Relays. [UK Motorcycles]

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From: Mark Olson on 30 Jul 2010 14:54

Krusty wrote:
> If I want to use a 6v supply to switch on a 9v thingy, can I use a
> normal 12v relay?

Maybe, as some relays will operate at less than their rated voltage,
but why not simply use a relay designed to work on 6V? What's the
application, how much current, etc.?

From: ian field on 30 Jul 2010 14:54

From: wessie on 30 Jul 2010 15:02

"Krusty" <dontwantany(a)nowhere.invalid> wrote in news:i2v6js$lra$1(a)news.eternal-september.org:

> If I want to use a 6v supply to switch on a 9v thingy, can I use a
> normal 12v relay?
>

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/Electronic-Components/Relays-Solenoids/PCB-Relays/Ultra-miniature-PCB-power-relay-10A/30804/kw/60-4275?source=googleps&utm_source=googleps

Seems there is a 6V version on that page too.

From: ian field on 30 Jul 2010 16:21

"Krusty" <dontwantany(a)nowhere.invalid> wrote in message
news:i2v6js$lra$1(a)news.eternal-september.org...
> If I want to use a 6v supply to switch on a 9v thingy, can I use a
> normal 12v relay?

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.

From: Mark Olson on 30 Jul 2010 16:33

ian field wrote:
> "Krusty" <dontwantany(a)nowhere.invalid> wrote in message
> news:i2v6js$lra$1(a)news.eternal-september.org...
>> If I want to use a 6v supply to switch on a 9v thingy, can I use a
>> normal 12v relay?
>
>
> 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.

All well and good if you're stranded at the south pole with limited resources,
but there must be shops about where Krusty can simply buy a suitable relay.

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