Relay confusion

Roger Bourne · 2006-06-29T03:03:43+00:00

Hello all,Today I was introduced to the wide world of magnetic relays (for SPDTswitching applications)... .Push a current through the coil of therelay--> creates magnetism --> makes the mechanical METAL switch flipposition, and TA-DAH you have an SPDT with practically nil "switch on"resistance capable of passing high currents (in the As).The problem that I have with relays is to do with their datasheetcontents. Now I am more or less familiar with inductors; theirinductance varies with frequency alongside their series resistance buttheir inductance+serires resistance is more or less constant at lowfrequencies.How is it then that the [coil inductance + the coil resistance]caracteristics vary with the rated voltage of the coil (I assume thatthe rated voltage is the voltage drop accross the inductor used to flipthe switch) in order to maintain a steady power consumption?What I am missing ?Does the "L+ ESR" of the coil really increase/decrease with its voltagedrop ? Seems odd.I know that I must be not understanding a fundamental principle, sincevarious datasheets all have the same varying coil inductance w.r.t.their rated voltage.This "pseudo-problem was discovered" when scouring randomly-chosenrelay datasheets in order to find how relays are typically driven. Arethere ICs out there whose task is to drive relays. I.e. they source asteady current of lets say 50 mA to one of the relay coil's terminalwhile the other coil terminal is tethered to a power supply node ? Atypical digital control signal will simply not do the trick. Is there atypical approach for driving relays?All manners of feedback will be appreciated.Kind Regards -Roger

Jun 29, 2006 20 Replies

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Roger Bourne 20 years ago

The reason why I have to multiplex the DDR power is the following: I have to test for the DDR memory output pins for open and shorts. Therefore, I have to create a *testing cct* which in order to test for open and shorts has to be able to place all the pins of the DDR (power pins included) to gnd.

-Roger

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