiring, and regulator circuits powering always-on relays.

Oscilloscopes

Oscilloscopes display time-based plots of voltage over shorter time intervals, showing dips and noise from milliseconds down to nanoseconds. The waveform visualization capabilities help characterize fast transients that may dropout relays despite normal voltages at slower sampling rates. This identifies behaviors like excessive ripple or intermittent connections that meters may overlook.

Data Loggers

Data loggers record a time series of measurements for minutes to years. This captures overall driving voltage trends, such as gradual declines indicating deteriorating power supplies. It also aids in diagnosing intermittent under-voltage events at high resolution across operational cycles. Analyzing the voltage profile exposure for always-on relays confirms whether transients pose issues.

Monitoring and Notification

Combining driving voltage measurement with alerts offers real-time notification of any undervoltage incidents that may jeopardize an always-on relay. This quickly identifies adverse electrical events before cascading into more severe outages. Notification allows rapid troubleshooting and preventative responses before relays unexpectedly dropout.

Adjusting Resistance to Increase Voltage Drop

The inherent resistance present in any conductor causes a slight unavoidable voltage drop across its length following Ohm’s Law. While normally a disadvantage, introducing additional resistance into the supply path to an always-on relay increases the intentional drop, allowing adjustment of the voltage across its coil.

Using Resistors to Drop Voltage

Fixed or variable resistors added in series to the driving voltage allows customizable voltage control to the relay. Series resistances between 20-500 ohms can produce several volts of drop needed to reduce coil voltage for lower-threshold always-on relays.

Using higher wattage resistors minimizes power dissipation and self-heating instabilities over long term usage. Variable or adjustable resistors enable tuning the voltage drop in response to variable loading or transient conditions.

Utilizing Longer Wiring with Higher Resistance

Similarly, extending the length of existing wiring increases its native resistance to effectively drop input voltage. Since fine gauge wire exhibits more ohms per unit length, spooling additional runs of miniscule 30 AWG wire lowers voltages substantially along its path. This alternative eschews additional components for simple result through leveraging inherent wiring resistance.

Carefully considering wiring placement also induces passive cooling that offsets self-heating in extremely thin wire. Voltage fluctuations along the conductor require monitoring in case ambient factors contribute undesired drift over time.

Selecting a Relay With Lower Pickup Voltage

For sensitive circuits with minimal headroom between operating and threshold voltages, choosing a relay with lower pickup parameters aligns device specifications closer to the available driving voltage.

Identifying Dropout Threshold Requirements

The minimum pickup voltage determines always-on relay selection, while the lower dropout voltage defines the functional limit before contact status becomes intermittent. Clarifying these threshold requirements for sensitive circuit loads informs appropriate relay selection.

Consulting Relay Specification Sheets

Datasheets provide operating voltage ranges and Electromechanical relays exhibit pickup voltages between 3-24V DC typically, while reed relays activate from low millivolt signals up to 48V DC. Solid state relays support triggering down to just 1.5V making them quite flexible. Identifying options matching target voltage parameters futureproofs selection.

Considering temperature derating info prevents ambient environments skewing expected voltage behaviors. Comparing pickup, dropout, and must release timepoints validates capability to operate reliably within small voltage windows.

Prototype Testing Various Relay Models

Empirically verifying relay pickup and hold voltages with bench level testing provides application assurance selected models fulfill needs. Monitoring coil voltage/current profiles during actuation also derisks production by ensuring models withstand project operating conditions with adequate margin.

Testing relays in target enclosures over temperature further confirms robustness to environmental influences that may alter device specifications in deployed settings.

Using a Solid-State Relay for Flexible Control

Solid-state relays based on optically isolated TRIAC, MOSFET, or SCR switching overcome inherent limitations of electromechanical contacts. Adjustable triggering and built-in drive circuitry offers enhanced control flexibility for always-on applications.

Low Current/Voltage Actuation Requirements

With no coils to energize, solid state relays allow microcontroller or logic level signals as low as 1.5-5V DC to activate loads up to 240V AC. On-resistances below 1 ohm prevent excessive power losses during conduction despite low trigger voltages.

Timer ICs, rectified AC signals, thermistor readings, and various other low voltage outputs conveniently interface for switching without bulky mechanical components or intermediary circuitry.

Consistent Switching Thresholds

Precise voltage thresholds crafted through semiconductor bandgap engineering provide reliable, consistent actuation unlike the variable pickup of electromagnetic types. Typical optocoupler turn-on varies less than 10% over lifetime eliminating dropout concerns.

Further logic integration adds programmable hysteresis for noise immunity along with timed switching delays to prevent chatter or unwanted state changes.

Withstanding Higher Frequency Operation

Without mechanical parts suffering wear or metal contacts enduring arcing, solid state relays support much faster, unlimited switching capacities exceeding tens of millions operations over lifetime. Always-on applications at high duty cycles run smoothly regardless.

This allows utilizing relays in moderate PWM roles, adjustable current regulation tasks, and similar frequent cycling situations impractical for conventional relays.

Example Arduino Sketch to Monitor and Log Voltage

This example Arduino sketch monitors the driving voltage powering an always-on relay using the analog input pin A0. It checks for any readings below the target 5V pickup threshold specified for the relay. Finally it saves a timestamped log of any undervoltage events detected to monitor reliability and troubleshoot issues over longer term operation.

// Set reference voltage for ADC to 5V 
const float vRef = 5.0;
 
// Relay pickup voltage
const float targetVoltage = 5.0;  

// 1000ms logging interval 
const int logInterval = 1000;

// Initialize variables  
int voltPin = A0;
float voltValue = 0.0;  

void setup(){

  // Begin Serial comms
  Serial.begin(115200);

}

void loop(){

  // Read the analog voltage value
  voltValue = analogRead(voltPin);  

  // Convert reading to voltage  
  voltValue = (voltValue * vRef) / 1023.0; 
   
  // Check if voltage dipped too low
  if(voltValue < targetVoltage){

    // Log undervoltage event 
    Serial.print("Undervoltage detected: "); 
    Serial.print(voltValue);
    Serial.print("V @ ");
    Serial.print(millis()/1000); 
    Serial.println(" sec");
  
  }

  // Check voltage every second
  delay(logInterval);

}

This sketch could be expanded to trigger visual indicators or alarms that notify users on-site if the relay driving voltage dips excessively. The addition of an SD card writer would provide robust long term logging histories for in-depth analysis. With some auxiliary circuitry for battery backup, key fault data remains available to troubleshoot unexpected shutdown events even during power failures.