How to find maximum coil current for relay based on nominal current value given in datasheet

Question

As per datasheet for Relay (Panasonic: JS-M Series); all nominal values are mentioned for coil current, voltage & power dissipation. Based on these values how to estimate maximum values for current, voltage & power dissipation for relays. e.g. Nominal Voltage: 12VDC Nominal Operating Current: 53.3mA Nominal Operating Power: 640mW

This relays is designed for 12V system; (Automotive Domain); Can it be used for 24V systems?

I have checked this relay on 31.5V power supply. It was working fine without any damage. So I am not sure whether to use same 12V relay for 24V system or not.

Answer 1

You should not do this. Assuming 150°C insulation the temperature rise at 85°C ambient is 65°C with 16V applied. Refer to figure 3 on the datasheet.

Even if your ambient temperature never exceeds 25°C (so the allowable temperature rise would be 60°C more), the maximum allowable voltage would be

Vmax = 16V\$\cdot \sqrt{\frac{125°C}{65°C}}\$ = 22VDC (Ta \$\le\$25°C).

The assumption in the above is that temperature rise is proportional to coil dissipation.

Use the proper voltage relay or add a ~1W resistor in series with the coil.

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Note that the resistance of the coil varies significantly from room temperature to the maximum coil temperature, however at the maximum it is roughly the same, whether you get there from high ambient temperature or from heating due to over-voltage.

Answer 2

If you look carefully at the data sheet, you should find everything that you need to know. The sheet lists +/- 10% tolerance for coil current and power. Allowable voltage range is listed as 10 to 16 volts. You stated that the relay works fine at 31.5 volts. What was the ambient temperature? How long did you operate the relay with that coil voltage? You should not expect a very long coil life with nearly twice the allowable coil voltage. However the relay is designed to operate in an 85C ambient. It may last quite a while at a lower temperature.