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This may seem like a dumb question, but I want to be sure. In the below datasheet, what is the DC voltage the relay can handle? I am assuming it is the Contact Capacity 24 VDC, 1 A.

Micro 1A SPDT Relay
(275-0240)                 Specifications             Faxback Doc. # 45621
Contact Form: ....................................................... SPDT
Contact Gap: ..................................................... 0.15 mm
Contact Pressure: ....................... 3 grs. Min. at center of contact
Contact Capacity: ........................................... 125 VAC, 1 A
                                                               24 VDC, 1 A
Coil Voltage: ...................................................... 5 VDC
Min. Puck-Up Voltage: ........................................... 3.75 VDC
Nominal Coil Current: .............................................. 90 mA
Coil Resistance: .......................................... 55 Ohm +/- 10%
Operating Time: ............... 10 milli Seconds Max. at nominal condition
Release Time: .................. 5 milli Seconds Max. at nominal condition
Temperature Range: ................... -25 to +55 Degrees C. @ 60% RH Max.
Humidity Range: ...................... 95% RH Max. from 5 to 35 Degrees C.
Dimensions (HWD): .............................. 0.35 x 0.41 x 0.29 Inches
                                                     (9.0 x 10.3 x 7.3 mm)

Specifications are typical; individual units might vary. Specifications are
subject to change and improvement without notice.

(EB 11/26/97)
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  • \$\begingroup\$ What makes you doubtful? \$\endgroup\$ – Kitana Nov 30 '14 at 12:54
  • \$\begingroup\$ Contact to me means pins in this context. I would expect to see Relay Capacity. \$\endgroup\$ – PhillyNJ Nov 30 '14 at 13:00
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    \$\begingroup\$ Not the pin, they talk about the switch contact inside the relais. The relais has two sides, the switch and the coil, so while beeing exactly you can not talk about the relais capacity. \$\endgroup\$ – Kitana Nov 30 '14 at 13:06
  • \$\begingroup\$ So 24vDc is the max Capacity the relay can handle? \$\endgroup\$ – PhillyNJ Nov 30 '14 at 13:08
  • \$\begingroup\$ For DC yes, for AC it's 125V. \$\endgroup\$ – Kitana Nov 30 '14 at 13:10
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What's given as the "Contact Capacity" on the data sheet is the amount of current the contacts can break and achieve their life expectancy with the specified open-circuit voltage across them when the break happens.

The voltage is higher for AC because even if the contacts open up at the AC peak when they're passing 1 ampere, at 60 Hz the arc/plasma generated by the contacts opening will automatically quench some time before the AC goes through zero volts, so less than 4.2 milliseconds from the break.

For switching DC, the open-circuit voltage limit is much lower than for AC because there is no zero crossing, therefore when the arc is generated it'll be sustained until the gap between the contacts increases enough to cause the resistance through the arc to increase to the point where the arc can no longer be sustained.

The criterion in either case is determined by the tiny area the contacts occupy when they're closed, and by the temperature rise they'll experience and how much metal will be melted when the contacts open/bounce.

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A 0.15 mm contact gap with dry air will arc over at ca. 500 V. What the relay needs to "handle" is really the current that it needs to stop and the inductance of the circuit. (A big inductor can generate the needed 500 V and maintain an arc for a while, however small the voltage that was used to build up a current in the inductor.)

The 1 A given in the datasheet is valid for some inductance that the manufacturer thinks is somehow reasonable. Some manufacturers give two current values, one for "non-inductive" loads and one for "inductive" loads, the former being something like a lightbulb and the latter something like a motor.

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  • \$\begingroup\$ -1 - Interesting but no answer to the question. \$\endgroup\$ – Kitana Nov 30 '14 at 13:29
  • \$\begingroup\$ +1 This IS at least part of an answer. It goes into more detail than the user may need and does not provide a clear conclusion BUT it is useful and informative and does not IMHO deserve a down vote. \$\endgroup\$ – Russell McMahon Nov 30 '14 at 23:22

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