I can only seem to find NC relays that are "reed relays" on DigiKey. I realize there are probably quite a few places that have NC relays, but I'm already ordering components from three seperate places and I'd rather not pay another six dollar shipping cost for just a relay.
They're two 'reeds' of thin strip metal in a sealed glass tube, can be worked with a permanent magnet or be put in the middle of a coil electro-magnet. Usually low-current only. Best bet is to look for a single-pole changeover relay, there are umpteen thousand available.
I see, thanks. I can't seem to find a changeover relay on DigiKey. I searched for "changeover relay", "change over relay" and "co relay" as well as using their filter thingy for relays and found no mention. Although I swear I saw a CO thing somewhere recently. Actually I just found an SPST NC relay on All Electronics for $1. Must have overlooked that. Thanks though.
How is SPDT normally closed? Unless you mean because you can have two sources switching it or...what? I'm building a PIMETA headphone amp, and I want a more reliable system for switching power from battery -> wall transformer than an NC 2.5mm coax plug, I've heard they're known to cause trouble over time and if the plug is abused at all. I'd rather not risk two 9v batteries getting charged by a high current source, you know?
3 switch connection points: common, NO, NC You could maybe use a diode to stop any risk of flow into the battery? A schottky will only lose about 0.4V from the supply.
But an NC relay would mean the arm is in contact and the circuit is closed, and powering it pulls the arm away and opens the circuit. NO would mean it's normally open and powering the relay pulls it closed and makes contact...how does changing poles change the physical state of the relay? I thought the point of SPDT was that you can have one switch switch two completely independand circuits at the same time. Yeah, that's probably a good idea. Infact I wonder if the schematic already has that in it, not sure. Come to think of it, the opamp I'm using requires at least 12v to work properly. I hope the 18v of two 9v batteries is enough...should be. I'm not too concerned with portability, but it would be nice for roadtrips and such.
That's Double-Pole, Single Throw; DPST SPDT, one set of contacts (Common & NC) are already made when the relay is un-energised; energising moves the Common arm over to the other contact (NO) so often called a change-over switch. http://forums.bit-tech.net/showpost.php?p=650624&postcount=2 for most possibilities
