If I want to use a 6v supply to switch on a 9v thingy, can I use a normal 12v relay?
Depends on the relay, find out the manu'/type # and google for the data sheet - it should list the minimum pull in voltage.
doubt it - the current through the coil would be 50%, if you treat the coil as a resistance, which is unlikely to generate a big enough magnetic field to actuate the plunger. 5V relays are readily available. E.g. http://www.rapidonline.com/Electron...w/60-4275?source=googleps&utm_source=googleps Seems there is a 6V version on that page too.
Ease of purchase. However a quick look through Maplin's & RS's websites suggest the minimum switching voltage is 7+ so I guess a quick trip to Halfords in the morning is out. 'Stuff', & pass (but not much).
If you don't need electrical isolation between the 6 & 9V supplies there's a trick of the trade that might work. You'd stand much more chance getting a 12V relay to pull in with 9V than 6, but I'm guessing the 9V supply doesn't have the Ah capacity to supply the relay. The solution is to charge an electrolytic capacitor to 9V via a current limiting resistor - also feed the capacitor via an anti-backflow diode from the 6V supply. If you're lucky the capacitor charge can pull the relay in and then the 6V via the diode is enough to hold the relay after its pulled in. The higher you make the resistor, the less current draw on the 9V supply - but the longer it takes to charge the capacitor enough to pull the relay in the next time. You can also speed up the charging time by making the capacitor smaller, but there comes a point where the charge is too small to do the job.
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There's more likely to be shops around selling 6V relays than shops selling electrolytic capacitors, current limiting resistors & anti-backflow diodes that's for sure.
You could harvest all the parts from a battery/mains radio or a lot of other stuff. In the greater order of things the components I mentioned are orders of magnitude more common than relays - but if you know where to look, a relay could be easier to find.
Go to your local Maplins. -- Malc Rusted and ropy. Dog-eared old copy. Vintage and classic, or just plain Jurassic: all words to describe me.
Or you could use a charge pump if you're up for a challenge http://www.geofex.com/circuits/+9_to_33.htm you'd need a cap as above, since it might not be able to develop the current
There's pretty much dozens of different ways of going down that route and most of them don't involve an obscure chip that the likes of Maplin probably won't have on the shelf. The difficulty of these circuits rises steadily with current requirement, so if 6V is enough to hold the relay once its pulled in, my suggestion of using a charge at higher voltage on a capacitor for initial pull in still has some merit. The voltage doubler can be as simple as a 2 transistor astable multivibrator driving a capacitor to a pair of diodes to charge the electrolytic. This will charge the electrolytic to almost double the input supply, this won't supply enough current to continuously hold the relay, but if the original 6V is enough, an anti-backflow diode from the 6v to the electrolytic takes care of holding the relay.
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