Relay Dissipation Input Form
Example Data Table
| Item |
Example Value |
Meaning |
| Coil voltage |
12 V |
Rated coil supply voltage. |
| Coil current |
75 mA |
Current drawn while energized. |
| Contact resistance |
50 milliohm |
Closed contact path resistance. |
| Pole currents |
2, 2, 1, 1 A |
Current through each 4PDT pole. |
| Thermal resistance |
35 C/W |
Temperature rise per watt. |
Formula Used
Coil power: Pcoil = Vcoil × Icoil, or Pcoil = Vcoil² / Rcoil.
Average coil power: Pcoil(avg) = Pcoil × coil duty × hold power.
4PDT contact power: Pcontact = (I1²R + I2²R + I3²R + I4²R) × load duty.
Driver power: Pdriver = Vdrop × Icoil × coil duty.
Switching power: Pswitch = switching energy × operations per second.
Design power: Pdesign = (Pcoil + Pcontact + Pdriver + Pswitch) × safety margin factor.
Temperature rise: ΔT = Pdesign × thermal resistance.
How to Use This Calculator
Enter the relay coil voltage first. Then choose current input or resistance input. Add the current carried by each of the four poles. Enter contact resistance from the datasheet when available. Add coil duty, load duty, driver loss, switching energy, and thermal data. Press the calculate button. Review the power breakdown and temperature estimate above the form. Use CSV or PDF export for records.
Advanced Guide to 4PDT Relay Heat
A 4PDT relay contains four changeover poles moved by one coil. It can route several circuits at the same time. Heat comes from more than the coil. Each closed contact has small resistance. Current through that resistance creates contact loss. Driver parts can also add loss. Repeated switching can add small transient energy.
Why Dissipation Matters
Dissipation decides temperature rise. Temperature rise affects coil life, plastic parts, and contact reliability. A relay mounted inside a panel may run hotter than a relay on an open bench. Nearby supplies, DIN rails, and closed enclosures can reduce cooling. A safe estimate should include duty cycle and margin.
Key Inputs
Start with coil voltage and coil current. Use coil resistance when current is unknown. Add the actual current in each pole. Do not assume every pole carries equal current. A 4PDT relay may switch logic on one pole and a motor load on another. Contact resistance must come from a datasheet or test. Milliohm values look small, but high current can still create heat.
Using Duty Cycle
Coil duty is the percentage of time the coil is energized. Load duty is the percentage of time current flows through contacts. Economizer circuits reduce hold current after pickup. This calculator handles that with a hold power percentage. Use 100 percent when no economizer exists.
Thermal Check
Thermal resistance converts watts into temperature rise. The final estimate adds coil, contact, driver, and switching losses. Then it applies safety margin. The estimated case temperature is ambient temperature plus calculated rise. Compare it with the allowable relay temperature.
Practical Advice
Use conservative values for contact resistance. Contacts age, oxidize, and warm during service. Measure coil current when possible. Check both normal load and worst load. If the result is close to the limit, choose a larger relay. You can also reduce current, improve airflow, or split loads across devices.
Interpreting Results
A low total wattage is not always safe. A high coil duty can dominate heat. Heavy contact current can dominate low voltage systems. Review the breakdown before making decisions. The margin result is the design number. Keep exported CSV or PDF records with your panel notes. It helps reviews and maintenance.
FAQs
What is 4PDT relay dissipation?
It is the heat power produced by the relay coil, closed contacts, driver device, and switching activity. The answer is normally expressed in watts.
Why are there four pole current fields?
A 4PDT relay has four switched paths. Each path can carry a different load current, so each path can produce a different contact loss.
Should I use coil current or coil resistance?
Use coil current when the datasheet lists it. Use coil resistance when current is missing. The calculator can estimate current from voltage and resistance.
What contact resistance should I enter?
Use the closed contact resistance from the relay datasheet. If contacts are old or critical, use a higher conservative value for design checks.
What does hold power mean?
Hold power describes reduced coil power after pickup. Use 100 percent for a normal relay circuit. Use a lower value for an economizer circuit.
Why include driver voltage drop?
A transistor, MOSFET, or driver module can dissipate heat while feeding the coil. This field adds that driver loss to the total estimate.
What is thermal resistance?
Thermal resistance estimates temperature rise per watt. A higher value means the relay gets hotter for the same power dissipation.
Is this result a final safety approval?
No. It is an engineering estimate. Always compare results with datasheets, enclosure tests, wiring rules, and real operating conditions.