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Current ratings PXI 2566
David100
Member
03-15-2004 10:47 AM
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I am confused by the PXI 2566. It says the switch rating carry current is 5 Amps but the data sheet for the relays (NAIS TQ-SMD) only claims 1 Amp AC and 2 Amps DC, and the manufacturer claims this lower number is the maximum carrry current they will warrant.
Life ratings are also confusing in your data sheetsd.End of life is stated at 1 ohms, but at 5 Amps it would be disipating 25 Watts in one relay,a real fire hazard.
You allow 9 channels to be in this state. PXI standard suggests only 25 watts per module so it seems not to make much sense. Even when the switches are new the power dispation in the relays is very high at 5 Amps and 0.1 ohms. You do not quote your expected life under these conditions.
Life ratings are also confusing in your data sheetsd.End of life is stated at 1 ohms, but at 5 Amps it would be disipating 25 Watts in one relay,a real fire hazard.
You allow 9 channels to be in this state. PXI standard suggests only 25 watts per module so it seems not to make much sense. Even when the switches are new the power dispation in the relays is very high at 5 Amps and 0.1 ohms. You do not quote your expected life under these conditions.
Guy Ferraro
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03-16-2004 04:22 PM
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Each channel of the PXI-2566 uses two Form C relays that are packaged and tied together. As per the relay datasheet, each of the relays in the 2 Form C package is specified for a maximum switching current of 2 A (DC or AC). The manufacturer of the relays, however, certifies that they can carry 5A when tied together in this way. Extensive thermal testing has shown that the PXI-2566 will meet specifications regardless of variations in manufacturing process or components.
During our life time testing experiments, we have verified that throughout the life of the relay the path resistance is fairly constant and only dramatically increases to above 1 ohm at the relay�s end of life. In fact, the path resistance remains well below 0.1 ohm during its useful life time. It is not recommended to keep using a relay past its specified lifetime. In the PXI-2566 specification we added a special note to that effect: "DC path resistance typically remains low for the life of the relay. At the end of relay life, the path resistance rises rapidly above 1 ohm. Load ratings apply to relays used within the specification before the end of relay life." This is a characteristic of all electromechanical relays.
When relays are used within their useful lifetime it is very conservative to assume a path resistance of 0.1 ohm. If 9 channels are carrying 5 A, the total power dissipation will only be 22.5 W. In practice, the path resistance is much lower. The typical data provided by the relay manufacturer shows that the contact resistance of one relay does not increase above 50 mohm over the relay's lifetime. The PXI-2566 uses two such relays in parallel. Traces and connectors add additional resistance, but the sum is still below 0.1 ohm.
Relay damage most often occurs during relay actuation. For this reason, we specify electrical life time in terms of closing and opening relays at a specified load. We specify DC electrical lifetime at 2 A 30 V DC and 1 A 30 V DC. Carry currents up to 5A will not, however, have a significant effect on the electrical life time of the relays used on the PXI-2566. A 5A carry lifetime specification, assuming the relay never switched current, would be very close to the mechanical lifetime.
Guy Ferraro
Switch Product Support Engineer
National Instruments
During our life time testing experiments, we have verified that throughout the life of the relay the path resistance is fairly constant and only dramatically increases to above 1 ohm at the relay�s end of life. In fact, the path resistance remains well below 0.1 ohm during its useful life time. It is not recommended to keep using a relay past its specified lifetime. In the PXI-2566 specification we added a special note to that effect: "DC path resistance typically remains low for the life of the relay. At the end of relay life, the path resistance rises rapidly above 1 ohm. Load ratings apply to relays used within the specification before the end of relay life." This is a characteristic of all electromechanical relays.
When relays are used within their useful lifetime it is very conservative to assume a path resistance of 0.1 ohm. If 9 channels are carrying 5 A, the total power dissipation will only be 22.5 W. In practice, the path resistance is much lower. The typical data provided by the relay manufacturer shows that the contact resistance of one relay does not increase above 50 mohm over the relay's lifetime. The PXI-2566 uses two such relays in parallel. Traces and connectors add additional resistance, but the sum is still below 0.1 ohm.
Relay damage most often occurs during relay actuation. For this reason, we specify electrical life time in terms of closing and opening relays at a specified load. We specify DC electrical lifetime at 2 A 30 V DC and 1 A 30 V DC. Carry currents up to 5A will not, however, have a significant effect on the electrical life time of the relays used on the PXI-2566. A 5A carry lifetime specification, assuming the relay never switched current, would be very close to the mechanical lifetime.
Guy Ferraro
Switch Product Support Engineer
National Instruments
