Step down 9v to 6v for relays

[Dj Task Manager]

Hello all, I want to run a load of 6v relays off a 9v battery.
From a quick google Ive got the impression that all i need is a resistor.
(....to drop by a 1/3, if the resistance of the relay is 100r then a 50r resistor will do it....)

Is it really that simple?

Cheerz!

[Gordonjcp]

Pretty much.  Unless they're tiny relays they'll draw quite a bit of current and eat your battery.

Depending on what you're doing, sometimes it's worth overdriving the relay coil for a moment, so it pulls in faster.  The suspension solenoid valve on Citroen XMs is actually rated for 4V continuous, but is fed 12V for 100ms and then gets PWMed at around 30% duty cycle to stop it burning out.  The big pulse of current for 100ms makes it pull in much more quickly.

[Dj Task Manager]

they are pretty tiny, they are these:

http://cgi.ebay.co.uk/6V-DC-SPCO-MINIATURE-RELAY-PCB-MOUNTING_W0QQitemZ360334088818QQcategoryZ36328QQcmdZViewItemQQ_trksidZp5197.m7QQ_trkparmsZalgo%3DLVI%26itu%3DUCI%26otn%3D4%26po%3DLVI%26ps%3D63%26clkid%3D6336449201901277422

I think I'll just use a resistor as im trying to make the circuit as simple as possible.

How many hours do you think you'd get out of a standard 9v battery if these are used as follows: momentary pulses every couple of seconds?


Ta!

P.S. No-one else bid on these please!!!  :-)

[Gordonjcp]

Hard to say... You'd need to try it and see.  What are you building?

[Dj Task Manager]

The relays are for the output part of a mechanical sequencer.
Ive been designing it for ages, I should be able to program 7 percussion parts and up to 9 note (or chord) melodies on a 32 step array.

Its going to be rad.

[phantompowers]

That sounds fuckin ace. Pardon my French.

[Dj Task Manager]

Ok I still havent got this sorted,

So I thought I'd spell out exactly what I need...

Im running a 5v relay off a 9v battery.
The relays specs as follows: 5v, current 56.2mA, resistance 89ohms, power consumption 280mV.

The thing to do seems to be to arrange a couple of resistors as a voltage divider.  I was just gearing up to workout what resistors I need when I read that a 9v battery will quickly drop to 7.2v, so do i also need a voltage regulator?...

Another concern is I really want to maximise battery life so I dont want whatever solution to eat power...

If anyone could draw me a schematic that would be freakin' 'mazin'


Cheerz

Mike

[Dj Task Manager]

...oh yeah and like columbo, one more thing...

I also want a standard LED to light when the relay is triggered.

Thanks again guys sorry for being so pathetic and needy!

[Gordonjcp]

7.2V would be unusably flat for a 9V battery.  You don't need a voltage divider, you just need to calculate a series resistor such that the current is limited to what the relay is rated for.  You don't say what you're using to turn the relay on and off, so we'll assume an "ideal" transistor with no voltage drop across it, or a switch.

So V=IR - from the magic triangle we get R = V/I, we know V is 9V - 5V = 4V and we know that I is 0.05A (give or take).  So that's 4/0.05 = 80 ohms, nearest preferred value is 82 ohms.  Good enough for rock'n'roll.

One thing to watch is that a 9V battery typically has a capacity of 150-200mAH so your 50mA relay alone will flatten it in three to four hours.

[Dj Task Manager]

You rightly assumed the circuit is being made with a switch.

...and finally do you recon i should just pop the LED (that shows the relay is being triggered) in series or parallel to the relay (and if in series, to the positive side of the relay or negative side? or does it not matter?), and does this alter the resistor calculation from previously?

Thanks once again Gordon, you truely are the oracle!

[Gordonjcp]

It would have to be in parallel, since the LED would drop around 1.6-2V and have an absolute maximum current of around 20mA ;-)

Wire it across the relay coil, with a resistor in series.  The exact value of resistor will depend on the voltage and current of the LED - but you know how to work out the resistor now, don't you?  Somewhere between 220 ohms and 820 ohms would probably be about right.

[Dj Task Manager]

brilliant!  got it!

cheers!

I'll post pictures when the machine is finished!

[Dj Task Manager]

hey look, heres my sequencer.  Its nearly finished!...
sequencer demo